Driving and Steering of Motor Vehicles

The present invention is concerned with arrangements for driving and steering motor vehicles, of the type m which vehicle wheels on the left and right hand sides of the vehicle are driven at different speeds m 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 m this way (note m 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" m question are pinions on which the tracks are carried) but so too are various types of vehicle used m construction, agriculture and horticulture, including for example πde-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 mside of the turn may be stationary, or may even be driven m 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 positπely controlled (e.g. through a conventional rack and pmion 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 πde- on lawnmowers, continuously variable transmissions of hydrostatic type have been used m 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 m Figure 1 comprises left and right hand control levers 2L, 2R movable by the dπ\er, controlling respective hydrostatic type CVTs 8L, 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 (engme-side) speed, so that at geared neutral the drive ratio is zero. A difference in the position of the levers 2L, 2R produces a turn Thus in

Figure Ia left hand lever 2L has been advanced further than right hand lever 2R. The left hand wheel is thus driven faster than the right, and a right turn results. Steerable front wheels 4L, 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 varymg-device ("vaπator") m conjunction with an epicyclic shunt gear to provide a geared neutral facility are described m 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 m Figure 1 a, are s ubsequently drawn b ackwards through i dentical distances by the driver, as represented m Figure Ib. The right-hand wheel is now driven faster, m reverse, than the left. The vehicle changes from going forward and turning right (Figure Ia) to reversing and turning left (Figure Ib) even though the displacement of one lever relative to the other has not changed. This is counter-mtuitive to some drivers.

In accordance w ith a first aspect of the present im ention. there is an arrangement for driving and steering a motoπsed v ehicle , 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, m 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 dπve arrangement(s) , through which the follower's position influences vehicle wheel speed ,

whereby movement of the speed control m a first direction causes an increase m the speed of both the left and the right vehicle wheels, movement of the speed control m a second direction opposite to the first causes a decrease m 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 dualelectπc motor(s) or an external combustion engine. The terms "right hand drive arrangement" and "left hand dπve arrangement" refer to the fact that the arrangements m 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 m a motor vehicle having

a right hand dπve 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 dπve 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 m 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 m 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 m 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 m 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 \ ehicle wheel at a drπe ratio w hich i s continuously \ arable 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 dπ\e from the engine to the respective driven \ elude wheels at respective, separately and continuously variable ratios, and a driver actuable control arrangement enabling the drner to control both ratios and thereby to control turning of the \ehicle, the vehicle haung at least one steerable wheel, and the or each steerable wheel(s) being mounted m the manner of a castor.

Specific e mbodiments o f t he p resent i nvention w ill now b e d escribed, b y w ay o f e xample only, with reference to the accompanying drawings, in which: -

Figures Ia-Ib illustrate the operation of a known steering arrangement;

Figures 2-8 are highly schematic representations of a first steering and drive arrangement m accordance with the present invention:

Figures 9 and 1 0 show, m p erspective and from above and below respectively, a physical construction of the first arrangement;

Figures 11 and 13-18 are highly schematic representations of a second steering and drive arrangement in accordance with the present invention;

Figure 12 is a schematic representation of a guide plate used m this arrangement;

Figures 19 and 20 show, in perspective and from above and belo% respectively, a physical construction of the second arrangement;

Figure 21 is an exploded view of this construction;

Figure 22 is an exploded view of an alternative physical construction of the second arrangement,

Figures 23 and 24 show the same construction in perspective, from below and above respectively;

Figures 25 a-e represent, m highly schematic form, various configurations which this construction can adopt m use; and

Figure 26 is a section through components of a third steering and drive arrangement embodying the present invention;

Figure 27 i s a s chematic r epresentation o f y et a further s teermg a nd d rive a rrangement i n accordance with the present invention;

Figures 28 and 29 are respectively plan and side views of parts of still a further steering and drive arrangement embodying the present invention.

