Patent Publication Number: US-2007096547-A1

Title: Method for vehicle front brake sizing

Description:
FIELD OF THE INVENTION  
      The present invention relates to motor vehicle design, and more particularly to a method for sizing the front brakes on a motor vehicle.  
     BACKGROUND OF THE INVENTION  
      Typically, when designing brake systems for motor vehicles, the designers use various prototype samples before arriving at a desired rotor and caliper combination. Then, once the desired rotor and caliper combination is finalized, the designers perform extensive thermal testing and thermal analysis to determine if the rotor and caliper combination can withstand the thermal energy dissipated while braking the motor vehicle. This generally results in increased design time and large prototype tooling costs.  
      Accordingly, it is desirable to provide a modeling method for front brake sizing which performs mathematical analysis to select and thermally validate a rotor and caliper combination based on the design criteria for the motor vehicle.  
     SUMMARY OF THE INVENTION  
      The present invention provides a method for selecting a brake system for an automobile using a processor. The method comprises gathering characteristics of the automobile and then calculating a maximum rotor size based on these characteristics. Next, a specific torque required to skid the automobile at a selected deceleration is calculated for the brake system at driver only weight, and then a brake caliper is selected based on the specific torque required and maximum brake rotor size. Finally, the selected rotor and brake caliper are evaluated to determine if thermal dissipation requirements for city driving conditions are met.  
      Further areas of applicability of the present invention will become apparent from the detailed description provided hereinafter. It should be understood that the detailed description and specific examples, while indicating the preferred embodiment of the invention, are intended for purposes of illustration only and are not intended to limit the scope of the invention. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The present invention will become more fully understood from the detailed description and the accompanying drawings, wherein:  
       FIG. 1  is a two-dimensional view of an automobile employing a brake system sized according to the principles of the present invention;  
       FIG. 2  is a cross-sectional view of a disc brake in the brake system taken along line  2 - 2  of  FIG. 1 ;  
       FIG. 3  is a front view of a rotor sized using the principles of the present invention;  
       FIG. 4  is a flowchart detailing a rotor and caliper sizing process according to one of various embodiments; and  
       FIG. 5  is a flowchart detailing a thermal validation process according to one of various embodiments. 
    
