Driving and steering of motor vehicles

An arrangement and a method are disclosed for driving and steering an engine driven vehicle of the type m which left and right hand wheels 120L, 120R are respectively driven from the engine through left and right hand transmissions 122L, 122R and a steering effect is exerted by changing the ratio of one transmission relative to the other. According to the invention a follower is operatively coupled to at least one of the transmissions and is movable circumferentially about an axis by means of a driver actuable steering control to change the relative speeds of the two driven wheels and radially with respect to the axis by means of a driver actuable speed control to increase/decrease the speeds of the two driven wheels in unison.

CROSS REFERENCES TO RELATED APPLICATIONS

This application is a national phase application under 35 U.S.C. §371 of International Application No. PCT/EP2006/065467 filed Aug. 18, 2006, which claims priority to Great Britain Application No. 0517200.2 filed Aug. 22, 2005, both of which are incorporated by reference without disclaimer.

The present invention is concerned with arrangements for driving and steering motor vehicles, of the type in which vehicle wheels on the left and right hand sides of the vehicle are driven at different speeds in accordance with a desired vehicle turning radius.

Vehicles can be steered by controlling the relative speeds of driven wheels on opposite sides of the vehicle. Track-laying vehicles such as tanks are of course steered in this way (note in this regard that although, for the sake of brevity, reference will be made throughout to vehicle “wheels”, this should be understood to encompass vehicles of track-laying type, wherein the “wheels” in question are pinions on which the tracks are carried) but so too are various types of vehicle used in construction, agriculture and horticulture, including for example ride-on lawnmowers. The principle is simple. If the wheel on one side of the vehicle is driven faster than the wheel on the other side, then the vehicle turns, with the slower moving wheel being on the inside of the turn. The wheel on the inside of the turn may be stationary, or may even be driven in the opposite direction from the other wheel. In the extreme case where the two wheels are driven at equal and opposite speeds, the vehicle can be caused to spin about its centre.

Where the vehicle has steerable wheels, such as the front wheels of a conventional tractor or ride-on lawnmower, the necessary steering effect may be achieved solely by control of driven wheel speed, with the steerable wheels being arranged as castors to follow the radius of turn determined by the driven wheels. Alternatively the steerable wheels may be positively controlled (e.g. through a conventional rack and pinion steering system) to produce the desired radius of turn, while the relative speeds of the driven wheels are set to produce the same radius. Compared with the alternative of guiding the vehicle by means of the steerable wheels only, this arrangement has the advantage of allowing the vehicle to turn very tightly and even to spin on the spot.

One way to achieve the necessary control over the speed and direction of rotation of the driven vehicle wheels involves the use of one engine but two essentially independent continuously variable transmissions (“CVTs”), through which the engine drives the left and right hand vehicle wheels respectively. In the case of small horticultural vehicles such as ride-on lawnmowers, continuously variable transmissions of hydrostatic type have been used in this role. The applicant now proposes to substitute infinitely variable transmissions of rolling-traction, toroidal-race type, but the principles involved in vehicle steering are common to both and the present invention is potentially applicable to either of these types of transmission, or indeed to others. Hydrostatic transmissions (well known to those skilled in the art) are also suitable, as are other types of mechanical CVT. An alternative way to achieve the necessary independent control over the speed of the driven vehicle wheels is to use two separate rotary drivers coupled to respective wheels and providing independently variable speeds. For example two separate electric motors coupled to the respective wheels through respective fixed ratio gearing could be used in this role.

The present invention is concerned with the manner in which control over such an arrangement is to be exercised. One known arrangement represented inFIG. 1comprises left and right hand control levers2L,2R movable by the driver, controlling respective hydrostatic type CVTs8L,8R. The drive ratio applied to left and right hand vehicle wheels is controlled by the position of the corresponding lever and is variable from maximum forward ratio, through a neutral ratio, referred to herein as “geared neutral”, at which wheel speed is zero, to maximum reverse. Note that throughout this document, drive ratio will be defined as output (wheel-side) speed divided by input (engine-side) speed, so that at geared neutral the drive ratio is zero. A difference in the position of the levers2L,2R produces a turn. Thus inFIG. 1aleft hand lever2L has been advanced further than right hand lever2R. The left hand wheel is thus driven faster than the right, and a right turn results. Steerable front wheels4L,4R are correspondingly inclined.