Figures 2-8 are schematic views, from above, of selected parts of a vehicle embodying the present invention. Left and right hand driven vehicle wheels are seen at 2OL and 2OR. Each is driven through a respective transmission 22L and 22R 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 m schematic form m Figure 2: a steering control 24, which may take the form of a conventional steering wheel, and a speed control 26, which may be formed as a foot pedal. The pedal is somewhat different from the accelerator control of a motor car, m 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 m which it causes both transmissions 22L, 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 controls 24, 26 determine the ratios adopted by the transmissions 22L, 22R, acting through a mechanism comprising left and right-hand guide plates 28L, 28R and left and right hand actuating levers 3OL, 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 plates 28L, 28R are actually placed one above the other, and are mounted for rotation about a common axis 32. Hence m Figure 2 only right-hand (upper) guide plate 28R is seen. However in Figures 3 to 8, for the sake of clarity, the two guide p lates are shown s ide-by-side, which allows both to be seen. The guide p lates e ach define a respective path for guiding a follower. In the illustrated embodiment, the path is simply a straight slot 34L, 34R m the guide plate and the follower 36L, 36R is formed as a pm πdmg m the slot. Each follower is earned on a respective one of the actuating levers 30L, 3OR, and each lever is mounted for rotation about a fixed fulcrum 38. The actuating are, m the present embodiment, "L" shaped, the follower being carried upon one limb while the other limb engages with the corresponding transmission 22L or 22R to set its ratio Lateral movement of the follower 36L or 36R causes lever 30L or 30R to rotate and produces a change m ratio of the relevant transmission 22L or 22R. Consequently the ratios provided by

the transmissions 22L, 22R are determined by the lateral positions of the respective followers 36L, 36R.

The guide plates can be moved together forward and backward by means of the driver's speed control 26. The fore-and-aft displacement o f the two guide p lates is a lways i dentical. The guide plates can also be rotated by means of the driver's steering control 24. 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.

Figure 3 shows a condition m which the steering control 24 is 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 paths 34L and 34 R inclined to the fore-and-aft direction by equal but opposite angles, which m the present embodiment are approximately 45 degrees. The speed control is set to zero indicating a demand for zero wheel movement. This speed c ontrol s etting c auses the guide plates 28L, R to adopt a fore-and-aft position such that the followers 36L, R he upon the rotational axes 32 of the guide plates 28L, R. This corresponds to positions of the actuating levers 3OL, R m which they cause the transmissions 22L, 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 m accordance with the expectation of the driver, who is used to controlling speed with one control and steering with another

Figure 4 shows the state of the system when the steering control 24 remains in die "straight ahead" p osition, b ut the speed control has been a dvanced by the dmer, to the l imit o f its tra\el. to demand maximum forward vehicle speed The two guide plates 28L, R have been

correspondingly advanced which, due to the inclination of their guide paths, has displaced both followers laterally Correspondingly the actuating levers 30L, R have been rotated, causing the transmissions 22L, R to adopt identical forward drive ratios, driving the vehicle forward m a straight line

Figure 5 shows 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 plates 28L, R causes the right hand guide plate 28R to rotate (in an anti-clockwise direction) but leaves the left hand guide plate m its default orientation. The inclination of the path 34R m the right hand guide plate is reduced Correspondingly the lateral displacement of its follower 36R, and the drive ratio from the associated transmission 22R, 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 control 26 In Figure 6, the speed control has been moved by the driver to place the vehicle m reverse, while maintaining the setting of the steeπng contiol 24 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 in Figure 7, causes the guide path 34 R 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 m 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 s teeπng c ontrol i n t his p osition, m ovmg t he s peed c ontrol from forward t o reverse - Figure 8 - changes the direction m which the vehicle spins

The drawings all show the steering control 24 set 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 plate 28L to rotate (in a clockwise direction, as viewed) to slow down the left hand wheel, leaving the right hand guide plate m its default orientation.