    
     DETAILED DESCRIPTION OF VARIOUS EMBODIMENTS  
      The following description of the various embodiments is merely exemplary in nature and is in no way intended to limit the invention, its application, or uses.  
      The present invention is generally related to a method for vehicle brake sizing. Although the following exemplary description refers to the sizing of front brakes for a vehicle, it will be understood that the present method may be applicable to sizing rear brakes and to other brake applications in general. Also, this methodology could be applied to brake applications including unvented rotors. It will also be understood that the motor vehicle referenced below is an exemplary vehicle, and the foregoing methodology, as applied to this motor vehicle, could be applied to any variety of motor vehicles. Further, the foregoing description is understood to not limit the appended claims.  
      With reference now to  FIG. 1 , a motor vehicle  10  is shown. Motor vehicle  10  generally includes a brake system  12  coupled to a plurality of wheels  14  with tires  20  mounted thereto. A center of gravity for the motor vehicle  10  is indicated by CG and a wheelbase W b  for the motor vehicle  10  is measured as the distance between a front axle  16  and a rear axle  18 . The motor vehicle  10  also has a driver only weight DOW calculated as the weight of the motor vehicle  10  containing only the driver (not specifically shown). The height from the center of gravity H CG  is measured from the center of gravity CG to a ground  22 . The motor vehicle  10  further has a tire static loaded radius SLR which is the distance between a center point  24  of the tire  20  and ground  22  with the weight of the motor vehicle  10  upon the tire  20 .  
      With additional reference to  FIGS. 2 and 3 , a disc brake  26  of the brake system  12  is shown in greater detail. The disc brake  26  generally includes a rotor  28  with a hub  30  protruding from a body  32  of the rotor  28 . The rotor  28  may define a plurality of openings  34  surrounding a central opening  36  for receipt of fasteners to secure the wheel  14  to the hub  30 . The central opening  36  is adapted to receive a spindle (not specifically shown) to rotatably couple the rotor  28  to the front axle  16 , such that a spindle center line S is equivalent to a rotor center line R. The rotor  28  is generally annular with an outer diameter OD. The body  32  of the rotor  28  includes two discs  40  separated by a vent width  42 . A plurality of vanes  44  may be formed between the two discs  40  to provide additional surface area and air flow through the rotor  28  to dissipate thermal energy generated during braking.  
      A caliper  46  may be disposed adjacent to the rotor  28 , with any desired rotor to caliper clearance RCC and desired caliper to rim clearance CWC. The caliper  46  includes a first brake pad  48  and a second brake pad  50 , each configured to contact the surface of the rotor  28  to stop the motor vehicle  10  when activated by a piston  52 . The first brake pad  48  may be secured with a bridge  54  on the caliper  46  through any appropriate mechanism, such as mechanical fasteners (not specifically shown). The bridge  54  may have any desired thickness T. The second brake pad  50  may be secured to a face  56  of the piston  52  via any suitable mechanism, such as mechanical fasteners (not shown). The piston  52  may be operated by hydraulic fluid provided by a master cylinder  58  and power brake booster  60  ( FIG. 1 ) coupled to the caliper  46 . However, the piston  52  may be operated by any suitable mechanism. The activation of the piston  52  causes the first and second brake pads  48 ,  50  to press against the rotor  28 , and will slow the rotation of the rotor  28  and thus wheel  14 .  
      The disc brake  26  can be coupled to the wheel  14  and tire  20  via the openings  34  provided on the hub  30  and the rotor  28 . The wheel  14  includes a rim  62  and a disc  64 . The rim  62  to supports the tire  20 . The rim  62  and disc  64  may have any desired thickness T R , T D  respectively. The rim  62  also includes a drop well depth D W , which is the distance between a theoretical cylindrical surface  66  of the wheel  14  and a surface  68  of the rim  62 . The distance from the surface  66  of the wheel  14  and the spindle center line S forms the tire and rim association guideline wheel diameter D/2. The tire  20  may be mounted to the rim  62 , and depending upon the tire  20 , will have a particular tire to ground friction coefficient μ T . Generally, the tire to ground friction coefficient μ T  can be 1.0.  
      With continuing reference to  FIGS. 1, 2  and  3  and additional reference to  FIG. 4 , a method for vehicle rotor and caliper sizing  100  is illustrated. This program  100  may be implemented upon a processor (not shown).  
      More specifically, with reference now to  FIG. 4 , the processor begins in step  110 . In step  112 , the operator inputs at least a few of the following parameters: the gross vehicle weight GVW in pounds (lbs), the front axle weight (lbs), driver only weight (lbs), wheelbase W b  (in), tire static loaded radius SLR (in), tire to ground friction coefficient μ T , (tire and rim guideline wheel diameter) D/2 (in), drop well depth D W  (in), rim thickness T R  (in), disc thickness T D  (in), caliper to wheel clearance CWC (in), caliper bridge thickness T (in), booster size I p  and rotor to caliper clearance RCC (in).  
      Next, in step  114 , the processor selects an initial size for the. caliper  46  based on the gross vehicle weight GVW. The size of the caliper  46  selected based on GVW provides a preliminary guideline for a caliper size from which the further calculations are based. Unless the operator inputs a different desired caliper size, the processor will use the smallest caliper available in the GVW range. In step  116 , the processor calculates the maximum rotor outer diameter OD. The maximum rotor outer diameter OD is given by the following equation: 
 
MaxRotorOD=2*{( D/ 2)− D   W   −T   R   −T   D −CWC− T −RCC)}
 
 wherein D is the tire and rim association guideline wheel diameter, D w  is the drop well depth, T R  is the rim thickness, T D  is the disc thickness, CWC is the caliper to wheel clearance, T is the caliper bridge thickness and RCC is the rotor OD to caliper bridge clearance. If the above values are unknown, or not inputted in step  112 , the processor uses default values. These default values are based on standard industry practice. 
 
