Patent Publication Number: US-10759416-B1

Title: Independent control of vehicle wheels

Description:
BACKGROUND 
     Vehicles often include a steering system and a braking system for controlling maneuvering of the vehicle. A steering system may be used to maneuver the vehicle along roads, causing the vehicle to travel through curves and provide general maneuverability, and a braking system may be used to slow or stop the vehicle. Used in combination with a powertrain, the steering and braking systems of the vehicle may be used to control the vehicle to maneuver between locations, avoid objects, and park the vehicle once the vehicle has arrived at a destination. However, situations may arise for which the steering and braking systems may be incapable of providing sufficient operational flexibility and/or safety. Thus, systems that provide enhanced control and/or maneuverability for a vehicle may be desired. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The detailed description is described with reference to the accompanying figures. In the figures, the left-most digit(s) of a reference number identifies/identify the figure in which the reference number first appears. The same reference numbers in different figures indicate similar or identical items. 
         FIG. 1  is a perspective view of an example vehicle including example vehicle systems shown in block diagram form. 
         FIG. 2  is a block diagram showing an example architecture for the vehicle systems and an example architecture for a drive system. 
         FIG. 3A  is a schematic overhead view of an example vehicle maneuvering through an example curve in the road. 
         FIG. 3B  is a schematic rear view of the vehicle shown in  FIG. 3A  with an example lateral force acting on the vehicle. 
         FIG. 3C  is a schematic rear view of the vehicle shown in  FIG. 3A  with another example lateral force acting on the vehicle. 
         FIG. 4A  is a schematic overhead view an example vehicle performing an example response to an example lateral force. 
         FIG. 4B  is a schematic overhead view an example vehicle performing another example response to an example lateral force. 
         FIG. 5A  is a schematic overhead view of an example vehicle maneuvering through an example curve according to an example first mode. 
         FIG. 5B  is a schematic overhead view of an example vehicle maneuvering through an example curve according to an example second mode. 
         FIG. 6A  is a schematic overhead view of an example vehicle maneuvering into an example parallel parking space according to an example first mode. 
         FIG. 6B  is a schematic overhead view of an example vehicle maneuvering into an example parallel parking space according to an example second mode. 
         FIG. 7A  is a schematic side view of an example vehicle including an example caster angle at the front end of the vehicle when the vehicle is operated in a first direction. 
         FIG. 7B  is a schematic side view of an example vehicle including an example caster angle at the front end of the vehicle when the vehicle is operated in a second direction opposite the first direction. 
         FIG. 8A  is a schematic front view of an example vehicle having wheels at a neutral camber angle. 
         FIG. 8B  is a schematic front view of an example vehicle having wheels at a negative camber angle. 
         FIG. 9A  is a schematic overhead view of an example vehicle having front wheels with toe-in. 
         FIG. 9B  is a schematic overhead view of an example vehicle having front wheels with toe-out. 
         FIG. 10A  is a schematic overhead view of an example vehicle maneuvering with a direction of travel that is misaligned with the longitudinal axis of the vehicle. 
         FIG. 10B  is a schematic overhead view of an example vehicle maneuvering with a direction of travel that is aligned with the longitudinal axis of the vehicle. 
         FIG. 11A  is a schematic overhead view of an example vehicle performing an example method for estimating tire-surface parameterization data associated with the surface on which the vehicle is traveling. 
         FIG. 11B  is a schematic overhead view of an example vehicle performing another example method for estimating tire-surface parameterization data associated with the surface on which the vehicle is traveling. 
         FIG. 12  is a flow diagram of an example process for preventing roll over of a vehicle. 
         FIG. 13  is a flow diagram of an example process for reducing or eliminating misalignment between the direction of travel of a vehicle and the longitudinal axis of the vehicle. 
         FIG. 14  is a flow diagram of an example process for estimating tire-surface parameterization data associated with the surface on which the vehicle is traveling 
         FIG. 15  is a flow diagram of another example process for estimating tire-surface parameterization data associated with the surface on which the vehicle is traveling. 
     
    
    
     DETAILED DESCRIPTION 
     Vehicles may include a steering system and a braking system for controlling maneuvering of the vehicle. However, situations may arise for which the steering and braking systems may be incapable of providing sufficient operational flexibility and/or safety. 
     This disclosure is generally directed to independent control of wheels and/or tires of a vehicle, which may provide enhanced safety, improved control, improved maneuverability, and/or acquisition of information relevant to operation of the vehicle or other vehicles. Some examples may facilitate preventing roll over of the vehicle, provide greater maneuverability or control when traveling through curves, provide ease of parking, provide greater directional stability, provide improved cornering ability, facilitate real-time vehicle alignment, and/or facilitate estimation of tire-surface parameterization data associated with the surface on which the vehicle is traveling. Tire-surface parameterization data may include, for example, friction-related data (e.g., static and/or dynamic coefficients of friction, including linear and non-linear tire behavior), tire cornering stiffness, side slip force, tire slip angles, or the like. 
     This disclosure is generally directed to a wheel control system configured to facilitate control of a vehicle having a longitudinal axis extending along a line between a first end of the vehicle and a second end of the vehicle opposite the first end of the vehicle. In some examples, the longitudinal axis of the vehicle may be aligned with, or contained in a plane, passing through the centerline of the vehicle. The wheel control system may include a first steering assembly configured to be coupled proximate to the first end of the vehicle and control wheel parameters of first wheels coupled proximate to the first end of the vehicle. The wheel control system may also include a wheel controller configured to control, based at least in part on a signal indicative of a rollover event, one or more parameters associated with the first wheels to prevent the vehicle from rolling over. The one or more parameters of the first steering assembly may include a steering angle and one or more of a caster angle, a camber angle, a toe angle, or a wheel speed. 
     The rollover event may include a lateral force acting on the vehicle in a first direction transverse to the longitudinal axis of the vehicle. In some examples, the wheel controller may be configured to determine, based at least in part on the signal indicative of a rollover event, that the lateral force exceeds a threshold force, and adjust wheel parameters of one or more of the first wheels or second wheels coupled proximate to the second end of the vehicle to reduce the lateral force to less than the threshold force. Wheel parameters of the second wheels may include one or more of a steering angle, a camber angle, a caster angle, or a toe angle. 
     The lateral force may result from a number of conditions. For example, the lateral force may result from lateral acceleration of the vehicle during cornering. In some examples, the wheel controller may be configured to control the first steering assembly to reduce the lateral acceleration of the vehicle to reduce the lateral force, so that the lateral force is less than the threshold force. The lateral force may result from other conditions, such as, for example, the wind. 
     The lateral force may result from a collision force. In some examples, the wheel controller may be configured to control the first steering assembly and a second steering assembly configured to be coupled proximate to the second end of the vehicle and control wheel parameters of second wheels coupled proximate to the second end of the vehicle to mitigate effects of the lateral force resulting from the collision force to prevent rollover of the vehicle. 
     In some examples, the wheel controller may be configured to cause the first steering assembly to pivot the first wheels at first steering angles relative to the longitudinal axis of the vehicle, and the second steering assembly to pivot the second wheels at second steering angles relative to the longitudinal axis of the vehicle. In some examples, the first and second steering angles may have the same sign (e.g., the first and second steering assemblies may pivot the respective wheels in the same direction relative to the longitudinal axis of the vehicle (i.e., both to the right, or both to the left)). In some examples, the first and second steering angles may have opposite signs (e.g., the first and second steering assemblies may pivot the respective wheels in opposite directions relative to the longitudinal axis of the vehicle (e.g., the first wheels pivot to the right, and the second wheels pivot to the left)). 
     In some examples, the wheel control system may include a braking system including brakes configured be coupled to one or more of at least one of the first wheels or at least one of the second wheels and a braking controller configured to control operation of the brakes. The braking controller may be configured to slow rotation of one or more of at least one of the first wheels or at least one of the second wheels of the vehicle based at least in part on the signal indicative of a rollover event. In some examples, the wheel control system may include an acceleration system including, for example, a motor configured to be coupled to one or more of at least one of the first wheels or at least one of the second wheels. The acceleration system may also include a motor controller configured to cause the motor to supply torque to one or more of at least one of the first wheels or at least one of the second wheels based at least in part on the signal indicative of a rollover event. 
     In some examples, the wheel control system may include a first steering assembly configured to be coupled proximate to the first end of the vehicle and control steering angles of first wheels coupled proximate to the first end of the vehicle. In some examples, the first steering assembly may be configured to independently control the steering angles of each of the first wheels of the vehicle. The wheel control system may also include a second steering assembly configured to be coupled proximate to the second end of the vehicle and control steering angles of second wheels coupled proximate to the second end of the vehicle. In some examples, the second steering assembly may be configured to independently control the steering angles of each of the second wheels of the vehicle. The wheel control system may further include a wheel controller configured to receive one or more signals indicative of a lateral force acting on the vehicle in a first direction transverse to the longitudinal axis of the vehicle. Such signal(s) may be generated via one or more sensors coupled to the vehicle and configured to generate such signals. In some examples, the wheel controller may be configured to determine, based at least in part on the one or more signals, when the lateral force exceeds a threshold force, for example, sufficient to cause the vehicle to roll over in the first direction. The wheel controller, in some examples, may be configured to control, when the lateral force exceeds the threshold force, one or more of the first steering assembly or the second steering assembly to reduce the lateral force to less than the threshold force, for example, to prevent the vehicle from rolling over. 
     In some examples, the wheel controller may be configured to determine, based at least in part on the one or more signals, when the lateral force exceeds a roll coefficient multiplied by a roll force sufficient to cause the vehicle to roll over in the first direction. The wheel controller, in some examples, may be configured to control, when the lateral force exceeds the roll coefficient multiplied by the roll force, one or more of the first steering assembly or the second steering assembly to reduce the lateral force to less than the roll coefficient multiplied by the roll force and prevent the vehicle from rolling over. In some examples, the roll coefficient may account for one or more safety factors. Other factors may be reflected in the roll coefficient. In some examples, the roll force may be predetermined, for example, based on characteristics of the vehicle (e.g., physical characteristics such as the weight and/or roll center of the vehicle), or the roll force maybe determined in real-time while the vehicle maneuvers. 
     In some examples, when the vehicle is traveling in a first forward travel direction, such that the first end of the vehicle is the front end of the vehicle, the wheel controller may be configured to cause the first steering assembly to pivot the first wheels coupled proximate to the front end of the vehicle at respective first steering angles relative to the first forward travel direction of the vehicle. In some examples, the first steering angles may range from about one degree to about 90 degrees from the first forward travel direction toward the first direction transverse to the longitudinal axis of vehicle. The wheel controller may also be configured to cause the second steering assembly to pivot the second wheels at second steering angles relative to the first forward travel direction of the vehicle. In some examples, the second steering angles may range from about one degree to about 90 degrees from the first forward travel direction toward the first direction transverse to the longitudinal axis of vehicle. For example, if the lateral force acting on the vehicle is toward the right side of the vehicle as viewed from the passenger compartment and facing in the first forward travel direction, the first steering angles may result in the first wheels pivoting toward the right, such that the front end of the vehicle will steer toward the right, and the second steering angles may result in the second wheels pivoting toward the right, such that the rear end of the vehicle will steer toward the right. 
     In some examples, when the vehicle is traveling in the first forward travel direction, such that the first end of the vehicle is the front end of the vehicle, the wheel controller may be configured to cause the first steering assembly to pivot the first wheels at first steering angles relative to the first forward travel direction of the vehicle. The first steering angles may range from about one degree to about 90 degrees from the first forward travel direction and in a direction opposite the first direction transverse to the longitudinal axis of vehicle. The wheel controller may also be configured to cause the second steering assembly to pivot the second wheels at second steering angles relative to the first forward travel direction of the vehicle. The second steering angles may range from about one degree to about 90 degrees from the first forward travel direction toward the first direction transverse to the longitudinal axis of vehicle. For example, if the lateral force acting on the vehicle is toward the right side of the vehicle as viewed from the passenger compartment and facing in the first forward travel direction, the first steering angles may result in the first wheels pivoting toward the left, such that the front end of the vehicle will steer toward the left, and the second steering angles may result in the second wheels pivoting toward the right, such that the rear end of the vehicle will steer toward the right. 
     This disclosure is also generally directed to a rollover prevention system including a wheel control system as outlined herein. In some examples, the rollover prevention system may include a braking system including respective brakes configured to be coupled to the first wheels of the vehicle. The braking system may also include a braking controller configured to control operation of the brakes. In some examples, the braking controller may be configured to cause rotation of at least one of the first wheels to slow in response to receipt of one or more signals indicative of the lateral force exceeding the roll coefficient multiplied by the roll force. In some examples, the braking system may be configured to independently control the brakes coupled to each of the first wheels of the vehicle. Some examples of the rollover prevention system may include an acceleration system including a motor configured to be coupled to the second wheels of the vehicle and a motor controller configured to cause the motor to supply torque to one or more of the second wheels of the vehicle in response to receipt of the one or more signals indicative the lateral force exceeding the threshold force, or, for example, the roll coefficient multiplied by the roll force. In some examples, the acceleration system may be configured to independently control the acceleration of each of the second wheels of the vehicle. In some examples, the wheel control system, the braking system, and/or the acceleration system may be used in combination to prevent the vehicle from rolling over (or reduce the likelihood of the vehicle rolling over). 
     This disclosure is also generally directed to a wheel control system configured to facilitate control of a vehicle. In some examples, the wheel control system may include a first steering assembly configured to be coupled proximate to the first end of the vehicle and control one or more parameters of first wheels coupled proximate to the first end of the vehicle. The wheel control system may also include a second steering assembly configured to be coupled proximate to the second end of the vehicle and control one or more parameters of second wheels coupled proximate to the second end of the vehicle. The wheel control system may further include a wheel controller configured to receive a signal indicative of a vehicle maneuver, and control operation of the first steering assembly and the second steering assembly to control the one or more parameters of the first wheels and the one or more parameters of the second wheels, respectively, based at least in part on the signal indicative of the vehicle maneuver. In some examples, the one or more parameters of the first and second steering assemblies may include a steering angle and one or more of a caster angle, a camber angle, a toe angle, or a wheel speed. 