The effect of this arrangement, as will be apparent, is that the positions of the levers 3OL, 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 plate 28. 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 stops 40, 41 m front of and behind the guide plates. Abutment of either guide plate 28L, R against one of the fixed stops limits their fore-and-aft travel, as seen for example m Figure 7. The guide plate's shape is selected to provide a suitable relationship between their rotational position and their maximum travel. Thus m the illustrated example, the guide plates 28 are elliptical. Their travel is maximised when both are m the "straight ahead" position (Figure 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 t he s teermg c ontrol 26, t he s peed s et b v c ontrol 26 m ust b e r educed - as t he d π\ er moves the wheel, the pedal will rise.

The vehicle's front vehicle wheels 42L and 42R could be m the form of castors, being free simply to follow the steering angle dictated by the speeds of the driven wheels, but m the present embodiment are controlled (e.g. through a rack-and-pmion gear, as in conventional steering arrangements) by driver's steering control 24.

Figures 9 and 10 illustrate an actual mechanical embodiment of the control system schematically represented m Figures 2 to 8. The left and right hand transmissions are once more seen at 22L and 22R. They each have a projecting ratio-control lever 44L, 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 lever 30L or 30R, so that the ratio- setting lever's position is determined by that of the actuating lever. The fixed fulcrum about which the actuating levers 3OL, R rotate is formed as a pm 38 received m through-going bores in b oth 1 evers. O ther c omponents o f the a rrangement h ave already b een described a nd a re given the same reference numerals here as m 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 dπver expects the steering control, when released, to seek i ts " straight ahead" p osition. A study o f the drawings will confirm that, if the ratio-control lever 44L, 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 to Figures 1 1 to 18 The arrangement is similar to that previously described m that continuously variable transmissions

122L, R are used to drive respective vehicle wheels 120L, 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 plate 128 which is rotatable by means of the steeπng control about a fixed axis indicated by the i ntersection of dotted l ines i n the drawings. A follower 136 is received m a guide path formed as a straight slot 134 m the guide plate 128, being thus constrained to m ove only back and forth along the slot, and this m ovement is controlled by the speed control. In the present embodiment, the movement is controlled by an opposed pair of Bowden type control cables 1 eading to the speed c ontrol (see Figure 12). Outer sheathes 150, 152 of the cables are led into bores m opposite end faces of the guide plate 128 and inner cables 151,153 are each coupled to opposite sides of the follower, thus acting in a "pull/pull" manner. Other types of control cable, a ble to push a s w ell a s pull, would make it possible to use a single cable, but there are m any event numerous other mechanisms which could be used to couple the follower 136 to the speed control 126, one of which will be described below.

The follower 136 is mounted upon a fore-and-aft extending limb 154 of a "T" shaped lever 130 having left and right limbs 156L, R which are operatively coupled to the respective transmissions 122L, 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 at 144L, R and are coupled to the lever's limbs 156L, R. The lever 130 pivots about a fulcrum 158, but this is not fixed. Instead it is able to move along the fore-and-aft direction in a fixed guideway 160. The arrangement is such that the follower's position dictates the positions of the ratio- control le\ers 144L, R. Moving the follower forward increase both ratios. Mσvmg it backward decreases b oth ratios L ateral movement o f the follower increases one ratio and decreases the other.

The operation of this embodiment will now be described Figure 11 shows its configuration when the speed control is set to zero, causing the follower 136 to be positioned on the axis of rotation of the guide plate 128, and the steering control is in the "straight ahead" position, so that the slot 134 is aligned along the foie-and-aft direction. The ratio-control levers are both at their "geared neutral" positions so the vehicle is stationary. Because the follower 136 is on the axis of rotation of the guide plate 128, any rotation of the steering control/guide plate 128 does 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.