In geared neutral, the engine is not physically de-coupled from the wheels (as by a clutch) but instead the transmission effectively provides infinite speed reduction, so that the relevant wheel is stationary despite rotation of the engine/transmission input. Hydrostatic transmissions can achieve this state. Numerous transmissions utilising a ratio varying-device (“variator”) in conjunction with an epicyclic shunt gear to provide a geared neutral facility are described in Torotrak (Development) Ltd.'s portfolio of patents. Recent examples include International Application PCT/GB03/00332, published under WO 03/064892, and International Application PCT/GB03/02332, published under WO 03/100295, but the principle has been known for many years.

The known steering arrangement is widely used and successful, but can create problems for some drivers. Consider what happens if the levers, starting from the positions represented inFIG. 1a, are subsequently drawn backwards through identical distances by the driver, as represented inFIG. 1b. The right-hand wheel is now driven faster, in reverse, than the left. The vehicle changes from going forward and turning right (FIG. 1a) to reversing and turning left (FIG. 1b) even though the displacement of one lever relative to the other has not changed. This is counter-intuitive to some drivers.

In accordance with a first aspect of the present invention, there is an arrangement for driving and steering a motorised vehicle the arrangement comprising

a right hand drive arrangement for driving a right hand vehicle wheel at a speed which is continuously variable through a range including forward and reverse speeds,

a left hand drive arrangement for driving a left hand vehicle wheel at a speed which is continuously variable through a range including forward and reverse speeds,

a driver actuable speed control, and

a driver actuable steering control,

wherein both the left and right hand drive arrangements are controlled, in dependence upon driver inputs, by means of the speed and steering controls acting either through a common mechanism, or through two mechanisms associated with the respective drive arrangements, comprising in either case

a guide defining a guide path which is rotatable about an axis by means of the steering control

a mechanical follower which is movable back and forth along the guide path by means of the speed control, and

a coupling between the follower and the drive arrangement(s), through which the follower's position influences vehicle wheel speed,

whereby movement of the speed control in a first direction causes an increase in the speed of both the left and the right vehicle wheels, movement of the speed control in a second direction opposite to the first causes a decrease in the speed of both the left and the right vehicle wheels, so that vehicle speed is controllable by means of the speed control, and movement of the steering control causes a change in the speed of one of the left and right vehicle wheels relative to the speed of the other, to provide for vehicle steering.

Rotary drive may be provided by an internal combustion engine but may be from a different type of rotary driver such as single or dualelectric motor(s) or an external combustion engine. The terms “right hand drive arrangement” and “left hand drive arrangement” refer to the fact that the arrangements in question are coupled to the right and left hand wheels respectively, and do not necessarily denote the relative spatial positions of the arrangements, nor is their physical construction necessarily wholly separate.

In accordance with a second aspect of the present invention, there a method of transmission control in a motor vehicle having

a right hand drive arrangement for driving a right hand vehicle wheel at a speed which is continuously variable through a range including forward and reverse,

a left hand drive arrangement for driving a left hand vehicle wheel at a speed which is continuously variable through a range including forward and reverse,

the method comprising

providing a follower which is operatively coupled to at least one of the transmissions such that its position determines transmission ratio,

receiving speed and steering control inputs from a driver,

controlling the follower's circumferential position relative to an axis in dependence upon the steering input, and

controlling the follower's radial distance from the axis in dependence upon the speed input,

wherein movement of the speed control in a first direction causes an increase of the speed of both the left and the right hand vehicle wheels, movement of the speed control in a second direction opposite to the first causes a decrease in the speed of both the left and the right hand vehicle wheels, so that vehicle speed is controllable by means of the speed control, and movement of the steering control causes a change in one ratio relative to the other, to provide for vehicle steering.