     In some examples, the signal indicative of the vehicle maneuver corresponds to the vehicle following a trajectory having a curvature greater than zero and a speed greater than zero. The signal indicative of a vehicle maneuver may include a signal indicative of the speed of the vehicle, and the wheel controller may be configured to control operation of the first steering assembly and second steering assembly according to a first mode based at least in part on the signal indicative of the speed of the vehicle and transition to operation according to a second mode when the signal indicative of the speed of the vehicle indicates the speed of the vehicle is increasing. In some examples, controlling the first steering assembly and the second steering assembly according to the first mode may include causing the first steering assembly to adjust one or more of the castor angle, camber angle, or toe angle. In some examples, controlling the first steering assembly and the second steering assembly according to the first mode may include causing the first steering assembly to pivot the first wheels at first steering angles in a first direction relative to the longitudinal axis of the vehicle, and causing the second steering assembly to pivot the second wheels at second steering angles in a second direction relative to the longitudinal axis of the vehicle, the first and second steering angles having opposite signs. According to some examples, controlling the first steering assembly and the second steering assembly according to the second mode may include causing the first steering assembly to pivot the first wheels at first steering angles in a first direction relative to the longitudinal axis of the vehicle, and causing the second steering assembly to pivot the second wheels at second steering angles in a second direction relative to the longitudinal axis of the vehicle, the first and second steering angles having the same sign. Operating in the first mode may provide more maneuverability at relatively lower speeds, and operating in the second mode may provide greater cornering stability at relatively higher speeds. In some examples, the transition between the first mode and the second mode may be gradual and may be related to vehicle speed and/or other factors, such as, for example, steering angle and/or braking. In some examples, the transition between the first and second modes may depend on a threshold speed, below which the vehicle operates according to the first mode, and above which the vehicle operates according to the second mode. In some examples, the transition from the second mode to the first mode may occur as the vehicle speed is reduced, for example, in a manner similar to the transition from the first mode to the second mode. In some examples, there may not be a unique threshold, but angles may vary continuously with respect to speed. 
     According to some examples, the signal indicative of a vehicle maneuver may include a signal indicative of initiating parallel parking into a parking space. For example, when the vehicle approaches the parking space in a first travel direction, the wheel controller may be configured to control operation of the first steering assembly and the second steering assembly to cause the vehicle to park in the parking space (e.g., without the vehicle traveling in a second travel direction opposite the first travel direction). In some examples, the wheel controller may be configured to control operation of the first steering assembly to provide steering angles of the first wheels that direct the first wheels toward the parking space and into the parking space, and control operation of the second steering assembly to provide steering angles of the second wheels that direct the second wheels toward parking space and into the parking space. The wheel controller may be configured to thereafter control operation of the first steering assembly to provide steering angles of the first wheels that direct the first wheels in a direction at least one of along a longitudinal axis of the parking space or toward a distal longitudinal border of the parking space and into the parking space, while controlling operation of the second steering assembly to provide steering angles of the second wheels that direct the second wheels toward the distal longitudinal border of the parking space and into the parking space. 
     In some examples, the wheel controller may be configured to receive one or more signals indicative of the speed of the vehicle and/or one or more signals indicative of initiating one or more of a lane change of the vehicle or a change in trajectory of the vehicle. In such examples, the wheel controller may be configured to control operation of the first steering assembly and the second steering assembly to control the steering angles of the first wheels and the steering angles of the second wheels, respectively, based at least in part on the one or more signals indicative of the speed of the vehicle and/or the one or more signals indicative of initiating one or more of a lane change of the vehicle or a change in trajectory of the vehicle. For example, the wheel controller may be configured to control operation of the first steering assembly and second steering assembly according to a first mode when the one or more signals indicative of the speed of the vehicle indicate the speed of the vehicle is greater than or equal to a threshold speed, and according to a second mode when the one or more signals indicative of the speed of the vehicle indicate the speed of the vehicle is less than the threshold speed. For example, controlling the first steering assembly and the second steering assembly according to the first mode may include causing the first steering assembly to pivot the first wheels at first steering angles in a first direction relative to the longitudinal axis of the vehicle, and causing the second steering assembly to pivot the second wheels at second steering angles in the first direction relative to the longitudinal axis of the vehicle. For example, when maneuvering through a curve that bends to the right, the wheel controller may be configured to cause the first and second steering assemblies to turn the first wheels and the second wheels toward the right, so the both the front end and rear end of the vehicle move toward the right and through the curve. In some examples, although first wheels and the second wheels are turned in the same direction, the first steering angles may differ from the second steering angles. For example, the first steering angles may be 15 degrees to the right, and the second steering angles may be 10 degrees to the right. This may provide improved stability at higher speeds when maneuvering through a curve. 
     In some examples, controlling the first steering assembly and the second steering assembly according to the second mode may include causing the first steering assembly to pivot the first wheels at first steering angles in a first direction relative to the longitudinal axis of the vehicle, and causing the second steering assembly to pivot the second wheels at second steering angles in a second direction relative to the longitudinal axis of the vehicle, such that the first direction and the second direction are different from one another. For example, the wheel controller may cause the first steering assembly to turn the first wheels to the right and the second steering assembly to turn the second wheels to the left. This may provide more maneuverability at lower speeds, which may be desirable, for example, in a parking lot. 
     This disclosure is also generally directed to a wheel control system configured to facilitate control of a wheel. The wheel control system may include a steering assembly configured to be coupled proximate to a first end of a vehicle and control steering angles of the wheel, the wheel coupled proximate to the first end of the vehicle. The wheel controller may be configured to receive a signal indicative of a state of the vehicle, and cause the wheel controller to change one or more wheel parameters of the first wheels based at least in part on the signal indicative of the state of the vehicle. The signal indicative of a state of the vehicle may include one or more of a signal indicative of the direction of travel of the vehicle, a signal indicative of a speed of the vehicle, a signal indicative of a steering angle of the vehicle, or a signal indicative of an instability associated with the vehicle. The wheel controller is configured to control, based at least in part on the one or more signals indicative of the state of the vehicle, the one or more wheel parameters of the first wheels by controlling operation of one or more of a camber adjustment assembly configured to control a camber angle of the wheel, a caster adjustment assembly to configured to control a caster angle of the wheel, and a toe adjustment assembly configured to control a toe angle of the wheel. 
     For example, the signal indicative of the state of the vehicle may include a signal indicative that the vehicle is traveling in the forward travel direction. The one or more wheel parameters may include caster angles, and the wheel controller may be configured to control operation of a caster adjustment assembly to cause the wheel to have positive caster angle. In some examples, the signal indicative of the state of the vehicle may include a signal indicative that the vehicle is traveling in a forward travel direction, such that the first end of the vehicle is the front end of the vehicle. The one or more wheel parameters include camber angles, and the wheel controller may be configured to control operation of a camber adjustment assembly to cause the first wheels to change camber angles based at least in part on the signal indicative that the vehicle is travelling in the first forward travel direction. 
     Some examples of the wheel control system may include a caster adjustment assembly coupled (directly or indirectly) to one or more of the first steering assembly or the second steering assembly. The caster adjustment assembly may be configured to change caster angles of the first wheels and/or the second wheels. In some examples, the vehicle may be a bi-directional vehicle configured to travel between locations with either end of the vehicle being the leading or front end. The vehicle may include sensors and/or a system configured to generate one or more signals indicative of which end of the vehicle is the front or leading end. The wheel control system may include a wheel controller configured to receive one or more signals indicative of a direction of travel of the vehicle, and cause the caster adjustment assembly to change the caster angles of one of the first wheels or the second wheels based at least in part on the one or more signals indicative of the direction of travel of the vehicle. For example, when the vehicle is traveling in a first forward travel direction, such that the first end of the vehicle is the front end of the vehicle, the wheel controller may be configured to control operation of the caster adjustment assembly to cause the first wheels coupled proximate to the first end of the vehicle to have positive caster angles. This may improve the on-center steering stability of the vehicle. In some examples, the caster angles of the wheels of the rear or trailing end of the vehicle may be adjusted. 
     In some examples, the vehicle may be configured to travel in a first forward travel direction, such that the first end of the vehicle is the front end of the vehicle, and travel in a second forward travel direction, such that the second end of the vehicle is the front end of the vehicle. In such examples, the wheel controller may be configured to receive one or more signals indicative of the first forward travel direction and control operation of the caster adjustment assembly to cause one of the first wheels or the second wheels to have positive caster angles based at least in part on the one or more signals indicative of the first forward travel direction of the vehicle. 
     Some examples of the wheel control system may include a camber adjustment assembly coupled (directly or indirectly) to one or more of the first steering assembly or the second steering assembly. The camber adjustment assembly may be configured to change camber angles of one or more of the first wheels and/or one or more of the second wheels. For example, when the vehicle is traveling in a first forward travel direction, such that the first end of the vehicle is the front end of the vehicle, the wheel controller may be configured to control operation of the camber adjustment assembly to cause the first wheels to have negative camber angles (e.g., respective planes passing through the centers of the wheels and the outer diameters will intersect above the vehicle). In some examples, the wheel controller may be configured to receive one or more signals indicative of initiating turning of the vehicle, and control operation of the camber adjustment assembly to cause the one or more of the first wheels and/or one or more of the second wheels to increase amounts of respective negative camber angles based at least in part on the one or more signals indicative of initiating turning of the vehicle. Negative camber angles may improve by the cornering grip of the tires, thereby improving cornering ability of the vehicle. 
     Some examples of the wheel control system may include a toe adjustment assembly coupled (directly or indirectly) to one or more of the first steering assembly or the second steering assembly. The toe adjustment assembly may be configured to change a toe angle of one or more of the first wheels and/or one or more of second wheels. For example, the wheel control system may include a controller configured to receive one or more signals indicative of the speed of the vehicle, and receive one or more signals indicative of initiating a turn for the vehicle. Based on one or more of these signals, the wheel controller may be configured to control operation of the toe adjustment assembly to change the toe angle of one or more of the first wheels and/or one or more of the second wheels. For example, the wheel controller may be configured to control operation of the toe adjustment assembly to change the toe angle of both the first wheels and/or both the second wheels, such that an amount of toe-in (e.g., respective planes passing through the centers of the wheels and the outer diameters will intersect in front of the vehicle) of the first wheels and/or the amount of toe-in of the second wheels is reduced upon receipt of the one or more signals indicative of initiating a turn for the vehicle. For example, the toe adjustment assembly may change the toe angle of one or more of the first wheels and/or one or more of the second wheels to have toe-out. Toe-out may improve the responsiveness of the vehicle to steering inputs. 
     In some examples, the wheel controller may be configured to control operation of the toe adjustment assembly based at least in part on the one or more signals indicative of speed, and the wheel controller may be configured to control operation of the toe adjustment assembly to change the toe angle of one or more of the first wheels and/or one or more the second wheels, such that an amount of toe-in of the first wheels and/or the second wheels is increased when the one or more signals indicative of speed indicates a vehicle speed greater than a threshold speed. Toe-in may improve straight-line stability of the vehicle. 
     In some examples of the wheel control system, the wheel control system may include a wheel controller configured to be coupled proximate to the first end of the vehicle and control steering angles and one or more of a toe angle, camber angle, or caster angle of first wheels coupled proximate to the first end of the vehicle. The wheel controller may also be configured to determine misalignment of one or more of at least one of the first wheels or at least one of the second wheels while the vehicle is travelling, and at least one of reduce or eliminate the misalignment. For example, the wheel controller may be configured to receive a signal indicative of the misalignment, and the signal indicative of the misalignment may be based at least in part on a signal generated by a localization system. In some examples, the localization system may be configured to determine one or more of a position or orientation of the vehicle based at least in part on one or more sensors coupled to the vehicle. The one or more sensors may include one or more of a global positioning system, an inertial measurement unit, a LIDAR, or a camera. 
     In some examples, the wheel controller may be configured to control toe angles of a first side wheel and a second side wheel of the first wheels (e.g., the left and right front wheels), so that the toe angles are the same magnitude in opposite directions. In some examples, the wheel controller may be configured to control camber angles of the first side wheel and the second side wheel of the first wheels, so that the camber angles are the same magnitude in opposite directions. In some examples, the wheel controller may be configured to control caster angles of the first side wheel and the second side wheel of the first wheels, so that the caster angles are the same magnitude in the same direction. This may result in alignment of the wheels of the vehicle. In some examples, the toe, caster, and/or camber angles of the first and second side wheels of the second wheels (e.g., the left and right rear wheels) may be aligned in an at least similar manner. 
     In some examples, the wheel control system may include a first steering assembly configured to be coupled proximate to the first end of the vehicle and control steering angles of first wheels coupled proximate to the first end of the vehicle. The wheel control system may also include a second steering assembly configured to be coupled proximate to the second end of the vehicle and control steering angles of second wheels coupled proximate to the second end of the vehicle. The wheel controller may be configured to receive a signal indicative of at least one of torque supplied to the first wheels and the second wheels, a lateral force associated with at least one of the first steering assembly or the second steering assembly, or a deviation from the direction of travel aligned with the longitudinal axis of the vehicle. In some examples, the wheel controller may be configured to control, based at least in part on the one or more signals, one or more of the steering angles, toe angles, the camber angles, or the caster angles of one or more of at least one of the first wheels or at least one of the second wheels. 
     In some examples, the wheel controller may be configured to receive a signal indicative of a direction of travel of the vehicle and a signal indicative of a direction defined by the longitudinal axis of the vehicle, and determine, based at least in part on the signal indicative of the direction of travel of the vehicle and the signal indicative of a direction defined by the longitudinal axis of the vehicle, misalignment of one or more of at least one of the first wheels or at least one of the second wheels. The wheel controller may also be configured to at least one of reduce or eliminate the misalignment based at least in part in part on the signals. 
     In some examples, the wheel control system may include a first steering assembly including a first steering rack configured to be coupled to each of the first wheels and control orientation of the first wheels relative to the longitudinal axis of the vehicle. The system may also include a second steering assembly including a second steering rack configured to be coupled to each of the second wheels and control orientation of the second wheels relative to the longitudinal axis. In some such systems, the wheel controller may be configured to control operation of the first steering rack and the second steering rack to align the first wheels and the second wheels with the longitudinal axis of the vehicle. For example, one or more of the first steering rack or the second steering rack may define respective zero positions intended to align the associated first wheels and second wheels with the longitudinal axis of the vehicle. In some examples, the wheel controller may be configured to adjust the zero positions of one or more of the first steering rack or the second steering rack, so that each of the first wheels and each of the second wheels is aligned with the longitudinal axis of the vehicle, for example, within technical tolerances. 
     In some examples, the wheel controller may be configured to at least one of control the first steering assembly to change the first steering angles of the first wheels in a first direction, or control the second steering assembly to change the second steering angles of the second wheels in a second direction having the same sign as the first direction. In some examples, the wheel controller may be configured to at least one of control the first steering assembly to change the first steering angles of the first wheels in a first direction, or control the second steering assembly to change the second steering angles of the second wheels in a second direction having the opposite sign as the first direction. 