Figures 13 and 14 both show configurations m which the steering control 124 is set for "straight ahead", hi Figure 13 the speed control has been moved to request maximum reverse speed, the follower 136 being correspondingly moved to the rear end of the slot 134. Consequently both ratio-control levers are displaced rearwardly by equal amounts, setting both transmissions 122L, R to the same reverse ratio. The vehicle moves backward m a straight line. In Figure 14, the driver has moved the speed control 126 to request full forward speed, the follower 136 is at the front end of the slot 134 and the transmissions 122L, R are set to identical forward ratios. The vehicle thus moves straight ahead.

In both of Figures 15 and 16, the steering control has been set to require a right turn and the guide plate 128 has been correspondingly rotated (clockwise, as viewed). Due to the lateral displacement of the follower which results from the inclination of the guide slot 134. the lever has pπoted about its fulcrum 158 causing the ratio-control levers 144L, R to adopt different positions. It will be apparent that whether the speed control is set for forward (Figure 15) or re\ erse (Figure 16) the required right turn results

Figures 17 and 18 show 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 control 126.

Figures 19 to 21 illustrate one possible construction of this type of arrangement. A mounting plate 162 has a fixed position m the vehicle, and receives a stub axle 164 formed on the upper surface of the guide plate 128 to pivotally mount the guide plate. The follower is formed as a stub 136 on the upper face of the "T" lever 130 running m a downwardly open slot 134 m the guide plate. The Bowden cable arrangement used to move the follower along the slot is omitted from these drawings. The fulcrum 158 is formed as a flanged spigot running in a through-going 1 ongitudmal s lot in the mounting p late 1 62 forming the guideway 1 60. The fulcrum/spigot 158 is screwed to the upper face of the lever 130. Parallel tines 164L, R on the left and right hand limbs of the lever 130 engage the ratio-control levers 144L, R of the transmissions 122L, R.

It was mentioned above that there are alternative mechanisms for controlling the position of the follower along its guide path. Figures 22 to 25 illustrate one such alternative mechanism. Compared w ith t he B owden c able a rrangement d escribed a bove, t his has t he a dvantage o f providing a p ositive m echanical c onnection b etween t he c ontrols a nd t he follower. 11 u ses movable racks to define the guide path and a pinion to form the follower, as will now be explained.

Gear wheel 200 is 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 wheel 200. Master and slave toothed racks 202. 204 are coupled to the gear wheel 200 such that they turn along w ith it, but are capable of moving

longitudinally relative to it. In the illustrated embodiments, this mounting is achieved through lugs 206,208 projecting from the gear wheel 200 and received as a sliding fit in longitudinal slots 210, 212 of the respective racks 202,204. A speed control rack 214 is connected to, and movable a long its longitudinal direction by, the driver's speed control, and meshes w ith a speed control pmion 216. Both the gear wheel 200 and the speed control pinion 216 are journalled on an axle 217 of a mounting pmion 218. The axle 217 is fixed in a mounting plate 219 such that mounting pmion 218 is likewise fixed. The gear wheel 200 has a domed upper region mto which the speed control pmion 216 projects, the dome being cut away to enable meshing of the speed control pmion 216 with the speed control rack 214. The mounting pmion 218 meshes with the slave rack 204 but runs in an un-toothed longitudinal recess 220 in the master rack 202, so that it does not restrict longitudinal motion of the master rack. The speed control pmion 216 meshes with the master rack 202, so that displacement of the speed control rack 214 produces a corresponding displacement of the master rack 202.

Follower pmion 224 meshes with lower regions of both master and slave racks 202, 204. It is rotatably mounted on a stub axle 225 carried by a "T" shaped lever 130 of the type already familiar from Figures 11 - 21. The lever is, as before, provided with a fulcrum in the form of a spigot 158 movable along a guideway formed as a slot 160 m the mounting plate 219, and its left and right limbs are coupled to the control levers 144L, R of the transmissions 122L, R. Note that although the follower pmion 224 is shown to be co-axial with the mounting pinion 218 etc. in some of the drawings, it is able to move away from this position m response to input from the driver's speed control.