In accordance with a third aspect of the present invention there is an arrangement for driving and steering a vehicle having an engine, the arrangement comprising

a right hand transmission which is for transmitting rotary drive between the engine and a right hand driven vehicle wheel at a drive ratio which is continuously variable through a range including forward and reverse ratios,

a left hand transmission which is for transmitting rotary drive between the engine and a left hand driven vehicle wheel at a drive ratio which is continuously variable through a range including forward and reverse ratios,

a driver actuable speed control,

a driver actuable steering control,

and a follower which is movable circumferentially about an axis by means of the steering control and is movable radially with respect to the axis by means of the speed control, the follower being operatively coupled to at least one of the transmissions,

whereby movement of the driver's steering control changes the radius of turn executed by the vehicle and movement of the speed control changes vehicle speed.

The use of independently, continuously variable transmissions allows for steering control through the driven wheels alone.

In accordance with a fourth aspect of the present invention, there is a vehicle having an engine, left and right hand driven wheels, left and right hand transmissions for transmitting rotary drive from the engine to the respective driven vehicle wheels at respective, separately and continuously variable ratios, and a driver actuable control arrangement enabling the driver to control both ratios and thereby to control turning of the vehicle, the vehicle having at least one steerable wheel, and the or each steerable wheel(s) being mounted in the manner of a castor.

FIGS. 2-8are schematic views, from above, of selected parts of a vehicle embodying the present invention. Left and right hand driven vehicle wheels are seen at20L and20R. Each is driven through a respective transmission22L and22R of continuously variable type, capable of providing a continuous range of ratios from forward to reverse.

The driver dictates the vehicle's speed and direction through two controls, which are represented in schematic form inFIG. 2: a steering control24, which may take the form of a conventional steering wheel, and a speed control26, which may be formed as a foot pedal. The pedal is somewhat different from the accelerator control of a motor car, in that it can rocked forwards, using the front of the foot, to select forward drive, or backwards, using the heel, to select reverse. The pedal is sprung toward a central position in which it causes both transmissions22L,22R to adopt geared neutral. Other types of user-operable control may of course be used in these roles. Also the two controls could be formed by a single assembly. For example the driver could be provided with a steering bar or wheel which is rotatable to steer and movable fore-and-aft to change speed.

The driver's inputs through the controls24,26determine the ratios adopted by the transmissions22L,22R, acting through a mechanism comprising left and right-hand guide plates28L,28R and left and right hand actuating levers30L,30R. The driver is typically provided with a separate control—e.g. a hand operated lever—for setting engine speed. In vehicles using speed governed diesel engines, the driver typically sets the engine speed with the lever and speed control is subsequently provided by means of the transmissions.

In the present embodiment, the guide plates28L,28R are actually placed one above the other, and are mounted for rotation about a common axis32. Hence inFIG. 2only right-hand (upper) guide plate28R is seen. However inFIGS. 3 to 8, for the sake of clarity, the two guide plates are shown side-by-side, which allows both to be seen. The guide plates each define a respective path for guiding a follower. In the illustrated embodiment, the path is simply a straight slot34L,34R in the guide plate and the follower36L,36R is formed as a pin riding in the slot. Each follower is carried on a respective one of the actuating levers30L,30R, and each lever is mounted for rotation about a fixed fulcrum38. The actuating levers are, in the present embodiment, “L” shaped, the follower being carried upon one limb while the other limb engages with the corresponding transmission22L or22R to set its ratio. Lateral movement of the follower36L or36R causes lever30L or30R to rotate and produces a change in ratio of the relevant transmission22L or22R. Consequently the ratios provided by the transmissions22L,22R are determined by the lateral positions of the respective followers36L,36R.

The guide plates can be moved together forward and backward by means of the driver's speed control26. The fore-and-aft displacement of the two guide plates is always identical. The guide plates can also be rotated by means of the driver's steering control24. The two guide plates are not rotated in unison. The movement of the guide plates, and the consequent manner of control of the transmissions, will now be explained with reference to the drawings.