     In some examples of the wheel control system, the wheel controller may be configured to receive one or more signals indicative of a direction of travel of the vehicle and one or more signals indicative of a direction defined by the longitudinal axis of the vehicle. In such examples, the wheel controller may be configured to determine, based at least in part on the one or more signals indicative of a direction of travel of the vehicle and the one or more signals indicative of the direction defined by the longitudinal axis of the vehicle, misalignment between the direction of travel of the vehicle and the direction defined by the longitudinal axis of the vehicle. The wheel controller, in some examples, may be configured to control the first steering assembly and the second steering assembly to reduce or eliminate the misalignment between the direction of travel of the vehicle and the direction defined by the longitudinal axis of the vehicle. 
     In some examples, the wheel control system may include, or be in communication with, a global positioning system, and the one or more signals indicative of the direction of travel of the vehicle may be based at least in part on one or more signals generated by the global positioning system. In some examples, the wheel control system may include, or be in communication with, a sensor configured to generate one or more signals indicative of the direction defined by the longitudinal axis of the vehicle the longitudinal axis of the vehicle, and the one or more signals indicative of the direction defined by the longitudinal axis of the vehicle may be based at least in part on one or more signals received from the sensor. In some examples, the wheel controller may be configured to control the first steering assembly to change the first steering angles of the first wheels in a first direction, and control the second steering assembly to change the second steering angles of the second wheels in a direction consistent with the first direction. For example, the first and second steering angles may be the same. 
     This disclosure is also generally directed to a system for estimating tire-surface parameterization data associated with a surface on which a vehicle travels. In some examples, the system may include a wheel controller configured to control at least one wheel parameter associated with a tire of the vehicle. The at least one wheel parameter may include at least one of a steering angle, caster angle, camber angle, toe angle, or wheel speed of the one or more wheels. The wheel controller may be configured to induce a change of at least one wheel parameter associated with the tire, and receive a signal indicative of the change of state of the vehicle. The change of state may include at least one of the direction of travel of the vehicle, the speed of the vehicle, the steering angle of the vehicle, or an instability of the vehicle. The wheel controller may also be configured to estimate tire-surface parameterization data associated with the tire and the surface based at least in part on the signal. For example, the wheel controller may be configured to estimate tire-surface parameterization data, such as, for example, friction-related data (e.g., static and/or dynamic coefficients of friction, including linear and non-linear tire behavior), tire cornering stiffness, side slip force, tire slip angles, or the like, associated with interaction between the tire and the surface. 
     In some examples, the system may include a first steering assembly configured to be coupled to a first tire proximate to a first end of the vehicle, and a first brake configured to be coupled to the first tire and slow rotation of the first tire. The system may also include a second steering assembly configured to be coupled to a second tire of the vehicle, and a tire-surface parameterization estimator. The wheel controller, in some examples, may be configured to activate at least one of the first steering assembly to change a steering angle associated with the first tire or activate the brake to slow rotation of the first tire. The wheel controller may also be configured to cause at least one of the second steering assembly to change a steering angle associated with the second tire to resist at least one of a change of speed or a change of direction of travel of the vehicle caused by activation first steering assembly or the brake. The tire-surface parameterization estimator may be configured to receive a signal indicative of at least one of the steering angle associated with the first tire or the speed of rotation of the first tire upon activation of at least one of the first steering assembly or the brake, respectively. The tire-surface parameterization estimator may also be configured to receive a signal indicative of the steering angle associated with the second tire, and estimate. The wheel controller may also be configured to estimate tire-surface parameterization data associated with the tire and the surface based at least in part on the signal data associated with the first tire and the surface based at least in part on the signals. 
     In some examples, the system may also include a motor configured to be coupled to the second tire and supply torque to the second tire. The wheel controller may be configured to activate at least one of the first steering assembly to change a steering angle associated with the first tire or activate the brake to slow rotation of the first tire, and cause at least one of the second steering assembly to change a steering angle associated with the second tire or the motor to supply torque to the second tire, to resist at least one of a change of speed or a change of direction of travel of the vehicle caused by activation first steering assembly or the brake. The tire-surface parameterization estimator may be configured to receive a signal indicative of at least one of the steering angle associated with the first tire or the speed of rotation of the first tire upon activation of at least one of the first steering assembly or the brake, respectively. The tire-surface parameterization estimator may also be configured to receive a signal indicative of at least one of the steering angle associated with the second tire or the torque applied to the second tire, and estimate tire-surface parameterization data associated with the first tire and the surface based at least in part on the signals. 
     In some examples, the wheel controller may be configured to activate the brake to cause the first tire to stop rotating, and the tire-surface parameterization estimator may be configured to receive a signal indicative of the speed of rotation of the first tire as the first tire slows to a stop and determine the tire-surface parameterization data based at least in part on the signal indicative of the speed of rotation as the first tire slows to a stop and the signal indicative of at least one of the steering angle associated with the second tire or the torque applied to the second tire. The tire-surface parameterization estimator, in some examples, may be configured to estimate the coefficient of static friction between the tire and the surface based as least in part on the tire-surface parameterization data. In some examples, the tire-surface parameterization estimator may be configured to determine the tire-surface parameterization data based at least in part on the signal indicative of at least one of the steering angle associated with the second tire or the torque applied to the second tire. In some examples, the tire-surface parameterization estimator may be configured to estimate a coefficient of dynamic friction between the tire and the surface based as least in part on the tire-surface parameterization data. 
     Some examples of the system may include a reporting module in communication with a communication network and configured to communicate, to one or more of network-based data stores or other vehicles, data correlating a location of the vehicle and the tire-surface parameterization-related data via the communication network. In such examples, the tire-surface parameterization data may be used by other vehicles and/or other parties. 
     In some examples of the system for estimating tire-surface parameterization, the system may include a brake coupled to a first tire and configured to slow rotation of the first tire, and a motor coupled to a second tire and configured to supply torque to the second tire. The system may also include a wheel controller configured to activate the brake to cause rotation of the first tire to slow, and cause the motor to supply torque to the second tire to resist slowing of the vehicle caused by activation of the brake. The system may also include a tire-surface parameterization estimator configured to receive one or more signals indicative of the speed of rotation of the first tire during the braking, and receive one or more signals indicative of a braking force on the first tire during the braking. Based at least in part on one or more of the signals indicative of the speed of rotation of the first tire or indicative of the braking force on the first tire, the tire-surface parameterization estimator may be configured to estimate tire-surface parameterization data associated with the surface. In some examples, the tire-surface parameterization data may include a friction coefficient (e.g., a static or a dynamic friction coefficient) between the surface and one or more tires of the vehicle. In some examples, the brake may include a brake that is part of a regenerative braking system. 
     In some examples, the tire-surface parameterization data may be used by the vehicle to determine performance parameters for the vehicle, such as, for example, maximum accelerations for increasing vehicle speed, maximum accelerations for reducing vehicle speed, maximum cornering speeds, etc. In some examples, the tire-surface parameterization data may be correlated to the location of the vehicle and may be communicated via a communication network for use by other parties and/or other vehicles. For example, the tire-surface parameterization data estimated by the vehicle may be used by other vehicles that are in communication with the communication network. Some examples of the system for estimating tire-surface parameterization data may also include a reporting module in communication with a communication network. The reporting module may be configured to communicate data correlating the location of the vehicle and the tire-surface parameterization data via the communication network. 
     Some examples of the system for estimating tire-surface parameterization data may include a steering assembly configured to be coupled proximate to an end of the vehicle and independently control a first steering angle of a first tire coupled proximate to the end of the vehicle and a second steering angle of a second tire coupled proximate to the end of the vehicle. For example, the steering assembly may be configured to control the first steering angle so that the first tire is at a first steering angle, while the second tire is at a second steering angle. In some examples, the steering assembly may be configured to change the first steering angle without changing the second steering angle, and/or change the second steering angle without changing the first steering angle. For example, the system for estimating tire-surface parameterization data may also include a wheel controller configured to cause the steering assembly to change the first steering angle, so that the first steering angle and the second steering angle differ from one another. The system for estimating tire-surface parameterization data may also include a tire-surface parameterization estimator configured to receive one or more signals indicative of a lateral force on the first tire, and estimate tire-surface parameterization data associated with the surface based at least in part on the one or more signals indicative of the lateral force on the first tire. 
     In some examples of the system for estimating tire-surface parameterization data, the steering assembly may include a steering actuator coupled to the first tire, and the one or more signals indicative of the lateral force on the first tire may be based at least in part on one or more signals indicative of a force acting on the steering actuator. Some examples of the system for estimating tire-surface parameterization data may include a motor configured to supply torque to cause the vehicle to move, and the one or more signals indicative of the lateral force on the first tire may be based at least in part on one or more signals indicative of a change in torque supplied by the motor to maintain the speed of the vehicle when the steering assembly changes the steering angle of the first tire, or a deceleration of the vehicle where no additional torque is supplied. Some examples of the system for estimating tire-surface parameterization data may include a sensor coupled to the steering assembly and configured to generate the one or more signals indicative of the lateral force on the first tire. 
     The techniques and systems described herein may be implemented in a number of ways. Example implementations are provided below with reference to the figures. 
       FIG. 1  is a perspective view of an example vehicle  100  including example vehicle systems  102  shown in block diagram form. The example vehicle  100  may be configured to travel via a road network  104  from one geographic location to a destination carrying one or more occupants. For the purpose of illustration, the vehicle  100  may be a driverless vehicle, such as an autonomous vehicle configured to operate according to a Level 5 classification issued by the U.S. National Highway Traffic Safety Administration, which describes a vehicle capable of performing all safety-critical functions for the entire trip, with an occupant not being expected to control the vehicle at any time. In such examples, because the vehicle  100  may be configured to control all functions from start to completion of the trip, including all parking functions, it may not include a driver and/or controls for driving the vehicle  100 , such as a steering wheel, an acceleration pedal, and/or a brake pedal. This is merely an example, and the example vehicle  100  may be any configuration of vehicle, such as, for example, a van, a sport utility vehicle, a cross-over vehicle, a truck, a bus, an agricultural vehicle, or a construction vehicle, such a vehicle operating according to any level of autonomy (e.g., Level 0 being fully manually operated to Level 5, as indicated above). 
     The vehicle  100  may be powered by one or more internal combustion engines, one or more electric motors, hydrogen power, any combination thereof, and/or any other suitable power sources. Although the example vehicle  100  has four wheels  106  and four tires  108  respectively mounted on the wheels  106 , the systems and methods described herein may be incorporated into vehicles having fewer or a greater number of wheels, tires, and/or tracks. The example vehicle  100  may have four-wheel steering and may operate generally with equal performance characteristics in all directions, for example, such that a first end  110  of the vehicle  100  is the front end of the vehicle  100  when traveling in a first travel direction  112 , and such that the first end  110  becomes the rear end of the vehicle  100  when traveling in the opposite, second travel direction  114 , as shown in  FIG. 1 . Similarly, a second end  116  of the vehicle  100  is the front end of the vehicle  100  when traveling in the second travel direction  114 , and such that the second end  116  becomes the rear end of the vehicle  100  when traveling in the opposite, first travel direction  112 . These example characteristics may facilitate greater maneuverability, for example, in small spaces or crowded environments, such as urban areas and parking lots. 
     As shown in  FIG. 1 , the vehicle  100  includes a chassis  118  coupled to the four wheels  106  and configured to carry and protect the occupants of the vehicle  100  as it travels from a location to a destination via a route on the road network  104 . The example vehicle systems  102  of the vehicle  100  include a drive system  120  configured to control maneuvering and/or related operations of the vehicle  100 . As shown in  FIG. 1 , the drive system  120  may include a wheel control system  122  configured to control operations associated with steering the wheels  106  of the vehicle  100 . In some examples, the wheel control system  122  may include a wheel controller  124  configured to control operation of one or more steering assemblies  126  coupled to the wheels  106  and chassis  118  of the vehicle  100 . For example, the wheel controller  124  may control operation of one or more steering assemblies  126  to change steering angles associated with one or more of the wheels  106 . The wheel control system  122  may also include a rollover prevention system  128  configured to prevent roll over of the vehicle  100  (or at least reduce the likelihood that the vehicle  100  will roll over). 
     The example wheel control system  122  may also include a braking system  130  configured to control vehicle operations related to slowing the rotation of one or more wheels  106  of the vehicle  100 . In some examples, the braking system  130  may include one or more brakes associated with each of the wheels  106 . The brakes, in some examples, may include regenerative braking devices, such as electric machines configured to convert the kinetic energy associated with rotation of the wheels  106  into electric energy. Additionally, or alternatively, the brakes may include a friction braking device, such as a caliper. The example wheel control system  122  also includes an acceleration system  132  coupled to the chassis and configured to cause one of more wheels  106  of the vehicle  100  to provide movement of the vehicle  100 , which may include supplying torque to one or more wheels  106  of the vehicle  100 . The acceleration system  132  may include one or more electric motors coupled to one or more of the wheels  106 . In some examples, the rollover prevention system  128  may incorporate the braking system  130  and/or the acceleration system  132 . 
     Some examples of the wheel control system  122  may include a system for estimating tire-surface parameterization data  134  associated with the surface on which the vehicle  100  travels (e.g., the road network  104 ). In some examples, the system for estimating tire-surface parameterization data  134  may include a tire-surface parameterization estimator  136  configured to receive one or more signals from components of the vehicle systems  102  and estimate tire-surface parameterization data associated with the surface on which the vehicle  100  travels. In some examples, the system for estimating tire-surface parameterization data  134  may incorporate one or more aspects of the wheel control system  122 , the braking system  130 , and/or the acceleration system  132 , for example, as described herein. In some examples, the wheel control system  122  may also include a reporting module  138  in communication with a communication network and configured to communicate data correlating the location of the vehicle  100  and the tire-surface parameterization data via the communication network. 
     In some examples, the wheel control system  122  may also include a caster adjustment assembly  140  coupled to one or more of the steering assemblies  126  and configured to change the caster angle associated with one or more of the wheels  106  of the vehicle  100 . This may facilitate improving the stability of the vehicle  100  under certain operating conditions. Some examples of the wheel control system  122  may also include a camber adjustment assembly  142  coupled to one or more steering assemblies  126  of the vehicle  100 . The camber adjustment assembly  142  may be configured to change the camber angle associated with one or more wheels  106  of the vehicle  100 , which may improve handling of the vehicle  100 , for example, as it travels through curves in the road network  104 . In some examples, the wheel control system  122  may also include a toe adjustment assembly  144  coupled to one or more of the steering assemblies  126 . The toe adjustment assembly  144  may be configured to change the toe angle associated with one or more of the wheels  106 , which may improve the stability and/or responsiveness of the handling of the vehicle  100 . 
