GB2236820A - Load responsive brake control - Google Patents

Abstract

A vehicle braking system includes a brake actuator (10) with integral valve means (30, 35) which, at a predetermined fluid pressure in the brake system, will vary the rate at which fluid pressure is applied to the brake actuator (10), said valve moans (30, 35) including a resilient loading means (45) set to the predetermined fluid pressure at which the rate of fluid pressure to the brake actuator (10) is varied, means (56) being provided to vary the load applied by said resilient loading means (45) in proportion to the load applied to the wheel (58) with which the brake actuator (10) is associated and means (60) for setting the resilient loading means (45) to apply a predetermined load at a standard wheel loading. <IMAGE>

Description

VEHICLE BRAKING SYSTEMS The present invention relates to vehicle braking systems and in particular vehicle braking systems of the type covered in UK Patent Application Nos. 8829317 and 8915094, in which at least one of the brakes of the vehicle has a hydraulic brake actuator with integral valve means to control the rate at which the braking effort is applied by the brake, the rate of application being varied at a fluid pressure which is proportional to the load applied to the wheel with which the brake is associated.

The present invention provides means for controlling the valve means in response to the load applied to the wheel.

According to one aspect of the present invention a vehicle braking system includes a brake actuator with integral valve means which, at a predetermined fluid pressure in the brake system, will vary the rate at which fluid pressure is applied to the brake actuator, said valve means including a resilient loading means to set the predetermined fluid pressure at which the rate of fluid pressure to the brake actuator is varied, means being provided to vary the load applied by said resilient loading means in proportion to the load applied to the wheel with which the brake actuator is associated and means for setting the resilient loading means to apply a predetermined load at a standard wheel loading.

Preferably such brake actuators are associated with both rear wheels of the vehicle and may be controlled independently in response to the load applied to each wheel or together in response to the mean load applied to the wheels on the common axle.

Various embodiments of the invention are now described, by way of example only, with reference to the accompanying drawings in which: Figure 1 illustrates in section a wheel cylinder assembly for a rear drum brake of a braking system in accordance with the present invention; Figure 2 is a sectional elevation along the line II - II of Figure 1; Figure 3 illustrates in plan view, a mechanism for resiliently loading the plunger of the brake actuator illustrated in Figures 1 and 2; Figure 4 illustrates a modification to the mechanism illustrated in Figure 3; Figure 5 illustrates an alternative modification to the mechanism illustrated in Figure 3;; Figure 6 illustrates in plan view an alternative mechanism for resiliently loading the plunger of the brake actuator illustrated in Figures 1 and 2 Figure 7 is a side elevation of the mechanism illustrated in Figure 6 Figure 8 illustrates in side elevation a further modification to the mechanism for resiliently loading the plunger of the brake actuator illustrated in Figure 1 and 2; Figure 9 illustrates in plan view a lever mechanism for resiliently loading the plunger of the brake actuator illustrated in Figures 1 and 2r Figure 10 illustrates in side elevation the mechanism illustrated in Figure 9; Figure 11 illustrates in partial rear elevation the mechanism illustrated in Figure 9;; Figure 12 illustrates in sectional side elevation a drum brake with cable mechanism for resiliently loading the plunger of the brake actuator illustrated in Figures 1 and 2; Figure 13 is a partial plan view of the drum brake illustrated in Figure 12; Figure 14 illustrates in side elevation a mechanism for operating the cable mechanism illustrated in Figures 12 and 13; Figure 15 is a partial view in the direction of arrow X in Figure 14; Figure 16 illustrates a modification to the mechanism illustrated in Figure 14; Figure 17 is a section along the line Y-Y in Figure 16; Figure 18 illustrates a modification to the mechanism illustrated in Figure 14; Figure 19 illustrates in side elevation a modification to the mechanism illustrated in Figure 8; Figure 20 illustrates in partial side elevation a modification to the mechanism illustrated in Figures 6 and 7; and Figure 21 is a side elevation of the backing plate of the brake associated with the mechanism illustrated in Figure 20.

Figures 1 and 2 illustrate a wheel cylinder assembly 10 for a drum brake the wheel cylinder assembly 10 comprises a body 11 which defines two cylinders 12 and 13. Pistons 14 are slidably sealed within the cylinders 12 and 13 and act against the shoes of the drum brake.