Hence the longitudinal position of the master rack 202 is controlled bv the speed control pmion 216 T he 1 ongitudmal p osition o f the slave rack 204 i s controlled by the mounting pinion 218. Moving the master rack 202 changes the radial position of the follower 214 - i.e. its distance from the axis about which the racks turn (which is the axis defined by the axle

217). However, turning the racks causes one of the racks 202, 204 to 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 m Figures 11-21. The racks together form a guide path which is rotatable, about the fixed axis defined by the axle 217, by means of the steering control. The radial position of the follower 224 (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 rack 214. This will now be illustrated by reference to Figure 25.

Figure 25a shows the configuration when the speed control is at zero and the steering control at "straight ahead". The axis of the follower pmion lies on the fixed axis 217, and correspondingly the lever 130 (omitted from Figure 25 for the sake of representational simplicity) is positioned to place both transmissions m geared neutral.

Figure 25b shows the configuration where the steering control remains at zero (the orientation of the master and slave racks 202, 204 is the same as m the previous drawing) but the speed control rack 214 (not seen m these drawings) has been advanced, and this motion has been transmitted through the speed control pmion 216 to the master rack 202. Consequently the follower pmion 224 has b een d isplaced forwardly ( m a d irection from r ight t o 1 eft, i n the drawings) from the fixed axis 217. As m previous embodiments, the effect of this forward displacement is to set the two transmissions to identical forward ratios, causing the vehicle to move m a straight line.

If the speed control setting of Figure 25b is maintained, but the drner the steering control to request a right turn, the configuration of Figure 25c is reached. The master and slave racks 202,204 have turned clockwise through mnetv degrees In the process, both

master and slave racks have rotated around their pinions - the fixed pinion controlling the slave rack 204 and the speed control pinion 216 controlling the master rack 202 - causing them to move equally and m opposite directions. Consequently the radial displacement of the follower pinion 224 from the fixed axis 217 is unchanged. The follower pinion is now displaced laterally relative to the vehicle (upwardly, as viewed m 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 m the configuration of Figure 25d. Compared to Figure 25b, the racks have turned through ninety degrees anticlockwise. Agam the radial displacement of the follower pinion 224 is unchanged.

Figure 25e shows the configuration when the steering control i s set to zero but the speed control rack is withdrawn to move the follower pinion 224 rearward relative to the vehicle (to the right in the drawing), setting both transmissions to identical reverse ratios and causing the vehicle to reverse m a straight line.

It will be apparent that m all of the above described embodiments, the speed control determines the radial distance of the follower or followers 36L, 36R, 136 from the axis about which the guide path 34L, 34 R, 134 rotates. 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

Figure 26 illustrates an arrangement which is largely functionally equivalent to that of Figures 22-25 but is more convenient m terms of assembly The arrangement once more has a master

rack 302 and a slave rack 304 but in this embodiment the racks are received and mounted by a two p art h ousmg 3 50, 3 52. The housing and the racks are able to rotate around axis 354. Mounting p imon 3 18 i s s patially fixed through a n i ntegral b oss 3 56 w hich i s s plmed i nto mounting plate 319. Housing part 350 has an integral collar 358 through which the housing is rotatably mounted upon the aforementioned boss 356. Running through an axial bore m the mounting pmion 318 is an integral shaft 360 of a speed control pinion 316, the shaft being sphned into an upper gear 362 through which speed control is exercised. The upper gear 362 is coupled to the driver's speed control through an arrangement (not shown) using either a chain or a further toothed rack. Rotation of the housing 350, 352 and of the racks it mounts is controlled through a steering gear 364 which 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 pmion 324 receives in an axial bore a stub axle 325 through which is mounted upon and serves to move a "T" shaped lever 330 coupled to the transmissions in the manner hereinbefore described with reference to Figures 21-24. The follower pmion 324 meshes with both master and slave racks 302, 304. Speed control pmion 316 meshes only with the master rack 302, so that moving this pmion, by means of the speed control, moves the follower pmion 324 radially. Fixed mounting pinion 318 meshes only with the slave rack 3041 o e nsure t hat w hen the h ousmg r otates, t he s lave r ack r etreats t o compensate for t he advance of the master rack, so that rotation of the housing does not m itself change the radial position of the follower pmion 324.