       FIG. 2  is a block diagram of an example architecture  200  including vehicle systems  102  for controlling operation of the systems that provide data associated with operation of the vehicle  100 , and that control operation of the vehicle  100 . In various implementations, the architecture  200  may be implemented using a uniprocessor system including one processor, or a multiprocessor system including several processors (e.g., two, four, eight, or another suitable number). The processor(s) may be any suitable processor capable of executing instructions. For example, in various implementations, the processor(s) may be general-purpose or embedded processors implementing any of a variety of instruction set architectures (ISAs), such as the x86, PowerPC, SPARC, FPGA, ASIC, or MIPS ISAs, or any other suitable ISA. In multiprocessor systems, each processor may commonly, but not necessarily, implement the same ISA. In some examples, the processor(s) may include a central processing unit (CPU), a graphics processing unit (GPU), or a combination thereof. 
     The example architecture  200  may include a non-transitory computer readable media configured to store executable instructions/modules, data, and/or data items accessible by the processor(s). In various implementations, the non-transitory computer readable media may be implemented using any suitable memory technology, such as static random access memory (SRAM), synchronous dynamic RAM (SDRAM), nonvolatile/Flash-type memory, or any other type of memory. In the illustrated implementation, program instructions and data implementing desired functions, such as those described above, are shown stored within the non-transitory computer readable memory. In other implementations, program instructions, and/or data may be received, sent, or stored on different types of computer-accessible media, such as non-transitory media, or on similar media separate from the non-transitory computer readable media. Generally speaking, a non-transitory, computer readable memory may include storage media or memory media, such as flash memory (e.g., solid state memory), magnetic or optical media (e.g., a disk) coupled to the architecture  200  via an I/O interface. Program instructions and data stored via a non-transitory computer readable medium may be transmitted by transmission media or signals such as electrical, electromagnetic, or digital signals, which may be conveyed via a communication medium such as a network and/or a wireless link, such as may be implemented via a network interface. 
     In some implementations, the I/O interface may be configured to coordinate I/O traffic between the processor(s), the non-transitory computer readable media, and any peripheral devices, the network interface, or other peripheral interfaces, such as input/output devices. In some implementations, the I/O interface may perform any necessary protocol, timing, or other data transformations to convert data signals from one component (e.g., the non-transitory computer readable media) into a format suitable for use by another component (e.g., processor(s)). In some implementations, the I/O interface may include support for devices attached through various types of peripheral buses, such as a variant of the Peripheral Component Interconnect (PCI) bus standard or the Universal Serial Bus (USB) standard, for example. In some implementations, the function of the I/O interface may be split into two or more separate components, such as a north bridge and a south bridge, for example. Also, in some implementations, some or all the functionality of the I/O interface, such as an interface to the non-transitory computer readable media, may be incorporated directly into the processor(s). 
     In the example architecture  200  shown in  FIG. 2 , the example vehicle systems  102  include a plurality of vehicle sensors  204 , for example, configured to sense movement of the vehicle  100  through the environment, sense environmental data, such as the ambient temperature, pressure, and humidity, and/or sense objects in the environment surrounding the vehicle  100 . In some examples, the vehicle sensors  204  may include sensors configured to identify a location on a map. The vehicle sensors  204  may include, for example, one or more light detection and ranging sensors (LIDAR), one or more cameras (e.g., RGB-cameras, intensity (grey scale) cameras, infrared cameras, depth cameras, stereo cameras, and the like), one or more radio detection and ranging sensors (RADAR), one or more sound navigation and ranging sensors (SONAR), one or more microphones for sensing sounds in the environment, such as sirens from law enforcement and emergency vehicles, and other sensors related to the operation of the vehicle  100 . Other sensors may include a speed sensor, sensors related to operation of internal combustion engines and/or electric motors, sensors related to the tires to detect tire temperature, tire pressure, and tread depth, and/or brake-related sensors for detecting brake temperatures and/or wear, and in vehicles having regenerative braking, sensors for detecting parameters related to operation of the regenerative braking system. The vehicle sensors  204  may also include, for example, inertial measurement units (IMUs), accelerometers, gyroscopes, and magnetometers. The vehicle sensors  204  may be configured to provide sensor data  206  representative of the sensed objects and signals to the vehicle systems  102  via, for example, an input/output (I/O) interface  208 . Other types of sensors and sensor data are contemplated. 
     The example vehicle systems  102  also include location systems  210  configured to receive location information, including position and/or orientation data (e.g., a local position or local pose) from the vehicle sensors  204  and/or external sources, and provide location data  212  to other portions of the vehicle systems  102  via the I/O interface  208 . The external sources may include global satellites for facilitating operation of a global positioning system (GPS) and/or a wireless network for communicating and receiving information related to the vehicle&#39;s location, such as map data. The location systems  210  may also include sensors configured to assist with navigation of the vehicle  100 , such as wheel encoders for sensing the rotation of the wheels, inertial navigation sensors, such as gyroscopes and/or accelerometers, magnetometers, and/or cameras for obtaining image data for visual odometry or visio-inertial navigation. 
     The example vehicle systems  102  may also include one or more of a planner  214 , an object data calculator  216 , an object classifier  218 , a collision predictor system  220 , a kinematics calculator  222 , and a safety system actuator  224 . The vehicle systems  102  may be configured to access one or more data stores including, but not limited to, an object type data store  226 . The object type data store  226  may include data representing object types associated with object classifications for objects detected in the environment. 
     The example vehicle systems  102  shown in  FIG. 2  also includes a vehicle controller  228  configured to receive vehicle control data  230 , and based on the vehicle control data  230 , communicate with the drive system  120 , which may include one or more of the wheel control system  122 , the wheel controller  124 , the one or more steering assemblies  126 , the rollover prevention system  128 , the braking system  130 , the acceleration system  132 , the system for estimating tire-surface parameterization data  134 , the tire-surface parameterization estimator  136 , the caster adjustment assembly  140 , the camber adjustment assembly  142 , and the toe adjustment assembly  144  to control operation of the vehicle  100 . For example, the vehicle control data  230  may be based at least in part on data received from one of more of the vehicle sensors  204  and one or more of the planner  214 , the object data calculator  216 , the object classifier  218 , the collision predictor system  220 , the kinematics calculator  222 , and the safety system actuator  224 , and control operation of the drive system  120 , to execute operation and maneuvering of the vehicle  100 . 
     In some examples, the planner  214  may be configured to generate data representative of a trajectory of the vehicle  100 , for example, using data representing a location of the vehicle  100  in the environment and other data, such as local pose data that may be included in the location data  212 . In some examples, the planner  214  may also be configured to determine projected trajectories predicted to be executed by the vehicle  100 . The planner  214  may, in some examples, be configured to calculate data associated with a predicted motion of an object in the environment, and may determine a predicted object path associated with the predicted motion of the object. In some examples, the object path may include the predicted object path. In some examples, the object path may include a predicted object trajectory. In some examples, the planner  214  may be configured to predict more than a single predicted object trajectory. For example, the planner  214  may be configured to predict multiple object trajectories based on, for example, probabilistic determinations or multi-modal distributions of predicted positions, trajectories, and/or velocities associated with an object. 
     In some examples, the object data calculator  216  may be configured to provide data representative of, for example, one or more of the pose (e.g., position and orientation) of an object in the environment surrounding the vehicle  100 , an object track associated with the object (e.g., a historic position, velocity, acceleration, and/or heading of the object over a period of time (e.g., 5 seconds)), and an object classification associated with the object (e.g., a pedestrian, a vehicle, a bicyclist, etc.). For example, the object data calculator  216  may be configured to receive data in the form of sensor signals received from one or more of the vehicle sensors  204  and determine data representing one or more of the position and/or orientation in the environment of the object, the object track, and the object classification. 
     In some examples, the object classifier  218  may be configured to access data from the object type data store  226 , which may be configured to store data representing object types, such as, for example, a species of an object classification, a subclass of an object classification, and/or a subset of an object classification. The object classifier  218 , in some examples, may be configured to analyze data representing an object track and data representing an object classification with data representing an object type, and determine an object type based at least in part on the object track and classification data. For example, a detected object having an object classification of an “automobile” may have an object type of “sedan,” “coupe,” “hatch-back,” “sports utility vehicle,” “pick-up truck,” or “minivan.” An object type may include additional subclasses, designations, or subsets. For example, a “sedan” that is parked may have an additional subclass designation of being “static” or “being dynamic” if moving. In some examples, such an object classifier  218  may also determine a predicted object behavior based on one or more of a portion of the sensor data  206  or the object type. 
     In some examples, the collision predictor system  220  may be configured to use the predicted object behavior, the data representing the object type, the data representing the trajectory of the object, and/or the data representing the trajectory of the vehicle  100 , to predict a collision between the vehicle  100  and the object. 
     In some examples, the kinematics calculator  222  may be configured to determine data representing one or more scalar and/or vector quantities associated with motion of objects in the environment, including, but not limited to, velocity, speed, acceleration, momentum, local pose, and/or force. Data from the kinematics calculator  222  may be used to compute other data, including, but not limited to, data representing an estimated time to impact between an object and the vehicle  100 , and data representing a distance between the object and the vehicle  100 . In some examples, the planner  214  may use data produced by the kinematics calculator  222  to estimate predicted object data. For example, the planner  214  may use current scalar and/or vector quantities associated with an object to determine a likelihood that other objects in the environment (e.g., cars, motorcyclists, pedestrians, bicyclists, and animals) are moving in an alert or controlled state, versus an un-alert or uncontrolled state. For example, the kinematics calculator  222  may be configured estimate the probability that other objects are moving as though they are being controlled and/or are behaving in a predictable manner, or whether they are not being controlled and/or behaving in an unpredictable manner, for example, by observing motion of the object over time and relative to other objects in the environment. For example, if the objects are moving erratically or without appearing to adjust to the presence or motion of other objects in the environment, this may be an indication that the objects are either uncontrolled or moving in an unpredictable manner. This may be inferred based on sensor data received over time that may be used to estimate or predict a future location of the object relative to a current or future trajectory of the vehicle  100 . 
     In some examples, the safety system actuator  224  may be configured to activate one or more safety systems of the vehicle  100  when a collision is predicted by the collision predictor system  220  and/or the occurrence of other safety related events, such as, for example, an emergency maneuver by the vehicle  100 , such as hard braking or a sharp acceleration. The safety system actuator  224  may be configured to activate an interior safety system (e.g., tensioning seat belts, deploying air bags, deploying curtains, providing audible alerts, and the like), an exterior safety system (e.g., including warning sounds and/or warning lights), the drive system  120 , which may be configured to execute an emergency maneuver to avoid a collision, and/or any combination thereof. For example, the drive system  120  may receive data for causing the wheel control system  122  of the vehicle  100  to change the travel direction of the vehicle  100 , and an acceleration system of the vehicle  100  to change the speed of the vehicle  100  to alter the trajectory of vehicle  100  from an initial trajectory to a trajectory for avoiding a collision. 
     Some examples of the vehicle systems  102  may operate according to the following example. Data representing a trajectory of the vehicle  100  in the environment may be received by the drive system  120 . Object data associated with an object in the environment may be calculated. Sensor data  206  from one or more of the vehicle sensors  204  may be used to calculate the object data. The object data may include data representing the location of the object in the environment, an object track associated with the object, such as whether the object is stationary or moving, and an object classification associated with the object, such as whether the object is another vehicle, a pedestrian, a bicyclist, an animal, or a stationary object. In some examples, the object data calculator  216 , based on the object data, may be used to determine data representing the object&#39;s location in the environment, data representing whether the object is moving, and data representing a classification associated with the object. 
     In some examples, the planner  214  may use the object data to determine a predicted path of the object in the environment, for example, based on data representing the location of the object and may process that data to generate data representing a predicted object path. Data representing the type of object may be determined based on the data representing whether the object is moving, data representing the object&#39;s classification, and/or data representing object&#39;s type. A pedestrian not in motion, a vehicle in motion, and traffic sign, a lane marker, or a fire hydrant, none of which is in motion, are examples of object types with an associated motion data. 
     In some examples, the collision predictor system  220  may be used to predict a collision between the vehicle  100  and an object in the environment based on the object type, whether the object is moving, the trajectory of the vehicle  100 , the predicted path of the object obtained from the planner  214 . For example, a collision may be predicted based in part on the object type due to the object moving, the trajectory of the object being in potential conflict with the trajectory of the vehicle  100 , and the object having an object classification that indicates the object is a likely collision threat. In some examples, such a collision prediction may also be based on a predicted object behavior. In some examples, each classification, or sub-classification, of objects may have a corresponding associated behavior. For example, a predicted behavior of a bicyclist is to travel in relatively straight lines having a maximum speed. 
     In some examples, the safety system actuator  224  may be configured to actuate one or more portions of a safety system of the vehicle  100  when a collision is predicted. For example, the safety system actuator  224  may activate one or more of the interior safety systems, one or more of the exterior safety systems, and one or more of the components of the drive system  120  (e.g., the wheel control system  122 , the braking system  130 , and/or the acceleration system  132 ) via the vehicle controller  228 . In some examples, the vehicle controller  228  may determine that the interior safety system will be activated based on some action of an object in the environment, and the vehicle control data  230  may include information configured to cause the vehicle controller  228  to activate one or more functions of the interior safety system, the exterior safety system, and the drive system  120 . 
       FIG. 3A  is a schematic overhead view of an example vehicle  100  maneuvering through an example curve  300  in the road  302  of the road network  104 . In the example shown, the vehicle  100  is traveling in a first travel direction  112 , such that the first end  110  of the vehicle  100  is the front end of the vehicle  100  as it maneuvers through the curve  300 . The vehicle  100  includes first wheels  106 A and  106 B coupled proximate to the first end  110  of the vehicle  100 , and second wheels  106 C and  106 D coupled proximate to the second end  116  of the vehicle  100 . In the example shown, a steering assembly  126  couples the first wheels  106 A and  106 B to the chassis  118  and controls the steering angles  304 A and  304 B of the first wheels  106 A and  106 B, respectively. The steering angles  304 A and  304 B may be measured relative to a longitudinal axis X of the vehicle  100 , which may correspond to the centerline of the vehicle  100 . For example, the longitudinal axis X may be contained in a plane extending through the centerline of the vehicle  100 . As shown in  FIG. 3A , the steering angles  304 A and  304 B may be measured relative to a neutral or “zero” position  306 , which is parallel to the longitudinal axis X of the vehicle  100  and which may be consistent with straight-ahead travel. 
     Although the steering assembly  126  is schematically depicted in a manner that suggests the steering assembly  126  is a single unitary part, the steering assembly  126  may include a plurality of parts, and in some examples, the steering assembly  126  may be configured so that the steering angle  304 A of the first wheel  106 A and the steering angle  304 B of the first wheel  106 B may be changed independently of one another, for example, as described herein. In some examples, the steering assembly  126  may be configured so that the steering angle  304 A of the first wheel  106 A and the steering angle  304 B of the first wheel  106 B may be changed together. 