      Next, in step  118 , the processor may calculate the effective radius R EEF  with respect to the rotor outer diameter from step  1   16 . The effective radius R EEF  denotes the radial location area of the rotor  28  wherein the force from the first and second brake pads  48 ,  50  is concentrated during braking. The effective radius R EEF  can be determined from the following equation: 
 
 R   Eff =(RotorOD)/2−(Piston diameter)/2 +c   0  
 
 wherein the rotor OD is in millimeters, the caliper piston diameter is in millimeters and c o  is a correction factor depending on type of caliper selected. 
 
      In step  120 , the processor calculates and plots the specific torque T SPEC  for selected calipers against the lining coefficient of friction μ L  for the various linings available for a range of calipers. An exemplary range for the lining coefficient of friction μ L  can be 0.2-0.6 depending upon the motor vehicle. The specific torque T SPEC  can be calculated by the following equation: 
 
 T   Spec =2 *A   C μ L   *R   Eff  
 
 wherein A C  is the caliper piston area in square inches (in 2 ), R EEF  is the effective radius from step  118  in inches (in), μ L  is the lining coefficient of friction. 
 
      With reference back to  FIGS. 1, 2 ,  3  and  4 , in step  122 , the processor calculates the brake torque required T BRAKE  at driver only weight (DOW) to skid the front tires  20  of the motor vehicle  10  at an assumed deceleration rate of 32 feet per second squared (1 G). The brake torque required T BRAKE  can be found by the following equation: 
 
 T   Brake =[{(DOW *H   cg   /wb )* D +FrtAxleWt}/2]*SLR*μ t  
 
 wherein DOW is the driver only weight in pounds, D is the deceleration rate 1 G, H CG  is the height of the center of gravity in inches, SLR is the tire static loaded radius in inches, W b  is the wheelbase in inches, and μ T  is the tire to ground friction coefficient. Generally, μ T  can be 1.0. 
 
      In step  124 , the processor acquires the line pressure p L  required for the brake system at power brake booster  60  run-out. The line pressure p L  may be found by using two different methods. First, if specific characteristics are known, the line pressure p L  can be determined from the following equation: 
 
 p   L   =F   Total   /A   MC =(( F   A +( F   p   *I   p *η p )− F   S )/ A   MC )*η MC  
 
 wherein F A  is the booster force, I p  is the pedal ratio, η MC  is the master cylinder efficiency, F p  is the pedal force, η p  is the pedal efficiency, F S  is the master cylinder spring force and A MC  is the master cylinder piston area. 
 
      Alternatively, the line pressure p L  can be set at a default value of psi based on historical data. Then, the processor in step  126  calculates the specific torque required T SPEC,REQD  for the brake system to skid the front tire  20  at a deceleration of 1 G. This can be calculated from the following equation: 
 
 T   Specific,Req&#39;d   =T   Brake   /p   L  
 
 wherein T BRAKE  is the brake torque required at driver only weight (DOW) determined in step  122  and μ L  is the line pressure determined from step  124 . 
 
      In step  128 , the user may input a desired target coefficient of friction μ L  for the lining. If no desired value for the lining coefficient of friction was provided in step  112 , then the processor assumes a default value based on historical data.  
      Then, based on the specific torque required T SPEC,REQD  calculated in step  126 , the processor generates a horizontal line on the plot from step  120  at the specific torque required (T SPEC,REQD ). Then the processor plots a vertical line on the plot from step  120  at the target lining coefficient of friction μ L . Based on this plot, the processor may then select a desired caliper  46  based on the specific torque required T SPEC,REQD , the desired coefficient of friction for the lining μ L , from step  128  and the specific torque T SPEC  calculated in step  120 . Generally, the processor can select the appropriate caliper  46  based on the nearest caliper that is above the intersection of the target coefficient of friction for the lining μ L  and the specific torque required T SPEC,REQD  by the brake system determined from the plot.  
      With reference back to  FIGS. 1, 2 ,  3  and  4 , after the caliper  46  has been selected, the processor in step  132  selects the standard rotor overall width and vent width associated with the selected size of the caliper  46  from step  130 .  
      Next, in step  134 , the processor uses standard design rules to generate the number of vanes  44  for the rotor  28 . First, the processor determines a rotor rub track inside diameters ID RUBTRACK  for the rotor  28 . The processor can assume that the inner diameter of the inboard rub track of the rotor  28  is equivalent to the inner diameter of the outboard rub track and that the wheel  14  is fully supported by the hub  30 . Based on these assumptions, the processor uses the following equation to calculate the rotor rub track inside diameter ID RUBTRACK : 
 