The wheel cylinder body defines a closed valve bore 25 intermediate of the two working chambers 12 and 13. The valve bore 25 has an inlet port 27 intermediate of its extremities and opening into cylinder 12 and an outlet port 28 adjacent the closed end and opening into cylinder 13.

An inlet 20 is provided to the wheel cylinder assembly 10 by means of which it may be connected to the brake system. The inlet 20 opens directly into the cylinder 12.

The bore 25 is stepped intermediate of passages 27 and 28 and a valve seat 30 is slidingly located in the bore 25, so that it will normally seat against a shoulder 31.

A plunger 35 is slidingly located in a bearing 36 which is located in the open end of bore 25 and is retained therein by circlip 37. The plunger 35 is stepped having a main body portion 38 which is slidingly located in the bearing 36, an intermediate reduced diameter portion 39 which extends through a control port 40 defined by the valve seat 30 and an enlarged diameter end portion 31 which is of greater diameter than the control port 40 and the main body portion 38. A head formation 42 is provided at the outer end of plunger 35 and a rubber bellows unit 43 extends between the plunger 35 and the wheel cylinder housing to prevent ingress of dirt.A seal 44 is also provided adjacent the inner end of bearing 36 and a spring 45 acts between a ring 46 abutting the seal 44 and a washer 47 abutting seat 30 to urge the seal 44 into engagement with the bearing 36 and the seat 30 to engagement with the shoulder 31. The ring 46 has an annular formation 48 which engages seal 44 and urges the inner lip formation 49 thereof into sealing engagement with the plunger 35.

As described in greater detail below, a resilient load is applied to the head 42 of plunger 35, this load being varied with the load applied to the wheel with which the drum brake is associated.

When the brake is applied, fluid under pressure from the master cylinder will flow through inlet 20 into cylinder 12 and through passage 27, bore 25, past open control port 40 and through passage 28 into cylinder 13. Fluid pressure in the cylinder 12 and 13 will thus initially increase at the same rate, forcing pistons 14 apart to move the brake shoes into engagement with the brake drum.

As pressure of fluid increases, because of the difference in diameters of portions 38 and 41 of plunger 35, a force will be applied to the plunger 35, urging it against the load applied to the head 42 thereof. When the fluid pressure has reached a predetermined value the load applied by the fluid pressure acting on the plunger 35 will exceed the resilient load applied thereto and the plunger 35 will move to the right, as illustrated in Figure 2, the end portion 41 closing control port 40, so that further increase in fluid pressure to cylinder 13 is interrupted. Continued increase in pressure from the brake system will continue to be applied directly to cylinder 12 and via piston 14 therein to one of the brake shoes. However, because of the area differential of portions 41 and 38 of plunger 35, the pressure to cylinder 13 will increase at a slower rate than the pressure from the brake system, this rate being a function of the difference in cross-sectional areas of portions 38 and 41 of plunger 35.

When the brake is released and the pressure of fluid from the brake system reduces, excess pressure in cylinder 13 will move valve seat 30 against the load applied by spring 45, thus opening the control port 40 so that fluid can flow out of cylinder 13 thus releasing the brake shoe controlled thereby.

Figure 3 illustrates one mechanism for applying a resilient load to the plunger 35. A lever 50 is pivotally mounted on a bracket 51 which is secured to the backing plate 52 of the disc brake. One end 53 of the lever 50 engages the head 42 of plunger 35 and the other end 54 is connected to the vehicle sub-frame 55 by means of a tension spring 56. The tension spring 56 will thus apply a load to the plunger 35 which will vary as the brake drum and trailing arm 57 to which it is attached, move relative to the sub-frame 55.

The mounting point of the spring 56 to the sub-frame 55 is located such that as the load on wheel 58 associated with the brake drum reduces and the wheel 58 moves downwardly relative to sub-frame 55, the length of spring 56 will decrease thus decreasing the load applied to the plunger 35. The spring 56 is connected to one or both of the sub-frame 55 and lever 50 by a straight portion which engages through a hole in the lever 50 and/or a bracket 59 mounted on the sub-frame 55. A collar 60 is then clamped on the straight portion of the spring 56 to provide an adjustable stop, by which extention of the spring 56 may be adjusted to set the resilient load applied to the plunger 35 for a standard wheel loading.