     As the vehicle  100  travels through the curve  300 , it may be subjected to a lateral force  308  acting in a first direction  310  transverse to the longitudinal axis X of the vehicle  100 . For example, the lateral force  308  may result from the lateral acceleration  312  of the vehicle  100  acting, for example, at the center of gravity  314  in a second direction  316  toward the center of the curve  300 . 
     As shown in  FIGS. 3B and 3C , the lateral force  308  may provide a tipping force tending to cause the vehicle  100  to roll over in the first direction  310  if sufficient to overcome the weight of the vehicle  100  and cause the wheels  106  on the side of the vehicle  100  toward the inside of the curve  300  to lift off the surface of the road  302 . As shown in  FIG. 3B , the lateral force  308  may result (e.g., solely) from the lateral acceleration  312  of the vehicle  100  as it travels through the curve  300 . Other sources of the lateral force  308  may act on the vehicle  100 , such as, for example, a collision force  318  resulting from contact with another object, such as another vehicle, or a high cross-wind, for example, as schematically depicted in  FIG. 3C . The other sources may act alone or in combination with a lateral force due to lateral acceleration, depending on whether the vehicle is traveling through a curve and the direction of the curve. 
       FIGS. 4A and 4B  are schematic overhead views of example vehicles  100  performing two example responses to example lateral forces  308  in an example environment  400 . Some examples of the vehicle  100  may include a wheel control system  122  (see  FIGS. 1 and 2 ) configured to facilitate control of the vehicle  100 . In the examples shown in  FIGS. 4A and 4B , the wheel control system  122  may include a first steering assembly  126 A coupled proximate to the first end  110  of the vehicle  100  and configured to control the steering angles  304 A and  304 B of the first wheels  106 A and  106 B coupled proximate to the first end  110  of the vehicle  100 . In some examples, the wheel control system  122  may also include a second steering assembly  126 B coupled proximate to the second end  116  of the vehicle  100  and configured to control the steering angles  304 C and  304 D of the second wheels  106 C and  106 D coupled proximate to the second end  116  of the vehicle  100 . The wheel control system  122  may also include a wheel controller  124  configured to receive one or more signals indicative of the lateral force  308  acting on the vehicle  100  in the first direction  310  transverse to the longitudinal axis X of the vehicle  100 . Such signal(s) may be generated via a sensor configured to generate such signals, such as, for example, one or more the vehicle sensors  204  (see  FIG. 2 ). For example, the signals may be generated by one or more of inertial measurement units (IMUs) accelerometers, gyroscopes, magnetometers, pose estimation, etc. In some examples, the wheel controller  124  may be configured to determine, based at least in part on the one or more signals, when the lateral force  308  exceeds a threshold force sufficient to cause the vehicle  100  to roll over in the first direction  310 . In some examples, the threshold force may be determined based on the moments of inertia of the vehicle  100  and/or a center of gravity of the vehicle  100 , as well as a function of vehicle speed, and an estimated friction associated with the surface. In some examples, the wheel controller  124  may be configured to control, when the lateral force  308  exceeds the threshold force, one or more of the first steering assembly  126 A or the second steering assembly  126 B to reduce the lateral force to less than the threshold force to prevent the vehicle  100  from rolling over (or at least reduce the likelihood that the vehicle  100  will roll over). 
     In some examples, the wheel controller  124  may be configured to determine, based at least in part on the one or more signals, when the lateral force  308  exceeds a roll coefficient multiplied by a roll force sufficient to cause the vehicle  100  to roll over in the first direction  310 . In some examples, the roll coefficient may range from greater than zero to 1.0. Some examples of the roll coefficient may account for one or more safety factors. Other factors may be reflected in the roll coefficient. In some examples, the roll force may be predetermined, for example, based on characteristics of the vehicle  100  (e.g., physical characteristics, such as the weight, moment of inertia, center of gravity, and/or the roll center of the vehicle  100 ), and/or the roll force maybe determined in real-time while the vehicle  100  maneuvers, for example, based on one or more signals received from the vehicle sensors  204  (see  FIG. 2 ). In some examples, the wheel controller  124  may be configured to control, when the lateral force  308  exceeds the roll coefficient multiplied by the roll force, one or more of the first steering assembly  126 A or the second steering assembly  126 B to reduce the lateral force to less than the roll coefficient multiplied by the roll force and prevent the vehicle  100  from rolling over (or at least reduce the likelihood that the vehicle  100  will roll over). 
     In some examples, the vehicle  100  may not calculate a lateral force, threshold force, or otherwise. The vehicle  100  may determine based on vehicle sensors  204  and drive system  102  that the vehicle  100  is progressing around a curve and that the orientation of the vehicle  100  is inclined with respect to a ground plane (i.e., it is rolling or beginning to roll). In such an example, signals indicative of the vehicle  100 &#39;s orientation (e.g., pose data from localizations, IMU data, gyroscope data, magnetometer data, and the like) may be sent to the wheel control system  122  for use in the rollover prevention system  128 . 
     Although the first and second steering assemblies  126 A and  126 B are schematically depicted in a manner that suggests they are single unitary parts, the first and second steering assemblies  126 A and  126 B may each include a plurality of parts, and in some examples, the first and second steering assemblies  126 A and  126 B may be configured so that the one or more steering angles  304  of the respective wheels  106  may be changed independently of one another, for example, as described herein. In some examples, the first and/or second steering assemblies  126 A and/or  126 B may be configured so that the steering angles  304  of two or more of the respective wheels  106  may be changed together. 
     In some examples, the lateral force  308  may result from the lateral acceleration  312  (see  FIG. 3B ), and the wheel controller  124  may be configured to control the first steering assembly  126 A and/or the second steering assembly  126 B to reduce the lateral acceleration  312  of the vehicle  100  to reduce the lateral force  308  to less than the threshold force. As mentioned herein, the lateral force  308  may result from other sources, such as a collision force  318  (see  FIG. 3C ), and the wheel controller  124  may be configured to control the first steering assembly  126 A and/or the second steering assembly  126 B to reduce the lateral force  308  resulting from the collision force  318  to less than the threshold force. 
     In some examples, the wheel controller  124  may be configured to control the first steering assembly  126 A and/or the second steering assembly  126 B to reduce the lateral acceleration  312  of the vehicle  100  to reduce the lateral force  308  to less than the roll coefficient multiplied by the roll force and prevent the vehicle from rolling over. As mentioned herein, the lateral force  308  may result from other sources, such as a collision force  318  (see  FIG. 3C ), and the wheel controller  124  may be configured to control the first steering assembly  126 A and/or the second steering assembly  126 B to reduce the lateral force  308  resulting from the collision force  318  to less than the roll coefficient multiplied by the roll force. 
     In some examples, the wheel controller  124  may be configured to control the first steering assembly  126 A and/or the second steering assembly  126 B to reduce a deviation from an expected orientation of the vehicle  100 . 
     For example, as shown in  FIG. 4A , when the vehicle is traveling in the first travel  112  direction, such that the first end  110  of the vehicle  100  is the front end of the vehicle  100 , the wheel controller  124  may be configured to cause the first steering assembly  126 A to pivot the first wheels  106 A coupled proximate to the front end of the vehicle  100  at respective first steering angles  304 A and  304 B relative to the respective zero positions  306 , for example, relative to the first travel direction  112  of the vehicle  100 . For example, as shown in  FIG. 4A , the first steering angles  304 A and  304 B may range from about 5 degrees to about 90 degrees from the zero positions  306 , for example, in a direction toward the first direction  310  of the lateral force  308 . In some examples, the wheel controller  124  may also be configured to cause the second steering assembly  126 B to pivot the second wheels  106 C and  106 D at respective second steering angles  304 C and  304 D relative to the respective zero positions  306 , for example, relative to the first travel direction  112  of the vehicle  100 . For example, the second steering angles  304 C and  304 D may range from about 5 degrees to about 90 degrees from the zero positions  306 , for example, in a direction toward the first direction  310  of the lateral force  308 . For example, if the lateral force  308  acting on the vehicle  100  is toward the right side of the vehicle  100  as viewed from the passenger compartment and facing in the forward direction (e.g., as shown in  FIG. 4A ), the first steering angles  304 A and  304 B may result in the first wheels  106 A and  106 B pivoting toward the right, such that the front end of the vehicle  100  will steer toward the right. In some examples, for example as shown in  FIG. 4A , the second steering angles  304 C and  304 D may result in the second wheels  106 C and  106 D pivoting toward the right, such that the second end  116  of the vehicle  100  will steer toward the right. In this example manner, the wheel control system  122  may reduce the lateral force  308  to less than the threshold force, or in some examples, the roll coefficient multiplied by the roll force, thereby preventing the vehicle  100  from rolling over (or at least reducing the likelihood that the vehicle  100  will roll over). 
     In some examples, when the vehicle  100  is traveling in a first travel direction  112 , such that the first end  110  of the vehicle is the front end of the vehicle  100 , the wheel controller  124  may be configured to cause the first steering assembly  126 A to pivot the first wheels  106 A and  106 B in the opposite direction relative to the example shown in  FIG. 4A . For example, as shown in  FIG. 4B , the first steering angles  304 A and  304 B may range from about 1 degree to about 90 degrees from the first travel direction  112  and in a direction opposite the first direction  310  transverse to the longitudinal axis X of vehicle  100  (e.g., relative to the zero positions  306 ). The wheel controller  124  may also be configured to cause the second steering assembly  126 B to pivot the second wheels  106 C and  106 D at second respective steering angles  304 C and  304 D relative to the first travel direction  112  of the vehicle  100 . For example, as shown in  FIG. 4B , the second steering angles  304 C and  304 D may range from about 5 degrees to about 90 degrees from the first travel direction  112  and toward the first direction  310  transverse to the longitudinal axis X of vehicle (e.g., relative to the zero positions  306 ). For example, if the lateral force  308  acting on the vehicle  100  is toward the right side of the vehicle  100  as viewed from the passenger compartment and facing in the first travel direction  112  (e.g., as shown in  FIG. 4B ), the first steering angles  304 A and  304 B may result in the first wheels  106 A and  106 B pivoting toward the left, such that the first end  110  of the vehicle  100  will steer toward the left, and the second steering angles  304 C and  304 D may result in the second wheels  106 C and  106 D pivoting toward the right, such that the second end  116  of the vehicle  100  will steer toward the right. 
     In some examples, the rollover prevention system  128  may be incorporated into the wheel control system  122  (see  FIGS. 1 and 2 ). In some examples, the rollover prevention system  128  may incorporate the braking system  130  including respective brakes  402 A and  402 B configured to be coupled to the respective first wheels  106 A and  106 B of the vehicle  100 . The braking system  130  may also include a braking controller  404  configured to control operation of the brakes  402 A and  402 B. In some examples, the braking controller  404  may be configured to cause rotation of at least one of the first wheels  106 A and  106 B to slow in response to receipt of one or more signals indicative of the lateral force exceeding the threshold force, or in some examples, the roll coefficient multiplied by the roll force. 
     Some examples of the rollover prevention system  128  may incorporate the acceleration system  132  (see  FIGS. 1 and 2 ). The acceleration system  132  may include one or more motors  406  coupled to the second wheels  106 C and  106 D of the vehicle  100 , and a motor controller  408  configured to cause the motor(s)  406  to supply torque to one or more of the second wheels  106 C or  106 D of the vehicle  100  in response to receipt of one or more signals indicative of the lateral force exceeding the threshold force, a deviation in orientation from an expected orientation, or in some examples, the roll coefficient multiplied by the roll force. 
     In some examples, the wheel control system  122 , the braking system  130 , and/or the acceleration system  132  may be used in combination to prevent the vehicle  100  from rolling over. For example, as schematically shown in  FIG. 4B , the first and second steering assemblies  126 A and  126 B, the brakes  402 A and  402 B, and the motor(s)  406  may operate in a coordinated manner (e.g., concurrently or substantially simultaneously) in order to prevent the vehicle  100  from rolling over. For example, the brakes  402 A and  402 B may reduce the speed of the first wheels  106 A and  106 B to cause respective braking forces  410  on the first wheels  106 A and  106 B. The motor(s)  406  may provide a driving torque to the second wheels  106 C and  106 D to cause respective forward-acting forces  412  on the second wheels  106 C and  106 D. In combination with the action of the first and second steering assemblies  126 A and  126 B, this may result in reducing the lateral force  308  acting on the vehicle  100 , thereby preventing the vehicle  100  from rolling over (or at least reducing the likelihood that the vehicle  100  will roll over). 
       FIGS. 5A and 5B  are schematic overhead views of example vehicles  100  maneuvering through example environments  500  that include curves  300  according to an example first mode of operation and an example second mode of operation, respectively. For example, the wheel control systems  122  associated with these example vehicles  100  shown in  FIGS. 5A and 5B  may be configured to facilitate control of the vehicles  100  according to different modes of operation based on parameters, such as, for example, the speed of the vehicle  100 . In some examples, the wheel controller  124  may be configured to receive one or more signals indicative of the speed of the vehicle  100 , and one or more signals indicative of initiating one or more of a lane change or a change in trajectory of the vehicle  100 , for example, to travel through a curve. In such examples, the wheel controller  124  may be configured to control operation of the first steering assembly  126 A and the second steering assembly  126 B to control the steering angles  304 A and  304 B of the first wheels  106 A and  106 B, and the steering angles  304 C and  304 D of the second wheels  106 C and  106 D, respectively, based at least in part on the one or more signals indicative of the speed of the vehicle  100  and the one or more signals indicative of initiating a lane change or a change in direction. 
     The wheel controller  124  may be configured to control operation of the first steering assembly  126 A and the second steering assembly  126 B according to different operating modes based on, for example, one more signals indicative of the speed of the vehicle  100 . For example, operating in a first mode may provide more maneuverability at relatively lower speeds, and operating in a second mode may provide greater cornering stability at relatively higher speeds. In some examples, the transition between the first mode and the second mode may be gradual and may be related to vehicle speed and/or other factors, such as, for example, steering angle and/or braking. In some such examples, any one or more of the angles may vary continuously with respect to speed. In some examples, the transition between the first and second modes may depend on a threshold speed, below which the vehicle operates according to the first mode, and above which the vehicle operates according to the second mode. In some examples, the transition from the second mode to the first mode may occur as the vehicle speed is reduced, for example, in a manner similar to the transition from the first mode to the second mode. 