ID RUBTRACK =HubFlangeOD+2*(RotortoHubClearance)+2* (RotorHatSideThickness)+(RotorHatODtoRubTrackIDGap) 
 
 wherein the hub flange OD, Rotor to Hub Clearance Rotor Hat Side Thickness and Rotor Hat OD to Rub track ID Gap may be default valves or based on user input. 
 
      After determining the rotor rub track inside diameters ID RUBTRACK  the processor determines a rub track height H RUBTRACK  for the rotor  28 . The rub track height H RUBTRACK  can be found from the following equation: 
 
Height RubTrack =(RotorOD−ID RUBTRACK )/2 
 
 wherein the rotor OD is the rotor outer diameter calculated in step  116 . 
 
      Based on the rub track height H RUBTRACK  the processor then creates a default configuration for the vanes  44 . The default configuration for the vanes  44  may be radial and configured according to a percentage of the total swept area. Vane width  70 , vane gap  72  and vane inset  74  are variable depending on manufacturing constraints. The processor may calculate the number of vanes  44  to the nearest prime number based on the following equation: 
 
Qty Vanes =π*(ID RUBTRACK +2*VaneInset)/(VaneWidth+VaneGap) 
 
 wherein the ID Rubtrack is the rotor inner diameter determined in step  116 . 
 
      After determining the quantity of the vanes  44 , the processor calculates the length for each of the vanes  44 . The length of each of the vanes  44  can be determined from the following equation: 
 
Length= H   RubTrack −2*(VaneInset) 
 
      After generating the quantity of rotor vanes and length  44  in step  134 , ,the processor ends the rotor and caliper sizing program  100  in step  136 . With reference now to  FIG. 7 , in step  200 , the processor begins the thermal validation program  199 .  
      Next, in step  202 , the processor determines the percent front work done at 5 feet per second squared (ft/sec 2 ) deceleration. Five ft/sec 2  is the typical deceleration rate for city driving conditions. The percentage of front work done can be determined through two methods. In the first method, a brake simulation program can be run to determine the percent work done. In a second method, a half-vehicle dynamometer test can be run on representative hardware to determine the percentage of front work done. In step  204 , the processor determines the front torque at 5 ft/sec 2  deceleration. The front torque can be determined by the following equation: 
 
FrontTorque=(VehicleWeight/Accel due to gravity)*Decelrate*TireSLR*% FrtWork 
 
 wherein the vehicle weight is in pounds, the acceleration due to gravity is in ft/sec 2  and the tire SLR is from step  112  (in feet) and the percent front work is from step  202 . 
 
      Next, in step  206 , the processor determines the front corner torque at 5 ft/sec 2  deceleration. The front corner torque can be determined from the following equation: 
 
FrontCornerTorque=FrontTorque/2 
 
 wherein the front torque is the front torque determined from step  204 . 
 
      Next, in step  208 , the processor can determine the effective surface area of the rotor  28 . The effective surface area of the rotor  28  is calculated based on the rub track area of the rotor plates, the interior rotor area not covered by vanes, the interior rotor area added by vanes and the interior rotor area correction coefficient as shown in the following equations: 
 
Effective Surface Area=2 A+D  
 
 A=rubtrack area of outboard rotor plate=(π/ 4)*(RubTrackOD 2 −RubTrackID 2 )
 
 B=interior rotor area not covered by vanes=(π/ 4)*(RubTrackOD 2 −RubTrackID 2 )−Qty vanes *(VaneWidth*Length)
 