In the modification illustrated in Figure 4, a tension spring 65 is secured at one end 66 to the vehicle sub-frame 55, the other end 67 of spring 65 being connected to lever 50 described above, by means of a flexible cable 68. The cable 68 passes through a cable guide 69 which is slidably located with respect to the vehicle sub-frame 55 on a plate 70, a spigot 71 and pinch bolt 72 engaging through an elongate hole 73 in the plate 70. With this modification, the load applied to the plunger 35 at a standard wheel loading, may be set to the desired predetermined value, by hanging an appropriate setting weight 75 on a plate 70 by means of hole 76 provided thereon, so that the spring 65 is extended to a degree sufficient to balance the setting weight and then tightening the pinch bolt 72 to clamp the plate 70 into position.The flexible cable 68 will then be held in tension by spring 65, so that the appropriate load is applied by lever 50 to the head 42 of plunger 35. This load again will vary with the load applied to the wheel, due to the variation in separation between the end 54 of lever 50 and the point at which end 66 of spring 65 is secured to the sub-frame 55.

In the modification illustrated in Figure 5, the cable guide 69 is mounted on a setting plate 80 which is attached by means of a pivot 82 to bracket 81 which is secured to sub-frame 55. A pinch bolt 83 locates through an arcuate hole 84 in a setting plate 80 and engages a captive bolt secured to the bracket 81. The load applied by this modified mechanism to the plunger 35, may be adjusted using a setting weight 75, in similar manner to that described with reference to Figure 4.

In the mechanism illustrated in Figures 6 and 7, the end 54 of lever 50 is interconnected to a plate 110, which is pivotally mounted with respect to sub-frame 55, by means of pivot 111, by a rod 112. The rod 112 locates through a hole 113 in lever 50 and a hole 114 in a flange portion 115 on plate 110. Semi-spherical washers 116 are located at each end of the rod 112 and bear against remote faces of the lever 50 and the plate 110, the semi-spherical washers 116 being retained on the rod by nuts 117. The holes 113 and 114 are frustoconical so that the rod 112 is able to rock on the semi-spherical washers 116, as the brake drum moves relative to the plate 110.

The plate 110 is connected at a point remote from pivot 111, to a setting plate 120 similar to that described with reference to Figure 5, by means of a tension spring 121.

The setting plate 120 is pivotally mounted on a bracket 122 by means of pivot 123 and a pinch bolt 124 is located through an arcuate hole 125 in the setting plate 120.

Bracket 122 is secured to the trailing arm 57, to the other end of which the brake drum is secured.

The plate 110 is located on the sub-frame 55 such that the rod 112 engages the plate 110 coaxially of the pivotal connection 126 of the trailing arm 57 to the sub-frame 55.

As a result, as the wheel 58 moves upwardly and downwardly relative to the sub-frame 55, the separation between the points at which the rod 112 engages the plate 110 and lever 50 will not vary. However, the movement of the trailing arm 57 will cause the spring 121 to expand and contract thus varying the load applied by plate 110 to rod 112 and thus the load applied to plunger 35. The load applied to plunger 35 at a standard wheel loading may be set by means of setting plate 120 as described above.

In the modification illustrated in Figures 20 and 21 a plate 110' is secured by pivot 111' to a bracket 122' secured to the trailing arm 57. The plate 110' is connected at a point remote from pivot 111' to a bracket 300 secured to the subframe 55, by means of tension spring 121'. The plate 110' is connected to lever 50 mounted on the backing plate 52 of the associated drum brake by means of a Bowden cable 301. The outer case 302 of the Bowden cable 301 is located between a lug 303 on bracket 122' and a lug 304 on bracket 51 which is attached to backing plate 52. Screw adjusting means 305 of conventional design is provided at the end of casing 302 which engages lug 303.

This modified mechanism operates in the same manner as that described above, movement of the trailing arm 57 causing spring 121' to expand or contract and the resulting variation in the load applied by the spring 121' being transmitted to the plunger 35 via plate 110', cable 301 and lever 50.