FIG. 3shows a condition in which the steering control24is in its “straight ahead” position—i.e. is set to produce no turn. The two guide plates are in their default orientations, with the guide paths34L and34R inclined to the fore-and-aft direction by equal but opposite angles, which in the present embodiment are approximately 45 degrees. The speed control is set to zero indicating a demand for zero wheel movement. This speed control setting causes the guide plates28L, R to adopt a fore-and-aft position such that the followers36L, R lie upon the rotational axes32of the guide plates28L, R. This corresponds to positions of the actuating levers30L, R in which they cause the transmissions22L, R both to adopt the geared neutral state, in which they provide zero ratio and hence zero output speed, despite rotation of the engine. Because the followers are at the plates' rotational axes, any movement of the steering control cannot move the followers, and so does not cause rotation of the vehicle wheels. This is in accordance with the expectation of the driver, who is used to controlling speed with one control and steering with another.

FIG. 4shows the state of the system when the steering control24remains in the “straight ahead” position, but the speed control has been advanced by the driver, to the limit of its travel, to demand maximum forward vehicle speed. The two guide plates28L, R have been correspondingly advanced which, due to the inclination of their guide paths, has displaced both followers laterally. Correspondingly the actuating levers30L, R have been rotated, causing the transmissions22L, R to adopt identical forward drive ratios, driving the vehicle forward in a straight line.

FIG. 5shows what happens if the driver then turns the steering control to demand a turn to the right. A mechanism (not shown) coupling the steering control to the guide plates28L, R causes the right hand guide plate28R to rotate (in an anti-clockwise direction) but leaves the left hand guide plate in its default orientation. The inclination of the path34R in the right hand guide plate is reduced. Correspondingly the lateral displacement of its follower36R, and the drive ratio from the associated transmission22R, are reduced. The right hand wheel is driven more slowly, while the left hand wheel's speed is unchanged, and a right turn results.

This turn is maintained regardless of the position of the speed control26. InFIG. 6, the speed control has been moved by the driver to place the vehicle in reverse, while maintaining the setting of the steering control24. The absolute speed of the right hand wheel remains smaller than that of the left hand wheel, so that the vehicle continues to turn to the right.

Turning the steering control still further, as inFIG. 7, causes the guide path34R to rotate beyond the point where it is parallel to the fore-and-aft direction. To put this another way, the angle of inclination of the guide path changes from positive to negative. Correspondingly, the direction of rotation of the right hand wheel is reversed. The two wheels thus rotate in opposite directions, producing a very tight radius of turn or even, where the right and left hand wheel speeds are equal but opposite as in the drawing, causing the vehicle to spin on the spot. With the steering control in this position, moving the speed control from forward to reverse—FIG.8—changes the direction in which the vehicle spins.

The drawings all show the steering control24set either to “straight ahead” or “right turn” positions. However the effect of the control mechanism is symmetrical. If the control is turned to the left of the straight ahead position, then it causes the left hand guide plate28L to rotate (in a clockwise direction, as viewed) to slow down the left hand wheel, leaving the right hand guide plate in its default orientation.

The effect of this arrangement, as will be apparent, is that the positions of the levers30L, R controlling the transmissions are each proportional to the speed control setting, but the constant of proportionality is determined by the respective steering control.

The range of ratios which can be demanded from either transmission by movement of the speed control thus depends upon the rotational position of the relevant guide plate28. Taking account of this, some form of end stop is needed to ensure that the transmissions are not driven beyond their upper and lower ratio limits. In the illustrated embodiment, this is achieved by use of fixed stops40,41in front of and behind the guide plates. Abutment of either guide plate28L, R against one of the fixed stops limits their fore-and-aft travel, as seen for example inFIG. 7. The guide plate's shape is selected to provide a suitable relationship between their rotational position and their maximum travel. Thus in the illustrated example, the guide plates28are elliptical. Their travel is maximised when both are in the “straight ahead” position (FIG. 3) where each ellipse's minor axis is aligned along the fore-and-aft direction. Rotating one of the plates increases its length along this direction and so reduces the distance it can travel without hitting one of the stops. This can mean that as the driver turns the steering control26, the speed set by control26must be reduced—as the driver moves the wheel, the pedal will rise.