 C=interior rotor area added by vanes=Qty   vane *(2*VaneWidth+2*Length)*VentWidth
 
 D=interior rotor area correction coefficient=(   CF )*(2 B+C )ˆ2,
 
 where CF=correction factor for a non-linear surface heat dissipation
 
      The processor may next determine the effective thermal mass of the front rotor in step  210 . The effective thermal mass is based on the rub track volume of the rotor plates and the total vane volume. The effective thermal “mass” of the rotor  28  can then be determined by the following equations: 
 
EffectiveThermal “Mass”=2 E+F  
 
 E=rubtrack volume of one rotor plate=(π/ 4) *(RubTrackOD 2 −RubTrackID 2 )*one Rotor Plate Thickness 
 
 F=total vane volume=Qty   vane *(VaneWidthLength*VentWidth) 
 
      In step  212 , the user can enter the effective lining volume. Next, in step  214 , the processor calculates the effective surface area factor of the rotor  28 . The effective surface area factor is based on the effective surface area calculated in step  208  and the front corner torque calculated in step  204 . The effective surface area factor can be given by the following equation: 
 
SurfaceAreaFactor=Effective Surface Area/Front Corner Torque at 5 ft/s 2  Decel 
 
      In step  216 , the processor calculates the effective thermal mass factor of the rotor  28 . The effective thermal mass factor is based on the effective thermal mass calculated in step  210  and the front corner torque calculated in step  206  and can be found by the following equation: 
 
Thermal “Mass” Factor=Effective Thermal Mass/Front Corner Torque at 5 ft/s 2  Decel 
 
      Then, in step  218  the processor determines the lining volume factor for the rotor  28 . The lining volume factor can be found from the following equation: 
 
LiningVolumeFactor=Effective Lining Volume/Front Corner Torque at 5 ft/s 2  Decel 
 
 wherein the effective lining volume was determined in step  212 . 
 
      Next, in step  220 , the processor compares the surface area factor found in step  214  to a surface area factor for a base line vehicle. The base line vehicle may be any suitable vehicle with similar characteristics to the motor vehicle  10 . If the surface area factor calculated in step  214  is greater than or equal to an acceptance criteria, the processor continues to step  222 . If, however, the surface area factor calculated in step  214  is less than the factor for the base line vehicle or alternate acceptance criteria, the processor goes to step  224  and returns to the rotor and caliper sizing program  100 .  
      Next, the processor compares the thermal mass factor calculated in step  216  to the thermal mass factor calculated for the base line vehicle in step  226 . If the thermal mass factor calculated in step  216  is greater than or equal to an acceptance criteria, then the processor continues to step  222 . If, however, the thermal mass factor calculated in step  216  is less than the thermal mass factor for the base line vehicle or alternative acceptance criteria, the processor jumps to step  224  and returns to the rotor and caliper sizing program  100 .  
      Next, in step  228 , the processor compares the lining volume factor calculated in step  218  to a lining volume factor for a base line vehicle. If the lining volume factor calculated in step  218  is greater than or equal to an acceptance criteria, then the validation is complete in step  230 . If, however, the lining volume factor calculated in step  218  is less than the lining volume factor calculated for the base line vehicle or alternative acceptance criteria, the processor jumps to step  224  and returns to the rotor and caliper sizing process. In step  230 , the processor ends the validation process and outputs the selected rotor  28  and caliper  46 .  
      The method for vehicle front brake sizing of the present invention enables automobile designers to quickly and easily determine the size of front brakes required for their vehicle. Thus, this method reduces design time and also reduces prototype part costs. In addition, performing thermal validation on the selected rotor  28  and caliper  46  under city driving conditions predicts the ability of the selected brake system  12  to dissipate the thermal energy generated by repeated braking in city traffic conditions and ensures acceptable brake lining life for the brake system.  
      The description of the invention is merely exemplary in nature and, thus, variations that do not depart from the gist of the invention are intended to be within the scope of the invention. Such variations are not to be regarded as a departure from the spirit and scope of the invention.