In order to adjust this mechanism to produce a predetermined load on plunger 35 at a standard axle loading, a setting weight is hung from plate 110' in similar manner to that described above. Slack in the cable 301 is then taken up by the screw adjustment means 305.

In the mechanism illustrated in Figure 8, a lever 130 is pivotally mounted on bracket 131 which is secured on the backing plate 52 of the drum brake. A hook formation 133 on the lever 130 engages through an aperture 134 on the bracket 131 to provide a pivot, while the edge 135 of a cut-away defining the hook formation 133 is arranged to engage a flange 136 on bracket 131 to limit pivotal movement of the lever 130. A compression spring 137 is located between a projection 138 on lever 130 and a cap 139 which engages the plunger 35. A hook formation 140 on the spring 137 engages over projection 138 and a retaining ring 141 maintains the spring 137 in position.

The end 142 of lever 130 remote from the pivot is connected to a plate 291 mounted on the vehicle sub-frame 55 by means of rod 290, which is held under tension so as to partially compress the spring 137 and apply a load to the plunger 35.

The plate 291 is mounted on subframe 55 by means of pivot 293, a pinch bolt 294 engaging through an arcuate hole 295 in plate 291, by means of which it may be clamped in position. The rod 290 is secured at one end to lever 130 by means, for example, of a snap retainer which locates in a rubber bush mounted in the end 142 of lever 130. A suitable formation on the other end of rod 290 engages a hook formation 292 on plate 291. The plate 291 may be adjusted to set a predetermined load on plunger 35 at a standard axle loading by hanging a setting weight W from hole 296 on plate 291 and clamping the plate 291 in the appropriate position by means of pinch bolt 294, lever 130 may alternatively be connected to the subframe 55 by other means of fixed length, for example a cable, with appropriate adjustment means.

In the modification illustrated in Figure 19, the lever 130' is formed from two plates 311 and 312 which are connected to the bracket 131' by common pivot 310. A pinch bolt 313 threadably engages plate 312 and extends through an arcuate hole 314 in plate 311. Plate 311 defines a formation 138' which serves to locate a spring assembly in engagement with plunger 35, as described above. The plate 311 also defines a stop formation 315, a plastic sleeve 316 being mounted on the stop formation 315. A setting screw 317 mounted on flange 136' of bracket 131' engages the plastic sleeve 316.

The plate 312 is connected to the vehicle subframe by a link of fixed length in suitable manner. With this modified embodiment the plunger 35 may be set to a predetermined load at a standard axle loading, by adjusting setting screw 317 to compress spring 137'. Slack in the link between plate 312 and the vehicle subframe may then be taken up by pivotting of plate 312, relative to plate 311, which may then be clamped together by pinch bolt 313.

In the mechanism illustrated in Figures 9, 10 and 11, a lever 150 is mounted intermediate of its ends to a support plate 151 by pivot 152, the support plate 151 being secured to the backing plate 52 of the drum brake. One end of the lever 150 is pivotally secured to the sub-frame 55 adjacent the pivot axis 153 of trailing arm 57, by means of pivot 154. The pivot 154 locates through an elongate hole 155 in lever 150, the lever 150 being slidable axially relative to the pivot 154, in order to accommodate small variations in the separation between pivot 154 and the drum brake as the wheel 58 moves relative to the sub-frame 55.

A second lever 160 is pivotally mounted to the support plate 151 about pivot 161. One end 162 of the lever 160 engages the plunger 35 and the other end of lever 163 is connected to the end of lever 150 remote from pivot 154 by means of a tension spring 164. A hook formation 165 at one end of the spring 164 engages a hole 166 in the end of lever 150 while a straight portion from 167 at the other end of spring 164 extends through a hole 168 in lever 160, a collar 169 being clamped to the straight portion 167 to act as a stop.

The main body of lever 160 is substantially parallel to the plane of the backing plate 52. An arm 170 which extends substantially at right angles to the backing plate 52 is pivotally connected to the main body of lever 160 by pivot 171 and a pinch bolt 172 engages through an arcuate hole 173 in arm 170. Spring 164 is connected to the arm 170, so that the load applied by the spring 164 at a standard wheel loading may be set using a setting weight which may be hung from a hole 174 in the arm 170, using a procedure similar to that described above.