Assembly of this arrangement involves placing all of the relevant parts m housing part 350, then adding housing part 352 to keep them m place. Note that although it is not apparent from the drawing, the housing 350, 352 forms an elongate enclosure containing the full length of the racks and lea\mg them room to mo\e longitudinally Stub axle 325 and a projecting hub 364 surrounding it project through an elongate slot m the housing part 352 to give them freedom to move longitudinally. Seals including 'O" rmg seals 366. 368 retain lubricant in

the housing 350, 352. Mounting the housing assembly on the mounting plate 319 is achieved by inserting the shaft 360 through its hole in the mounting plate and securing the upper gear 362 m place upon the shaft 360 to resist its subsequent withdrawal.

Figure 27 i llustrates a v ariant o f the 1 ever a rrangement o f F igures 1 1-21. C omponents a re given the same reference numerals m Figure 27 as in the earlier drawings. In Figures 11-21 the movable fulcrum 158 lies on a line joining the ends of the lateral limbs 156L, R of the lever 130 - i.e. it lies at the junction of the "T" shape of the lever. However m Figure 27 the fulcrum 158 is positioned away from this line, on the far side of it from the follower 136. 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 rum dictated by the driven wheels. However it is a common practice to provide the vehicle with conventional steerable wheels c oupled to the steering c ontrol, so that the driven wheels and the steerable wheels work m 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 c ontrolhng t he steerable w heels (typically o f 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.

Figures 28 a nd 29 s how a v ersion o f t he t ransrmssion a rrangement d esigned t o match t he characteristics of an Ackerman type steering gear The mechanism seen at 400 is of the same

general type seen m Figure 26, and serves to control the position of a "T" shaped lever 402 which is the equivalent of the lever 130 seen m Figures 19-24. Note that m this embodiment the outer ends of this lever couple to the ratio control levers of the vaπators (-which are not seen in this drawing) through spherical heads 403 received m complementarity shaped slots 404, which is a slight modification of the version described earlier. However the major difference of the present arrangement concerns an arrangement of gears 406, 408 through which t he m echamsm 400 i s c oupled to t he d river ' s s teermg c ontrol. The g ear w heel 406 serves the same purpose as gear wheel 200 seen in Figures 22 to 25: it serves to rotate the mechanism 400 and so, by turning the lever 402, to provide the required steering effect. The driver is able to turn the gear wheel 406 by means of the steering control (not seen m this drawing), which is operatively coupled to steering gear 408 which m its turn meshes with the gear wheel 406. Gear wheel 406 and steeπng gear 408 are non-circular, and their shapes are chosen to provide the required relationship between the position of the driver's steering control and the ratios p rovided b y the two transmissions. The determination of the shapes required for the two gears is a straightforward numerical exercise based upon the characteristic (steeπng 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 wheel 206 which has three curved sides, as seen. The gears are shaped to remain m mesh at all times, so that the shape of one determines the shape of the other.

It is to be understood that the 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, b ut c ould i nstead b e i mplemented u smg o ther types o f transmission to vary wheel speed Hydrostatic or mechanical transmissions would be suitable The geometr) of the control mechanism may be altered to match functional or packaging requirements. For

example, the control lever 130 of Figure 22 is "T" shaped, but m practice a cruciform shape could be chosen, so that the pinion 158 would he beyond the line forming the ends 164L, 164R of the cross bar of the lever.