     In the example shown in  FIGS. 5A and 5B , the wheel controller  124  is configured to control operation of the first steering assembly  126 A and the second steering assembly  126 B according to a first mode when the one or more signals indicative of the speed of the vehicle indicate the speed of the vehicle  100  is less than or equal to a threshold speed. As shown in  FIG. 5B , the wheel controller  124  may be configured to control operation of the first steering assembly  126 A and the second steering assembly  126 B according to a second mode when the one or more signals indicative of the speed of the vehicle indicate the speed of the vehicle  100  is greater than the threshold speed. For example, controlling the first steering assembly  126 A and the second steering assembly  126 B according to the first mode may include causing the first steering assembly  126 A to pivot the first wheels  106 A and  106 B at first steering angles  304 A and  304 B in a first direction relative to the longitudinal axis X of the vehicle  100  (e.g., relative to the zero positions  306 ), and causing the second steering assembly  126 B to pivot the second wheels  106 C and  106 D at second steering angles  304 C and  304 D in a second direction relative to the longitudinal axis X of the vehicle  100  (e.g., relative to the zero positions  306 ), such that the first direction and the second direction are different from one another. For example, the wheel controller  124  may cause the first steering assembly  126 A to turn the first wheels  106 A and  106 B to the right, and the second steering assembly  126 B to turn the second wheels  106 C and  106 D to the left, for example, as shown in  FIG. 5A . This may provide more maneuverability at lower speeds, which may be desirable in, for example, a parking lot. 
     In some examples, controlling the first steering assembly  126 A and the second steering assembly  126 B according to the second mode may include causing the first steering assembly  126 A to pivot the first wheels  106 A and  106 B at first steering angles  304 A and  304 B in a first direction relative to the longitudinal axis X of the vehicle  100  (e.g., relative to the zero positions  306 ), and causing the second steering assembly  126 B to pivot the second wheels  106 C and  106 D at second steering angles  304 C and  304 D in the first direction relative to the longitudinal axis X of the vehicle  100  (e.g., relative to the zero positions  306 ), for example, as shown in  FIG. 5B . For example, when maneuvering through a curve  300  that bends to the right, the wheel controller  124  may be configured to cause the first and second steering assemblies  126 A and  126 B to turn the first wheels  106 A and  106 B and the second wheels  106 C and  106 D toward the right, so that both the first end  110  and second end  116  of the vehicle  100  move toward the right and through the curve  300 . Although the first wheels  106 A and  106 B and the second wheels  106 C and  106 D are turned in the same direction, the first steering angles  304 A and  304 B may differ from the second steering angles  304 C and  304 D. For example, the first steering angles  304 A and  304 B may be about 15 degrees to the right, and the second steering angles  304 C and  304 D may be about 10 degrees to the right. Other steering angles are contemplated. This example mode of operation may provide improved stability at higher speeds when maneuvering through a curve. 
     In some examples, a steering strategy may be selected, for example, to optimize stability for a vehicle during execution of a trajectory for the vehicle. For example, it may be based on factors such as center of gravity, speed, radius of curvature of the instantaneous section of road, moments of inertia, and the like, as would be appreciated. As a non-limiting example, the wheel controller  124  may continuously, and smoothly, vary operation between the first mode and the second mode with respect to speed. 
       FIGS. 6A and 6B  are schematic overhead views of example vehicles  100  in environments  600  maneuvering into example parallel parking spaces  602  according to an example first mode and an example second mode, respectively. For example,  FIGS. 6A and 6B  show example parallel parking spaces  602 , for example, bordered at each of opposite ends by vehicles  604 A and  604 B, and a distal longitudinal border  606  of the parallel parking space  602  (e.g., an edge of the road  302 ) on which the vehicle  100  is traveling. In some examples, the wheel controller  124  may be configured to receive a signal indicative of initiating parallel parking into a parallel parking space  602 , such as those shown in  FIGS. 6A and 6B . For example, the wheel controller  124  may control operation of the first steering assembly  126 A and the second steering assembly  126 B to control the steering angles  304 A and  304 B of the first wheels  106 A and  106 B, and the steering angles  304 C and  304 D of the second wheels  106 C and  106 D, respectively, based at least in part on the one or more signals indicative of initiating parallel parking. In some examples, when the vehicle  100  is traveling in a first travel direction  112 , such that the first end  110  of the vehicle  100  is the front end of the vehicle  100 , the wheel controller  124  may be configured to control operation of the first steering assembly  126 A and the second steering assembly  126 B to cause the vehicle  100  to park in the parallel parking space  602  without the vehicle  100  traveling in a second travel direction  114  opposite the first travel direction  112 . Although the vehicle  100  may be sufficiently close to the longitudinal border  606  to be considered properly parked, it is contemplated that the vehicle  100  may be maneuvered in the second travel direction  114  to park closer to the longitudinal border  606  and/or to provide additional room for one or more of the other vehicles  604 A and  604 B to exit their respective parking spaces. 
     In the example first mode shown in  FIG. 6A , at position A, the vehicle  100  pulls forward in the first travel direction  112 , alongside the vehicle  604 A bordering a proximate end  608  of the parallel parking space  602  with the first wheels  106 A and  106 B and the second wheels  106 C and  106 D in the zero positions  306 . As the vehicle  100  continues to pull forward in the first travel direction  112 , the vehicle sensors  204  (see  FIG. 2 ) may generate sensor signals indicative of the end of the vehicle  604 A bordering the proximate end  608  of the parallel parking space  602 , the end of the other vehicle  604 B bordering the distal end  610  of the parallel parking space  602 , and the longitudinal border  606 . As the vehicle  100  moves into position B shown in  FIG. 6A , in the example first mode of operation, the wheel controller  124  is configured to control operation of the first steering assembly  126 A to provide steering angles  304 A and  304 B of the first wheels  106 A and  106 B that point the first wheels  106 A and  106 B toward the longitudinal border  606  and into the parallel parking space  602 , while controlling operation of the second steering assembly  126 B to provide steering angles  304 C and  304 D of the second wheels  106 C and  106 D that point the second wheels  106 C and  106 D toward the longitudinal border  606  and into the parallel parking space  602 . In some examples, the wheel controller  124  may cause the steering assemblies  126 A and  126 B to adjust one or more of the steering angles  304 A- 304 D as the vehicle  100  pulls into the parallel parking space  602  and toward the longitudinal border  606  to position C shown in  FIG. 6A  based on, for example, one or more signals received from the planner  214  and/or one or more signals received from the vehicle sensors  204  (see  FIG. 2 ). In this example manner, the wheel control system  122  may operate to maneuver the vehicle  100  into the parking space  602  and into position C without changing directions from the first travel direction  112  to the opposite, second travel direction  114 . Though not illustrated in the drawings, a similar procedure may be employed for navigating the vehicle  100  out of the parking space  602 . 
       FIG. 6B  shows an example second mode for parking the vehicle  100  in the parallel parking space  602 . In the example shown, at position A, the vehicle  100  pulls forward in the first travel direction  112 , alongside the vehicle  604 A bordering a proximate end  608  of the parallel parking space  602  with the first wheels  106 A and  106 B and the second wheels  106 C and  106 D in the zero positions  306 . As the vehicle  100  continues to pull forward in the first travel direction  112  to position B, where the first end  110  of the vehicle  100  is generally even with the end of the vehicle  604 A bordering the proximate end  608  of the parallel parking space  602 , the vehicle sensors  204  may generate sensor signals indicative of the end of the vehicle  604 A bordering the proximate end  608  of the parallel parking space  602 , the end of the other vehicle  604 B bordering the distal end  610  of the parking space  602 , and the longitudinal border  606 . As the vehicle  100  moves into position B shown in  FIG. 6B , in the example second mode of operation, the wheel controller  124  is configured to control operation of the first steering assembly  126 A to provide steering angles  304 A and  304 B of the first wheels  106 A and  106 B that point the first wheels toward the longitudinal border  606  and into the parallel parking space  602 , while controlling operation of the second steering assembly  126 B to provide steering angles  304 C and  304 D of the second wheels  106 C and  106 D that point the second wheels  106 C and  106 D away from the parallel parking space  602 , for example, as shown. This causes the second end  116  of the vehicle  100  to pivot outward relative to the other vehicle  604 A, so that as the vehicle  100  pulls forward farther into the parallel parking space  602 , the second end  116  of the vehicle  100  clears the other vehicle  604 A, for example, as the vehicle  100  moves from position A to position B. Thereafter, the wheel controller  124  causes the first steering assembly  126 A to pivot the first wheels  106 A and  106 B, so that they are pointed toward the vehicle  604 B at the distal end  610  of the parking space  602 . The wheel controller  124  also causes the second steering assembly  126 B to pivot the second wheels  106 C and  106 D, so that they are at steering angles  304 C and  304 D that cause the second wheels  106 C and  106 D to be pointing toward the longitudinal border  606 , for example, when the vehicle  100  is at position C in  FIG. 6B . Thereafter, the wheel controller  124 , according to the example second mode, causes the second steering assembly  126 B to pivot the second wheels  106 C and  106 D toward the other vehicle  604 B at the distal end  610  of the parallel parking space  602  as the vehicle  100  continues to pull forward in the first travel direction  112  and into the parallel parking space  602 . As above, it will be appreciated that a similar procedure may be used to navigate the vehicle  100  out of the parking space  602 . 
       FIGS. 7A and 7B  are schematic side views of an example vehicle  100  including example caster angles CS. In some examples, the vehicle  100  is a bi-directional vehicle, for example, such as the bi-directional vehicle described previously herein. In the example shown in  FIGS. 7A and 7B , the wheel control system  122  includes first and second steering assemblies  126 A and  126 B coupled proximate to the first end  110  and the second end  116  of the vehicle  100 , respectively. The wheel control system  122  also includes a first caster adjustment assembly  700 A coupled to the first steering assembly  126 A (directly or indirectly) and/or a second caster adjustment assembly  700 B coupled the second steering assembly  126 B (directly or indirectly). In some examples, the caster adjustment assemblies  700 A and  700 B may be configured to change caster angles CS of the first wheels  106 A and  106 B and/or the second wheels  106 C and  106 D, respectively. For example, the wheel controller  124  may be configured to receive one or more signals indicative of the direction of travel of the vehicle  100 , and cause the caster adjustment assembly  700 A to change the caster angles CS of the first wheels  106 A and  106 B and/or the caster adjustment assembly  700 B to change the caster angles CS of the second wheels  106 C and  106 D based at least in part on the one or more signals indicative of the direction of travel of the vehicle. For example, when the vehicle  100  is traveling in the first travel direction  112  (see  FIG. 7A ), such that the first end  110  of the vehicle  100  is the front end of the vehicle  100 , the wheel controller  124  may be configured to control operation of the first caster adjustment assembly  700 A to cause the first wheels  106 A and  106 B to have positive caster angles CS, for example, as shown in  FIG. 7A . Alternatively, or in addition, when the vehicle  100  is traveling in the second travel direction  114  (see  FIG. 7B ), such that the second end  116  of the vehicle  100  is the front end of the vehicle  100 , the wheel controller  124  may be configured to control operation of the second caster adjustment assembly  700 B to cause the second wheels  106 C and  106 D to have positive caster angles CS, for example, as shown in  FIG. 7B . 
     In some examples, the wheel controller  124  may be configured to control operation of caster adjustment assemblies at both ends of the vehicle  100 , for example, to improve the handling of the vehicle  100 . For example, providing or increasing the amount of positive caster angle at the front end of the vehicle  100  may improve the on-center stability of the front end of the vehicle  100 . However, the caster adjustment assembly  700  may be used to either reduce or eliminate the amount of positive caster angle at the rear end of the vehicle  100 , for example, to provide a desired level of responsiveness of the rear end of the vehicle  100 . 
       FIGS. 8A and 8B  are schematic front views of an example vehicle  100  having first wheels  106 A and  106 B and/or second wheels  106 C and  106 D at a neutral (or zero) camber angles CB ( FIG. 8A ) and negative camber angles CB ( FIG. 8B ). The wheel control system  122  may include a first camber adjustment assembly  800 A coupled to the first steering assembly  126 A (directly or indirectly), and/or a second camber angle adjustment assembly  800 B coupled to the second steering assembly  126 B (directly or indirectly). In some examples, the camber adjustment assemblies  800 A and  800 B may be configured to change camber angles CB of the first wheels  106 A and  106 B and/or the second wheels  106 C and  106 D, respectively. For example, the wheel controller  124  may be configured to receive one or more signals indicative of the direction of travel of the vehicle  100 , and cause the first camber adjustment assembly  800 A to change the camber angles CB of the first wheels  106 A and  106 B and/or cause the second camber angle adjustment assembly  800 B to change the camber angles of the second wheels  106 C and  106 D, based at least in part on the one or more signals indicative of the direction of travel of the vehicle  100 . For example, when the vehicle  100  is traveling in a first travel direction  112 , such that the first end  110  of the vehicle  100  is the front end of the vehicle  100 , the wheel controller  124  may be configured to control operation of the first camber adjustment assembly  800 A to cause the first wheels  106 A and  106 B to have negative camber angles CB, for example, as shown in  FIG. 8B . In some examples, the wheel controller  124  may be configured to receive one or more signals indicative of initiating turning of the vehicle  100 , and control operation of the first camber adjustment assembly  800 A to cause the first wheels  106 A and  106 B to increase amounts of respective negative camber angles CB based at least in part on the one or more signals indicative of initiating turning of the vehicle  100 , and/or control operation of the second camber adjustment assembly  800 B to cause the second wheels  106 C and  106 D to increase amounts of respective negative camber angles CB based at least in part on the one or more signals indicative of initiating turning of the vehicle  100 . In some examples, camber angles of the wheels may also be adjusted based on knowledge of the driving surface as may be made available from map data. In some examples, the camber adjustment assemblies  800 A and  800 B may be configured to change the camber angle(s) CB of one or more of the wheels  106 A- 106 D independently from the other wheels. Increasing the negative camber angle may improve the road holding and/or handling of the vehicle  100 , particularly when the vehicle  100  is traveling through a curve  300  in the road  302  because the contract patches between the road  302  and the tires  108  on the side of the vehicle  100  that are on the outside of the curve  300  may be larger when the wheels  106  have a negative camber angle. 