The vehicle's front vehicle wheels42L and42R could be in the form of castors, being free simply to follow the steering angle dictated by the speeds of the driven wheels, but in the present embodiment are controlled (e.g. through a rack-and-pinion gear, as in conventional steering arrangements) by driver's steering control24.

FIGS. 9 and 10illustrate an actual mechanical embodiment of the control system schematically represented inFIGS. 2 to 8. The left and right hand transmissions are once more seen at22L and22R. They each have a projecting ratio-control lever44L,44R whose outer end is movable fore-and-aft to change the transmission's ratio. This outer end is located between a parallel pair of tines formed on the actuating lever30L or30R, so that the ratio-setting lever's position is determined by that of the actuating lever. The fixed fulcrum about which the actuating levers30L, R rotate is formed as a pin38received in through-going bores in both levers. Other components of the arrangement have already been described and are given the same reference numerals here as in previous drawings.

A possible concern relating to the above-described embodiment is that weighting applied to the steering control by the transmissions may be inappropriate. The driver expects the steering control, when released, to seek its “straight ahead” position. A study of the drawings will confirm that, if the ratio-control lever44L, R on the inside of the turn seeks its “geared neutral” position, as it does while power flow is from the engine to the relevant wheel, then the steering control is urged away from “straight ahead”—i.e. the turn would tend to tighten, rather than to straighten.

A further embodiment of the present invention, not subject to this disadvantage, will now be described. The principles can best be appreciated by reference toFIGS. 11 to 18. The arrangement is similar to that previously described in that continuously variable transmissions122L, R are used to drive respective vehicle wheels120L, R at independent, continuously variable ratios. However in place of the two guide plates of the previous embodiment, the present version has a single guide plate128which is rotatable by means of the steering control about a fixed axis indicated by the intersection of dotted lines in the drawings. A follower136is received in a guide path formed as a straight slot134in the guide plate128, being thus constrained to move only back and forth along the slot, and this movement is controlled by the speed control. In the present embodiment, the movement is controlled by an opposed pair of Bowden type control cables leading to the speed control (seeFIG. 12). Outer sheathes150,152of the cables are led into bores in opposite end faces of the guide plate128and inner cables151,153are each coupled to opposite sides of the follower, thus acting in a “pull/pull” manner. Other types of control cable, able to push as well as pull, would make it possible to use a single cable, but there are in any event numerous other mechanisms which could be used to couple the follower136to the speed control126, one of which will be described below.

The follower136is mounted upon a fore-and-aft extending limb154of a “T” shaped lever130having left and right limbs156L, R which are operatively coupled to the respective transmissions122L, R such that their fore-and-aft positions dictate the transmissions' ratios. In the drawings, uppermost ends of ratio-control levers of the transmissions are seen at144L, R and are coupled to the lever's limbs156L, R. The lever130pivots about a fulcrum158, but this is not fixed. Instead it is able to move along the fore-and-aft direction in a fixed guideway160. The arrangement is such that the follower's position dictates the positions of the ratio-control levers144L, R. Moving the follower forward increase both ratios. Moving it backward decreases both ratios. Lateral movement of the follower increases one ratio and decreases the other.

The operation of this embodiment will now be described.FIG. 11shows its configuration when the speed control is set to zero, causing the follower136to be positioned on the axis of rotation of the guide plate128, and the steering control is in the “straight ahead” position, so that the slot134is aligned along the fore-and-aft direction. The ratio-control levers are both at their “geared neutral” positions so the vehicle is stationary. Because the follower136is on the axis of rotation of the guide plate128, any rotation of the steering control/guide plate128does not move the follower or change the geared neutral ratios of the transmissions, so nothing the driver does with the steering control alone will cause the vehicle to move.

FIGS. 13 and 14both show configurations in which the steering control124is set for “straight ahead”. InFIG. 13the speed control has been moved to request maximum reverse speed, the follower136being correspondingly moved to the rear end of the slot134. Consequently both ratio-control levers are displaced rearwardly by equal amounts, setting both transmissions122L, R to the same reverse ratio. The vehicle moves backward in a straight line. InFIG. 14, the driver has moved the speed control126to request full forward speed, the follower136is at the front end of the slot134and the transmissions122L, R are set to identical forward ratios. The vehicle thus moves straight ahead.