With this mechanism, as the suspension and wheel 58 move upwardly and downwardly relative to sub-frame 55, the end of lever 50 will be pivoted away and towards the lever 160, respectively. The spring 164 will consequently be extended and contracted to vary the load on plunger 35.

Normally wheel cylinders with fluid pressure regulating valves will be provided on both the rear wheels of the vehicle. With the mechanisms described above, independent mechanisms will be provided on each side of the vehicle to control the valves independently in response to movement of the wheels with which they are associated.

Alternatively, the valves associated with each of the rear brakes may be controlled by common mechanism, so that pressure to the two brake actuators will be modulated to the same degree in response to the mean load applied to the wheels on the common axle.

In the mechanism illustrated in Figures 12 to 16, the vehicle sub-frame 55 comprises a pair of longitudinal members 200 which are interconnected by a tubular cross-member 201, to which they are welded. The sub-frame 55 is mounted to the vehicle at the opposite ends of each longitudinal member 200 by rubber mounting blocks.

A trailing arm 57 is pivotally mounted on each of the longitudinal members 200 about a common pivot axis 202. The outer ends of the trailing arms support the wheels 58 and associated drum brake. The trailing arms 57 are interconnected by an anti-roll bar 203 which serves to reduce movement of one trailing arm 57 relative to the other and thus rolling of the vehicle.

As illustrated in Figure 14, a bracket 210 is welded to the tubular cross-member 201 intermediate of longitudinal members 200. A lever 211 is pivotally mounted by pivot 213 to a plate 212, the plate being secured to the bracket 210.

A setting plate 214 is pivotally mounted on lever 211 by pivot 215 and a pinch bolt 216 locates through an arcuate hole 217 in the setting plate 214. One end of a tension spring 220 engages a hole 221 in setting plate 214, the other end of the spring 220 being attached to the anti-roll bar 203 by means of a clamp 222.

A cranked strip 225 is secured to the lever 211 to provide a bifurcated formation, a notch 226 being provided on one edge of strip 225 corresponding to a hole 227 in lever 211, for location of a barrel nipple 228 at one end of a Bowden cable 229. The outer case 230 of the Bowden cable 229 is adjustably located by means of a flange 231 on plate 212, in known manner.

The other end of Bowden cable 229 is connected to a swivel plate 235 mounted on a base plate 236 by pivot 237, the base plate 236 being attached to the bracket 210. A nipple 238 at said other end of the Bowden cable 229 engages a retaining formation 239 on swivel plate 235, the retaining formation 239 being displaced radially of the pivot 237.

The outer case 230 of Bowden cable 229 abuts a lug 240 extending from the base plate 236.

Two further retaining formations 241 and 242 are provided on swivel plate 235 disposed diametrically of the pivot 237, by which nipples 243 of two further Bowden cables 244 and 245 may be secured to the swivel plate 235. Lugs 246 and 247 extending from the base plate 236 are provided for location of the outer cases 248 of cables 244 and 245.

As illustrated in Figures 12 and 13, the ends of each of the cables 244 and 245 remote from the swivel plate 235 are connected to a lever 250 which is pivotally mounted on a bracket 251 secured to the backing plate 52 of the drum brake associated with each of the rear wheels. The lever 250 has a bifurcated hook formation 252 at one end which is engaged by a barrel nipple 253 on the end of the associated Bowden cable 244, 245. The other end 256 of the lever 250 engages the plunger 35. A stop formation 257 on bracket 251 locates the outer casing 248 of the associated Bowden cable 244, 245.

With this assembly, as the trailing arms 57 move upwardly and downwardly under varying loads, the anti-roll bar 203 pivots round with the trailing arms 57, thus causing the spring 220 to be expanded and contracted respectively, thus increasing and decreasing the load on plunger 35 in response to axle loading. The load applied to the plunger 35 at a standard axle loading may be set using the setting plate 214, in the manner described above.

In the modification illustrated in Figures 16 and 17, a lug 260 on lever 211 engages in a slot 261 in the swivel plate 235, said slot 261 being offset radially from the pivot 237. With this arrangement, pivoting of the lever 211 will be transmitted directly to the swivel plate 235 and from there to the levers 250 and plunger 35 by cables 244 and 245.