       FIGS. 9A and 9B  are schematic overhead views of an example vehicle  100  having front wheels with toe-in and toe-out, respectively. The wheel control system  122  may include a first toe adjustment assembly  900 A coupled to the first steering assembly  126 A (directly or indirectly), and/or a second toe adjustment assembly  900 B coupled to the second steering assembly  126 B (directly or indirectly). In some examples, the first toe adjustment assembly  900 A may be configured to change toe angles TA of the first wheels  106 A and  106 B, and/or the second toe adjustment assembly  900 B may be configured to change toe angles TA of the second wheels  106 C and  106 D. For example, the wheel controller  124  may be configured to receive one or more signals indicative of one or more of the speed of the vehicle  100  and/or initiating a turn of the vehicle  100 , and cause the toe adjustment assemblies  900 A and/or  900 B to change the toe angles TA of the first wheels  106 A and  106 B and/or the second wheels  106 C and  106 D based at least in part on the one or more signals. For example, the wheel controller  124  may be configured to control operation of the toe adjustment assemblies  900 A and/or  900 B to change the toe angle(s) TA, such that an amount of toe-in of one or more of the wheels  106 A- 106 D is reduced upon receipt of the one or more signals indicative of initiating a turn for the vehicle  100 . For example, the first toe adjustment assembly  900 A may change the toe angles TA of one or more of the first wheels  106 A and  106 B, from having toe-in (see  FIG. 9A ) to instead have toe-out (see  FIG. 9B ). In some examples, the wheel controller  124  may be configured to control operation of the toe adjustment assemblies  900 A and/or  900 B based at least in part on the one or more signals indicative of speed, such that an amount of toe-in of the first wheels  106 A and/or  106 B (and/or the second wheels  106 C and/or  106 D) is increased when the one or more signals indicative of speed indicates a vehicle speed greater than a threshold speed. Increasing the amount of toe-in may result in the vehicle  100  having greater stability when traveling in a straight line, for example, at greater speeds. Decreasing the amount of toe-in (e.g., such that the toe angle TA results in slight toe-out (see  FIG. 9B )) may increase the responsiveness of the vehicle  100  when turning. Thus, the wheel controller  124  may control operation of the toe adjustment assembly  900  to tailor the handling of the vehicle  100  based at least in part on the speed of the vehicle  100  and/or whether the vehicle  100  of traveling through a curve. It should be appreciated that any various combinations of castor, camber, and toe may be selected for each wheel to optimize performance of the vehicle  100 . 
       FIG. 10A  is schematic overhead view of an example vehicle  100  maneuvering with a direction of travel  1000  that is misaligned with the longitudinal axis X of the vehicle  100 , and  FIG. 10B  is a schematic overhead view of the example vehicle  100  maneuvering with a direction of travel  1000  that is aligned with the longitudinal axis X of the vehicle  100 . In some examples of the wheel control system  122 , the wheel controller  124  may be configured to receive one or more signals indicative of the direction of travel  1000  of the vehicle  100  and one or more signals indicative of a direction  1002  defined by the longitudinal axis X of the vehicle  100 . In such examples, the wheel controller  124  may be configured to determine, based at least in part on the one or more signals indicative of a direction of travel  1000  and the one or more signals indicative of the direction  1002  defined by the longitudinal axis X, misalignment between the direction of travel  1000  and the direction  1002  defined by the longitudinal axis X The wheel controller  124 , in some examples, may be configured to control the first steering assembly  126 A and the second steering assembly  126 B to reduce or eliminate the misalignment between the direction of travel  1000  and the direction  1002 . In some examples, the longitudinal axis X of the vehicle  100  may be aligned with (or contained in) a plane passing through the centerline of the vehicle  100 . 
     In some examples, the vehicle  100  may include active suspension, for example, configured to be actively and dynamically adjustable at one or more of the wheels  106 A- 106 D of the vehicle  100  based on, for example, the state of the vehicle  100  (e.g., based on vehicle speed, whether the vehicle  100  is accelerating, braking, and/or turning, and/or the relative smoothness of the surface of the road). In some such examples, the wheel controller  124  may be configured to detect misalignment, for example, when the active suspension is adjusted at one or more of the four wheels  106 A- 106 D and by detecting differences in the response(s) to the adjustment(s) at the different wheels  106 A- 106 D. 
     In some examples, the first steering assembly  126 A may include a first steering rack configured to be coupled to each of the first wheels  106 A and  106 B and control orientation of the first wheels  106 A and  106 B relative to the longitudinal axis X of the vehicle  100 . The second steering assembly  126 B may include a second steering rack configured to be coupled to each of the second wheels  106 C and  106 D and control orientation of the second wheels  106 C and  106 D relative to the longitudinal axis X In some such systems, the wheel controller  124  may be configured to control operation of the first steering rack and the second steering rack to align the first wheels  106 A and  106 B and the second wheels  106 C and  106 D with the longitudinal axis X of the vehicle  100 . For example, one or more of the first steering rack or the second steering rack may define respective zero positions intended to align the associated first wheels  106 A and  106 B and second wheels  106 C and  106 D with the longitudinal axis X of the vehicle  100 . In some examples, the wheel controller  124  may be configured to adjust the zero positions of one or more of the first steering rack or the second steering rack, so that each of the first wheels  106 A and  106 B and each of the second wheels  106 C and  106 D is aligned with the longitudinal axis X of the vehicle, for example, within technical tolerances 
     In some examples, the wheel control system  122  may include, or be in communication with, the location systems  210  (see  FIG. 2 ), which may, for example, include a global positioning system, and the one or more signals indicative of the direction of travel  1000  of the vehicle  100  may be based at least in part on one or more signals generated by the global positioning system. In some examples, the wheel control system  122  may be in communication with one or more of the vehicle sensors  204  (see  FIG. 2 ), which may include a sensor configured to generate one or more signals indicative of the direction  1002  defined by the longitudinal axis X of the vehicle  100 . In some examples, the wheel controller  124  may be configured to control the first steering assembly  126 A to change the first steering angles  304 A and/or  304 B of the first wheels  106 A and/or  106 B in a first direction, and control the second steering assembly  126 B to change the second steering angles  304 C and/or  304 D of the second wheels  106 C and/or  106 D in a direction consistent with the first direction. For example, the first and second steering angles  304 A- 304 D may be substantially the same. In some examples, the resulting steering angles  304 A- 304 D may be zero, for example, such that each of the wheels  106 A- 106 D is aligned with the zero position  306 , for example, as shown in  FIG. 10B . In some examples, the resulting steering angles  304 A- 304 D may not necessarily be zero, and the wheels  106 A- 106 D may not necessarily be aligned with the zero position  306 . In this example manner, the wheel control system  122  may be used to align the direction of travel  1000  of the vehicle  100  and the direction  1002  defined by the longitudinal axis X of the vehicle  100 . 
       FIG. 11A  is a schematic overhead view of an example vehicle  100  including an example system for estimating tire-surface parameterization data  134  (see  FIGS. 1 and 2 ) associated with a surface on which the vehicle  100  is traveling, and  FIG. 11B  is a schematic overhead view of the example vehicle  100  including an example of a system for estimating tire-surface parameterization data  134  associated with a surface on which the vehicle  100  is traveling according to another method. For example, as shown in  FIG. 11A , some examples of the system for estimating tire-surface parameterization data  134  may incorporate one or more of the wheel control system  122 , the braking system  130  including brakes  402 A and  402 B configured to be coupled to the respective first wheels  106 A and  106 B, and brakes  402 C and  402 D configured to be coupled to the respective second wheels  106 C and  106 D. For example, the braking system  130  may also include a braking controller  404  configured to control operation of each of the brakes  402 A- 402 D. In some examples, the braking controller  404  may be configured to control activation of one or more of the brakes  402 A- 402 D independently of one another. In some examples, the braking controller  404  may be configured to activate one or more of the brakes  402 A and  402 B to cause rotation of one or more the first wheels  106 A or  106 B to slow or stop, and/or to activate one or more of the brakes  402 C and  402 D to cause rotation of one or more the second wheels  106 C or  106 D to slow or stop. Some examples of the vehicle  100  may also include an acceleration system  132 . The acceleration system  132  may include one or more motors  406  coupled to the first wheels  106 A and  106 B and/or one or more motors  406  coupled to the second wheels  106 C and  106 D, and a motor controller  408  configured to cause the motor(s)  406  to supply torque to one or more of the first wheels  106 A and  106 B and/or one or more of the second wheels  106 C or  106 D. 
     In some examples, the wheel control system  122  may be configured to activate one of the brakes  402 A,  402 B,  402 C, or  402 D to cause rotation of one of the tires  108 A,  108 B,  108 C, or  108 D to slow, and cause the one or more motors  406  to supply torque to another one or more of the tires  108 A,  108 B,  108 C, or  108 D to resist slowing of the vehicle  100  caused by activation of the brake  402 A,  402 B,  402 C, or  402 D. For example, the wheel control system  122  may be configured to activate the brake  402 B to cause rotation of the associated tire  108 B to slow, and cause the one or more motors  406  to supply torque to another one or more of the tires  108 A,  108 C, or  108 D to provide forward-acting forces  412  to resist slowing of the vehicle  100  caused by activation of the brake  402 B. In such an example, appropriate combinations of braking and torque may be provided so as not to incur any yaw of the vehicle. The wheel control system  122  may also include a tire-surface parameterization estimator  136  (see  FIGS. 1 and 2 ) configured to receive one or more signals indicative of the speed of rotation of the tire  108 B during the braking, and receive one or more signals indicative of the braking force  410 B on the tire  108 B during the braking. Based at least in part on one or more of the signals indicative of the speed of rotation of the tire  108 B and/or indicative of the braking force on the tire  108 B, the tire-surface parameterization estimator  136  may be configured to estimate tire-surface parameterization data associated with the surface. In some examples, the tire-surface parameterization data may include a friction coefficient (e.g., a static or a dynamic friction coefficient) between the surface and one or more tires  108  of the vehicle  100 . In some examples, the brakes  402  may include one or more brakes that are part of a regenerative braking system. 
     In some examples of the system for estimating tire-surface parameterization data  134 , the first tires  108 A and  108 B may be coupled proximate to a first end  110  of the vehicle  100  (e.g., via the wheels  106 A and  106 B, respectively, and suspension), and the second tires  108 C and  108 D may be coupled proximate to a second end  116  of the vehicle  100  (e.g., via a wheels  106 C and  106 D, respectively, and suspension). In some examples, the wheel control system  122  may be configured to activate one of the second brakes  402 C and  402 D to cause rotation of one of the respective second tires  108 C and  108 D to slow or stop, cause the motor(s)  406  coupled to the first wheels  106 A and  106 B to supply torque to one or more of the first tires  108 A and  108 B to provide forward-acting forces  412  to resist slowing of the vehicle  100  caused by activation of the second brake  402 C or  402 D. The tire-surface parameterization estimator  136  may be configured to receive one or more signals indicative of the speed of rotation of the second tire  108 C or  108 D during braking, and receive one or more signals indicative of a braking force  410 C or  410 D on the second tire  108 C or  108 D during the braking. The tire-surface parameterization estimator  136  may be configured to estimate tire-surface parameterization data associated with the surface based at least in part on the one or more signals indicative of the speed of rotation of the second tire  108 C or  108 D and the one or more signals indicative of the braking force  410 C or  410 D. Some examples of the system for estimating tire-surface parameterization data may also include a reporting module  138  (see  FIGS. 1 and 2 ) in communication with a communication network. The reporting module  138  may be configured to receive one or more signals from the location systems  210  (see  FIG. 2 ) and communicate data correlating the location of the vehicle  100  and the tire-surface parameterization data via the communication network. 
     In some examples, the tire-surface parameterization data  134  may be communicated to, for example, the vehicle controller  228 , which may cause one or more of the vehicle systems  102  to operate based at least in part on the tire-surface parameterization data  134 . For example, the performance envelopes of one or more of acceleration, braking, and/or steering may be modified based at least in part on the tire-surface parameterization data  134 , for example, to reduce the acceleration rates, braking rates, and/or steering rates in relatively lower friction situations, or raise the acceleration rates, braking rates, and/or steering rates in relatively higher friction situations. In some examples, suspension compliance and/or tire pressures may be adjusted based at least in part on the tire-surface parameterization-related data. 
     As shown in  FIG. 11B , some examples of the system for estimating tire-surface parameterization data  134  may incorporate the steering assembly  126 A or  126 B, which may, in some examples, independently control the first steering angle  304 A of the first tire  108 A, and the second steering angle  304 B of the second tire  108 B. In some examples, the steering assembly  126 A may be configured to change the second steering angle  304 B without changing the first steering angle  304 A, and/or change the first steering angle  304 A without changing the second steering angle  304 B. For example, the system for estimating tire-surface parameterization data  134  may also incorporate the wheel controller  124 , and cause the second steering angle  304 B and the first steering angle  304 A to differ from one another. For example, as shown in  FIG. 11B , the steering assembly  126 A may be configured to control the second steering angle  304 B, so that the second tire  108 B is at a second steering angle  304 B, while the first tire  108 A is at a first steering angle  304 A that differs from the second steering angle  304 B. Though depicted in  FIG. 11B  for illustrative purposes, it should be appreciated that a caster angle, camber angle, and/or toe angle of any wheel  106  may be adjusted instead of, or in addition to, a steering angle  304 , so as to impose a force on one or more of the wheels  106 . In some examples, changes to any of the one or more wheels  106  for measuring friction should not impact a direction of travel, speed, or acceleration of the vehicle  100 . The tire-surface parameterization estimator  136  may be configured to receive one or more signals indicative of a lateral force  1102  acting on the second tire  108 B and estimate tire-surface parameterization data associated with the surface based at least in part on the one or more signals indicative of the lateral force  1102  acting on the second tire  108 B. Such a force may be calculated, for example, by measuring, or otherwise determining, an additional amount of torque to apply to maintain a trajectory and speed, a reduction in speed, a deviation from an expected trajectory, a steering angle to maintain an expected trajectory, or the like. 
     In some examples, the steering assembly  126 A may include a steering actuator coupled to one or more of the first tire  108 A or the second tire  108 B, and the one or more signals indicative of the lateral force  1102  acting on the first tire  108 A or the second tire  108 B may be based at least in part on one or more signals indicative of a force acting on the steering actuator. Some examples of the system for estimating tire-surface parameterization data  134  may include a sensor coupled to the steering assembly and configured to generate the one or more signals indicative of a lateral force  1102  on the first tire  108 A or the second tire  108 B. Some examples of the system for estimating tire-surface parameterization data  134  may incorporate one or more of the motors  406 , and the one or more signals indicative of the lateral force  1102  acting on the first tire  108 A or the second tire  108 B may be based at least in part on one or more signals indicative of a change in torque supplied by the motor(s)  406  to maintain a speed of the vehicle  100  when the steering assembly  126 A changes the steering angle  304 A or  304 B of the first tire  108 A or the second tire  108 B. 
     In various implementations, the parameter values and other data illustrated herein may be included in one or more data stores, and may be combined with other information not described or may be partitioned differently into more, fewer, or different data structures. In some implementations, data stores may be physically located in one memory or may be distributed among two or more memories. 
     Those skilled in the art will appreciate that the example architecture  200  is merely illustrative and is not intended to limit the scope of the present disclosure. In particular, the computing system and devices may include any combination of hardware or software that can perform the indicated functions, including computers, network devices, internet appliances, tablet computers, PDAs, wireless phones, pagers, etc. The architecture  200  may also be connected to other devices that are not illustrated, or instead may operate as a stand-alone system. In addition, the functionality provided by the illustrated components may in some implementations be combined in fewer components or distributed in additional components. Similarly, in some implementations, the functionality of some of the illustrated components may not be provided and/or other additional functionality may be available. 