In both ofFIGS. 15 and 16, the steering control has been set to require a right turn and the guide plate128has been correspondingly rotated (clockwise, as viewed). Due to the lateral displacement of the follower which results from the inclination of the guide slot134, the lever has pivoted about its fulcrum158causing the ratio-control levers144L, R to adopt different positions. It will be apparent that whether the speed control is set for forward (FIG. 15) or reverse (FIG. 16) the required right turn results.

FIGS. 17 and 18show that, with full lock on the steering control, the vehicle can be made to spin on the spot in either direction, depending on the setting of the speed control126.

FIGS. 19 to 21illustrate one possible construction of this type of arrangement. A mounting plate162has a fixed position in the vehicle, and receives a stub axle164formed on the upper surface of the guide plate128to pivotally mount the guide plate. The follower is formed as a stub136on the upper face of the “T” lever130running in a downwardly open slot134in the guide plate. The Bowden cable arrangement used to move the follower along the slot is omitted from these drawings. The fulcrum158is formed as a flanged spigot running in a through-going longitudinal slot in the mounting plate162forming the guideway160. The fulcrum/spigot158is screwed to the upper face of the lever130. Parallel tines164L, R on the left and right hand limbs of the lever130engage the ratio-control levers144L, R of the transmissions122L, R.

It was mentioned above that there are alternative mechanisms for controlling the position of the follower along its guide path.FIGS. 22 to 25illustrate one such alternative mechanism. Compared with the Bowden cable arrangement described above, this has the advantage of providing a positive mechanical connection between the controls and the follower. It uses movable racks to define the guide path and a pinion to form the follower, as will now be explained.

Gear wheel200is externally toothed to engage with a mechanism (omitted from the drawings for simplicity) leading to the driver's steering control. This mechanism uses a toothed rack or gear (not shown) movable by means of the steering control. Movement of the steering control by the driver thus rotates the gear wheel200. Master and slave toothed racks202,204are coupled to the gear wheel200such that they turn along with it, but are capable of moving longitudinally relative to it. In the illustrated embodiments, this mounting is achieved through lugs206,208projecting from the gear wheel200and received as a sliding fit in longitudinal slots210,212of the respective racks202,204. A speed control rack214is connected to, and movable a long its longitudinal direction by, the driver's speed control, and meshes with a speed control pinion216. Both the gear wheel200and the speed control pinion216are journalled on an axle217of a mounting pinion218. The axle217is fixed in a mounting plate219such that mounting pinion218is likewise fixed. The gear wheel200has a domed upper region into which the speed control pinion216projects, the dome being cut away to enable meshing of the speed control pinion216with the speed control rack214. The mounting pinion218meshes with the slave rack204but runs in an un-toothed longitudinal recess220in the master rack202, so that it does not restrict longitudinal motion of the master rack. The speed control pinion216meshes with the master rack202, so that displacement of the speed control rack214produces a corresponding displacement of the master rack202.

Follower pinion224meshes with lower regions of both master and slave racks202,204. It is rotatably mounted on a stub axle225carried by a “T” shaped lever130of the type already familiar fromFIGS. 11-21. The lever is, as before, provided with a fulcrum in the form of a spigot158movable along a guideway formed as a slot160in the mounting plate219, and its left and right limbs are coupled to the control levers144L, R of the transmissions122L, R. Note that although the follower pinion224is shown to be co-axial with the mounting pinion218etc. in some of the drawings, it is able to move away from this position in response to input from the driver's speed control.

Hence the longitudinal position of the master rack202is controlled by the speed control pinion216. The longitudinal position0f the slave rack204is controlled by the mounting pinion218. Moving the master rack202changes the radial position of the follower224—i.e, its distance from the axis about which the racks turn (which is the axis defined by the axle217). However, turning the racks causes one of the racks202,204to advance while the other retreats an identical distance, so that the radial position of the follower is unchanged. Hence the operation of this arrangement is analogous to that of the embodiment illustrated inFIGS. 11-21. The racks together form a guide path which is rotatable, about the fixed axis defined by the axle217, by means of the steering control. The radial position of the follower224(i.e, the distance of its centre from the fixed axis) is unchanged by rotation of the guide path and depends only on the position of the speed control rack214. This will now be illustrated by reference toFIG. 25.

FIG. 25ashows the configuration when the speed control is at zero and the steering control at “straight ahead”. The axis of the follower pinion lies on the fixed axis217, and correspondingly the lever130(omitted fromFIG. 25for the sake of representational simplicity) is positioned to place both transmissions in geared neutral.

FIG. 25bshows the configuration where the steering control remains at zero (the orientation of the master and slave racks202,204is the same as in the previous drawing) but the speed control rack214(not seen in these drawings) has been advanced, and this motion has been transmitted through the speed control pinion216to the master rack202. Consequently the follower pinion224has been displaced forwardly (in a direction from right to left, in the drawings) from the fixed axis217. As in previous embodiments, the effect of this forward displacement is to set the two transmissions to identical forward ratios, causing the vehicle to move in a straight line.

If the speed control setting ofFIG. 25bis maintained, but the driver moves the steering control to request a right turn, the configuration ofFIG. 25cis reached. The master and slave racks202,204have turned clockwise through ninety degrees. In the process, both master and slave racks have rotated around their pinions—the fixed pinion controlling the slave rack204and the speed control pinion216controlling the master rack202—causing them to move equally and in opposite directions. Consequently the radial displacement of the follower pinion224from the fixed axis217is unchanged. The follower pinion is now displaced laterally relative to the vehicle (upwardly, as viewed in the drawing) to produce a right turn.

Still maintaining the same speed control setting, but moving the steering control to request a left turn, results in the configuration ofFIG. 25d. Compared toFIG. 25b, the racks have turned through ninety degrees anticlockwise. Again the radial displacement of the follower pinion224is unchanged.

FIG. 25eshows the configuration when the steering control is set to zero but the speed control rack is withdrawn to move the follower pinion224rearward relative to the vehicle (to the right in the drawing), setting both transmissions to identical reverse ratios and causing the vehicle to reverse in a straight line.

It will be apparent that in all of the above described embodiments, the speed control determines the radial distance of the follower or followers36L,36R,136from the axis about which the guide path34L,34R,134rotates. The displacement of the follower produced by moving the steering control is a function of this radial distance. Rotating the guide path causes the ratio of one transmission relative to the other to change, whereas moving the follower along the guide path changes both ratios in the same sense.

FIG. 26illustrates an arrangement which is largely functionally equivalent to that ofFIGS. 22-25but is more convenient in terms of assembly. The arrangement once more has a master rack302and a slave rack304but in this embodiment the racks are received and mounted by a two part housing350,352. The housing and the racks are able to rotate around axis354. Mounting pinion318is spatially fixed through an integral boss356which is splined into mounting plate319. Housing part350has an integral collar358through which the housing is rotatably mounted upon the aforementioned boss356. Running through an axial bore in the mounting pinion318is an integral shaft360of a speed control pinion316, the shaft being splined into an upper gear362through which speed control is exercised. The upper gear362is coupled to the driver's speed control through an arrangement (not shown) using either a chain or a further toothed rack. Rotation of the housing350,352and of the racks it mounts is controlled through a steering gear364which is carried upon the housing and coupled to the driver's steering control through an arrangement (not shown) using either a chain or a further toothed rack. A follower pinion324receives in an axial bore a stub axle325through which is mounted upon and serves to move a “T” shaped lever330coupled to the transmissions in the manner hereinbefore described with reference toFIGS. 21-24. The follower pinion324meshes with both master and slave racks302,304. Speed control pinion316meshes only with the master rack302, so that moving this pinion, by means of the speed control, moves the follower pinion324radially. Fixed mounting pinion318meshes only with the slave rack304to ensure that when the housing rotates, the slave rack retreats to compensate for the advance of the master rack, so that rotation of the housing does not in itself change the radial position of the follower pinion324.