     Those skilled in the art will also appreciate that, while various items are illustrated as being stored in memory or storage while being used, these items or portions of them may be transferred between memory and other storage devices for purposes of memory management and data integrity. Alternatively, in other implementations, some or all the software components may execute in memory on another device and communicate with the illustrated architecture  200 . Some or all the system components or data structures may also be stored (e.g., as instructions or structured data) on a non-transitory, computer-accessible medium or a portable article to be read by an appropriate drive, various examples of which are described above. In some implementations, instructions stored on a computer-accessible medium separate from the architecture  200  may be transmitted to the architecture  200  via transmission media or signals such as electrical, electromagnetic, or digital signals, conveyed via a communication medium such as a wireless link. Various implementations may further include receiving, sending, or storing instructions and/or data implemented in accordance with the foregoing description on a computer-accessible medium. Accordingly, the techniques described herein may be practiced with other control system configurations. Additional information about the operations of the modules of the vehicle  100  is discussed below. It should be appreciated that any examples described herein may be performed in sequence, substantially simultaneously, and repeatedly, and/or in no particular order. 
       FIGS. 12-15  are flow diagrams of example processes illustrated as a collection of blocks in a logical flow graph, which represent a sequence of operations that can be implemented in hardware, software, or a combination thereof. In the context of software, the blocks represent computer-executable instructions stored on one or more computer-readable storage media that, when executed by one or more processors, perform the recited operations. Generally, computer-executable instructions include routines, programs, objects, components, data structures, and the like that perform particular functions or implement particular abstract data types. The order in which the operations are described is not intended to be construed as a limitation, and any number of the described blocks can be combined in any order and/or in parallel to implement the processes. 
       FIG. 12  is a flow diagram of an example process  1200  for preventing roll over of a vehicle. At  1202 , the example process  1200  may include receiving one or more signals indicative of a lateral force acting on the vehicle in a first direction transverse to the longitudinal axis of the vehicle. The lateral force may be detected and the one or more signals may be generated via one of more of the vehicle sensors. The one or more signals may be received by a wheel controller, such as, for example, a wheel controller consistent with those described herein. The lateral force may result from the lateral acceleration of the vehicle and/or contact with an object, such as, for example, a collision with another vehicle. In some examples, rollover or predicted rollover, may be detected by other methods/devices, such as, for example, detected orientation of the vehicle compared with expected orientation, accelerometers, gyroscopes, magnetometers, inertial measurement units, and/or other measurements relative to the ground. 
     At  1204 , the example process  1200  may include determining, based at least in part on the one or more signals, when the lateral force exceeds a threshold force sufficient to cause the vehicle to roll over in the first direction transverse to the longitudinal axis of the vehicle. In some examples, at  1204 , the example process  1200  may include determining, based at least in part on the one or more signals, when the lateral force exceeds a roll coefficient multiplied by a roll force sufficient to cause the vehicle to roll over in the first direction transverse to the longitudinal axis of the vehicle. In some examples, the roll coefficient may account for one or more safety factors. Other factors may be reflected in the roll coefficient. In some examples, the roll force may be predetermined, for example, based on characteristics of the vehicle (e.g., physical characteristics such as the weight and/or roll center of the vehicle, a moment of inertia, a speed of the vehicle, etc.), or the roll force maybe determined in real-time while the vehicle maneuvers. 
     At  1206 , the example process  1200  may include controlling, when the lateral force exceeds the threshold force, one or more of the first steering assembly or the second steering assembly to reduce the lateral force to less than the threshold force to prevent the vehicle from rolling over in the first direction. In some examples, the wheel controller may send one or more signals to the first and second steering assemblies so that the steering assemblies pivot the first and second wheels at respective first and second steering angles to prevent the vehicle from rolling over. In some examples, at  1206 , the example process  1200  may include controlling, when the lateral force exceeds the roll coefficient multiplied by the roll force, one or more of the first steering assembly or the second steering assembly to reduce the lateral force to less than the roll coefficient multiplied by the roll force and prevent the vehicle from rolling over in the first direction. In some examples, the wheel controller may send one or more signals to the first and second steering assemblies so that the steering assemblies pivot the first and second wheels at respective first and second steering angles to prevent the vehicle from rolling over. 
     For example, if the lateral force results from the lateral acceleration of the vehicle as it maneuvers through a curve, the method may include controlling the first steering assembly and the second steering assembly to reduce the lateral acceleration of the vehicle to reduce the lateral force to less than the threshold force, or in some examples, the roll coefficient multiplied by the roll force, and prevent the vehicle from rolling over. If the lateral force results from a collision force, the method may include controlling the first steering assembly and the second steering assembly to reduce the lateral force from the collision force to less than the threshold force, or in some examples, the roll coefficient multiplied by the roll force. 
     In some examples of the process  1200 , when the vehicle is traveling in a first forward travel direction, such that the first end of the vehicle is the front end of the vehicle, it may include pivoting the first wheels at first steering angles relative to the first forward travel direction of the vehicle. For example, the first steering angles may range from about 5 degrees to about 90 degrees from the first forward travel direction toward the first direction transverse to the longitudinal axis of vehicle. The process  1200  may also include pivoting the second wheels at second steering angles relative to the first forward travel direction of the vehicle, and the second steering angles may range from about 5 degrees to about 90 degrees from the first forward travel direction toward the first direction transverse to the longitudinal axis of vehicle. In such examples, the first wheels and the second wheels may pivot to be pointed in generally the same direction, for example, as described herein. 
     In some examples, the process  1200  may include pivoting the first wheels at first steering angles relative to the first forward travel direction of the vehicle, such that the first steering angles range from about 5 degrees to about 90 degrees from the first forward travel direction and in a direction opposite the first direction transverse to the longitudinal axis of vehicle. Some such examples may also include pivoting the second wheels at second steering angles relative to the first forward travel direction of the vehicle, such that the second steering angles range from about 5 degrees to about 90 degrees from the first forward travel direction toward the first direction transverse to the longitudinal axis of vehicle. In such examples, the first wheels and the second wheels may pivot to be pointed in generally different directions. 
     In some such examples, the process  1200  may further include, at  1208 , activating one or more brakes coupled to the first wheels of the vehicle to reduce a rate of rotation of at least one of the first wheels in response to receipt of one or more signals indicative that the lateral force exceeds a threshold force sufficient to cause the vehicle to roll over in the first direction. Combined with pivoting the first wheels in a direction opposite the first direction transverse to the longitudinal axis of vehicle and pivoting the second wheels toward the first direction transverse to the longitudinal axis of vehicle, this may reduce the lateral force to a level below the threshold force, for example, by biasing the vehicle to rotate about the first wheels. Some examples of the process  1200 , may also include, at  1210 , causing a motor coupled to the second wheels of the vehicle to supply torque to at least one of the second wheels in response to receipt of one or more signals indicative that the lateral force exceeds the threshold force. The torque may be supplied to cause the second wheels increase the speed of rotation. This may result in causing the vehicle to rotate about the first wheels. In some examples, the process  1200 , at  1208 , may include activating one or more brakes coupled to the first wheels of the vehicle to reduce a rate of rotation of at least one of the first wheels in response to receipt of one or more signals indicative that the lateral force exceeds a roll coefficient multiplied by a roll force sufficient to cause the vehicle to roll over in the first direction. This may reduce the lateral force to a level below the roll coefficient multiplied by the roll force, for example, by biasing the vehicle to rotate about the first wheels. Some examples of the process  1200 , may also include, at  1210 , causing a motor coupled to the second wheels of the vehicle to supply torque to at least one of the second wheels in response to receipt of one or more signals indicative that the lateral force exceeds the roll coefficient multiplied by the roll force. The torque may be supplied to cause the second wheels increase the speed of rotation. This may result in causing the vehicle to rotate about the first wheels. 
       FIG. 13  is a flow diagram of an example process  1300  for reducing or eliminating misalignment between the direction of travel of a vehicle and the direction defined by the longitudinal axis of the vehicle. At  1302 , the process  1300  may include receiving one or more signals indicative of a direction of travel of a vehicle. For example, the one or more signals indicative of the direction of travel of the vehicle may include or be based at least in part on one or more signals generated and/or received from the location systems of the vehicle. For example, the location systems of the vehicle may include one or more signals generated by a global positioning system associated with the vehicle. 
     At  1304 , the process  1300  may include receiving one or more signals indicative of a direction defined by the longitudinal axis of the vehicle, which may be aligned with or contained in a plane passing through the centerline of the vehicle. In some examples, the one or more signals indicative of the direction defined by the longitudinal axis of the vehicle may include receiving one or more signals from vehicle sensors associated with the vehicle, such as, for example, one or more sensors configured to generate one or more signals indicative of the direction defined by the longitudinal axis of the vehicle. 
     At  1306 , the example process  1300  may include determining, based at least in part on the one or more signals indicative of the direction of travel of the vehicle and the one or more signals indicative of the direction defined by the longitudinal axis of the vehicle, a misalignment between the direction of travel of the vehicle and the direction defined by the longitudinal axis of the vehicle. This determination may be made by, for example, a wheel controller. 
     The example process  1300  may also include, at  1308 , controlling a first steering assembly coupled proximate to a first end of the vehicle and a second steering assembly coupled proximate to a second end of the vehicle opposite the first end to reduce or eliminate the misalignment between the direction of travel of the vehicle and the direction defined by the longitudinal axis of the vehicle, for example, as discussed herein. For example, the first and second steering assemblies may cause the first wheels and the second wheels, respectively, to pivot to new steering angles that result in reducing or eliminating the misalignment. 
       FIG. 14  is a flow diagram of an example process  1400  for estimating tire-surface parameterization data associated with a surface on which a vehicle travels. At  1402 , the process  1400  may include activating a brake coupled to a first tire coupled to the vehicle to cause rotation of the first tire to slow. In some examples, a brake controller may send one or more signals to a brake causing the brake to be activated, thereby slowing the first tire. 
     At  1404 , the process  1400  may include causing a motor coupled to a second tire to supply torque to the second tire to resist slowing of the vehicle caused by activation of the brake. In some examples, a wheel controller may send one or more signals to the motor to cause the motor to supply torque to the second wheel to counteract the braking force and to prevent the vehicle from noticeably slowing when the brake is activated. 
     At  1406 , the process  1400  may include receiving one or more signals indicative of the speed of rotation of the first tire during the braking. For example, the vehicle may include one or more vehicle sensors, such as, for example, one or more wheel encoders configured to detect the speed of rotation of the first tire. In some examples, the one or more signals may be received by a tire-surface parameterization estimator, for example, as described herein. 
     At  1408 , the process  1400  may include receiving one or more signals indicative of a braking force on the first tire during the braking. The vehicle sensors may include, for example, a torque sensor coupled to the first wheel on which the first tire is mounted, and the torque sensor may generate one or more signals indicative of the braking force. In some examples, the one or more signals indicative of a braking force may be received by the tire-surface parameterization estimator, for example, as described herein. 
     The process  1400 , at  1410 , may include estimating the tire-surface parameterization data associated with the surface based at least in part on the one or more signals indicative of the speed of rotation of the first tire and the one or more signals indicative of the braking force on the first tire. For example, the tire-surface parameterization estimator may receive the signals and estimate the tire-surface parameterization data based at least in part on the signals. In some examples, the tire-surface parameterization data may include the friction coefficient between the surface and one or more of the tires of the vehicle. The friction coefficient may be the static friction coefficient or the dynamic friction coefficient. 
     In some examples, the tire-surface parameterization data may be used by the vehicle (e.g., by the vehicle systems  102 ) to determine performance parameters for the vehicle, such as, for example, maximum accelerations for increasing vehicle speed, maximum accelerations for reducing vehicle speed, maximum cornering speeds, etc. In some examples, the process may also include communicating data correlating the location of the vehicle and the tire-surface parameterization data via a communication network. In this example manner, the tire-surface parameterization data estimated by the vehicle may be used by other vehicles that are in communication with the communication network. 
       FIG. 15  is a flow diagram of another example process  1500  for estimating tire-surface parameterization data associated with the surface on which the vehicle travels. At  1502 , the process may include causing a steering assembly coupled proximate to an end of the vehicle to change a first steering angle of a first tire coupled proximate to the end of the vehicle, so that the first steering angle and a second steering angle of a second tire coupled proximate to the vehicle differ from one another. For example, the steering angle of the first tire may be pivoted out of the direction of travel of the vehicle to cause the first tire to scrub along the surface, thereby generating a lateral force at the first tire as the vehicle continues to travel in the direction it was traveling prior to causing the first tire to pivot. 
     At  1504 , the process  1500  may include receiving one or more signals indicative of a lateral force on the first tire. In some examples, the tire-surface parameterization estimator may receive the one or more signals from a sensor coupled to the steering assembly and/or a suspension assembly associated with the first tire and configured to generate the one or more signals. 
     The process  1500 , at  1506 , may include estimating the tire-surface parameterization data associated with the surface based at least in part on the one or more signals indicative of the lateral force acting on the first tire. For example, the tire-surface parameterization estimator may estimate the tire-surface parameterization data based on one or more of the weight of the vehicle, the vertical load on the first tire, or the one or more signals indicative of the force acting on the steering assembly and/or the suspension assembly. In some examples, the tire-surface parameterization data may be associated with a local map and/or a global map, for example, identifying locations corresponding to the tire-surface parameterization data. 
     It should be appreciated that the subject matter presented herein may be implemented as a computer process, a computer-controlled apparatus, a computing system, or an article of manufacture, such as a computer-readable storage medium. While the subject matter described herein is presented in the general context of program modules that execute on one or more computing devices, those skilled in the art will recognize that other implementations may be performed in combination with other types of program modules. Generally, program modules include routines, programs, components, data structures, and other types of structures that perform particular tasks or implement particular abstract data types. 
     Those skilled in the art will also appreciate that aspects of the subject matter described herein may be practiced on or in conjunction with other computer system configurations beyond those described herein, including multiprocessor systems, microprocessor-based or programmable consumer electronics, minicomputers, mainframe computers, handheld computers, mobile telephone devices, tablet computing devices, special-purposed hardware devices, network appliances, and the like. 
     Based on the foregoing, it should be appreciated that technologies for operating an electrical system have been presented herein. Moreover, although the subject matter presented herein has been described in language specific to computer structural features, methodological acts, and computer readable media, it is to be understood that the invention defined in the appended claims is not necessarily limited to the specific features, acts, or media described herein. Rather, the specific features, acts, and media are disclosed as example forms of implementing the subject matter recited in the claims. 
     The subject matter described above is provided by way of illustration only and should not be construed as limiting. Furthermore, the claimed subject matter is not limited to implementations that solve any or all disadvantages noted in any part of this disclosure. Various modifications and changes may be made to the subject matter described herein without following the examples and applications illustrated and described, and without departing from the spirit and scope of the present invention, which is set forth in the following claims.