Assembly of this arrangement involves placing all of the relevant parts in housing part350, then adding housing part352to keep them in place. Note that although it is not apparent from the drawing, the housing350,352forms an elongate enclosure containing the full length of the racks and leaving them room to move longitudinally. Stub axle325and a projecting hub364surrounding it project through an elongate slot in the housing part352to give them freedom to move longitudinally. Seals including “O” ring seals366,368retain lubricant in the housing350,352. Mounting the housing assembly on the mounting plate319is achieved by inserting the shaft360through its hole in the mounting plate and securing the upper gear362in place upon the shaft360to resist its subsequent withdrawal.

FIG. 27illustrates a variant of the lever arrangement ofFIGS. 11-21. Components are given the same reference numerals inFIG. 27as in the earlier drawings. InFIGS. 11-21the movable fulcrum158lies on a line joining the ends of the lateral limbs156L, R of the lever130—i.e. it lies at the junction of the “T” shape of the lever. However inFIG. 27the fulcrum158is positioned away from this line, on the far side of it from the follower136. The effect of the change is to modify the relationship between follower position and transmission ratios, and such adjustments to the geometry allow a desired steering characteristic to be achieved.

Steering may be provided solely through the transmissions and the adjustment they provide of the relative speeds of the driven vehicle wheels. In this case other wheels may be arranged to steer themselves in the manner of castors, to follow the radius of turn dictated by the driven wheels. However it is a common practice to provide the vehicle with conventional steerable wheels coupled to the steering control, so that the driven wheels and the steerable wheels work in unison to cause the vehicle to turn. In this case the steering characteristics (steering control position vs vehicle turn radius) of (a) the transmission arrangement and (b) the arrangement controlling the steerable wheels (typically of the type having the well known Ackermann geometry) must be matched if wheel slip is to be avoided. This can in principle be achieved through modification of either arrangement.

FIGS. 28 and 29show a version of the transmission arrangement designed to match the characteristics of an Ackerman type steering gear. The mechanism seen at400is of the same general type seen inFIG. 26, and serves to control the position of a “T” shaped lever402which is the equivalent of the lever130seen inFIGS. 19-24. Note that in this embodiment the outer ends of this lever couple to the ratio control levers of the variators (which are not seen in this drawing) through spherical heads403received in complementarily shaped slots404, which is a slight modification of the version described earlier. However the major difference of the present arrangement concerns an arrangement of gears406,408through which the mechanism400is coupled to the driver's steering control. The gear wheel406serves the same purpose as gear wheel200seen inFIGS. 22 to 25: it serves to rotate the mechanism400and so, by turning the lever402, to provide the required steering effect. The driver is able to turn the gear wheel406by means of the steering control (not seen in this drawing), which is operatively coupled to steering gear408which in its turn meshes with the gear wheel406. Gear wheel406and steering gear408are non-circular, and their shapes are chosen to provide the required relationship between the position of the driver's steering control and the ratios provided by the two transmissions. The determination of the shapes required for the two gears is a straightforward numerical exercise based upon the characteristic (steering control position vs vehicle turn radius) of the Ackermann steering device and the characteristic (ratio control lever position vs ratio) of the transmissions. In the present embodiment this yields a shape for the gear wheel206which has three curved sides, as seen. The gears are shaped to remain in mesh at all times, so that the shape of one determines the shape of the other.

It is to be understood that the above described embodiments are presented by way of example rather than limitation and numerous possible variations will present themselves to the skilled person. For example, the invention is not necessarily limited to toroidal-race, rolling-traction type transmissions, but could instead be implemented using other types of transmission to vary wheel speed. Hydrostatic or mechanical transmissions would be suitable. The geometry of the control mechanism may be altered to match functional or packaging requirements. For example, the control lever130ofFIG. 22is “T” shaped, but in practice a cruciform shape could be chosen, so that the pinion158would lie beyond the line forming the ends164L,164R of the cross bar of the lever.