In the modification illustrated in Figure 18, a lever 270 is pivoted to plate 271 about a pivot 272, the plate 271 being secured to the bracket 210. The lever 270 is connected at one side of the pivot 272 to a clamp 222 on anti-roll bar 203 by means of tension spring 220. A pulley 273 is pivotally mounted to the lever 270 on the other side of the pivot 272. A cable 275 is mounted over the pulley 273, opposite ends of the cable 275 being connected to the levers 250' associated with the drum brakes on the opposite rear wheels of the vehicle.

The cable 275 passes through a pair of lugs 276 which are provided on a plate 277 secured to bracket 210. The lugs 276 serve to locate outer casing portions 278 and 279 which extend between the lugs 276 and the brackets 251' on each of the drum brake backing plates 52. Screw adjustment means 280 is provided on at least one of the outer casing portions 278.

Variation in the load applied by the spring 220 during suspension movement, will be transmitted to the plungers 35 associated with the two drum brakes, by cable 275. The pulley 273 will ensure that the load is applied equally to the plunger 35 associated with each of the brakes. The load applied to the plungers 35 may be set for a standard axle load by hanging a setting weight a; on lever 270 by means of formation 285 and then taking up slack in the cable 275 by the screw adjustment means 280.

Various modifications may be made without departing from the invention. For example, while the above embodiments refer to controlling valve means associated with the wheel cylinder of a drum brake, the invention is equally applicable to fluid modulating valves formed integrally of the caliper of a disc brake. Furthermore, while the pressure modulating valve described above is configured as a proportioning valve, it may be in the form of a pressure limiting valve.

Claims (26)

1. A vehicle braking system including a brake actuator with integral valve means which, at a predetermined fluid pressure in the brake system, will vary the rate at which fluid pressure is applied to the brake actuator, said valve means including a relisient loading means to set the predetermined fluid pressure at which the rate of fluid pressure to the brake actuator is varied, means being provided to vary the load applied by said resilient loading means in proportion to the load applied to the wheel with which the brake actuator is associated and means for setting the resilient loading means to apply a predetermined load at a standard wheel loading.

2. A vehicle braking system according to Claim 1 in which a lever is pivotally mounted in fixed relationship to the plunger, one end of the lever acting upon the plunger and the other end of the lever being interconnected to a fixed point on the vehicle sub-frame, a tension spring being provided between the lever and sub-frame to load the lever into engagement with the plunger, the point on the sub-frame to which the lever is connected being positioned such that the separation between said point and the lever varies as the vehicle suspension moves under variation of the load applied to the wheel, so that as the load increases, the separation will increase and the load applied to the plunger will increase.

3. A vehicle braking system according to Claim 2 in which the spring is connected to the lever via a flexible member, means being provided to deflect the flexible member so that the extension of the spring may be set to apply a predetermined load to the plunger at a standard wheel loading.

4. A vehicle braking system according to Claim 3 in which a cable interconnects the spring to the lever, the cable passing through a cable guide which is mounted on a setting plate, the setting plate being adjustably clamped with respect to the vehicle sub-frame so that deflection of the cable may be adjusted.

5. A vehicle braking system according to Claim 1 in which a lever is pivotally mounted in fixed relationship to the plunger, a compression spring acting between one end of the lever and the plunger, the other end of the lever being connected to a fixed point on the vehicle sub-frame, the separation between said point and the lever varying with the movement of the vehicle suspension, so that as the load applied to the wheel increases, the spring will be compressed to increase the load applied to the plunger.

6. A vehicle braking system according to Claim 5 in which a flexible member connects the lever to the sub-frame, means being provided to deflect the flexible member so that compression of the spring may be set to apply a predetermined load to the plunger at a standard axle load.

7. A vehicle braking system according to Claim 6 in which a cable interconnects the lever to the sub-frame, the cable passing through a cable guide which is mounted on a setting plate, the setting plate being adjustably clamped with respect to the vehicle sub-frame so that deflection of the cable may be adjusted.

8. A vehicle braking system according to Claim 1 in which the brake and associated wheel are mounted on an arm, the arm being pivotally mounted to a sub-frame, a lever is mounted in fixed relationship to the plunger, one end of the lever acting on the plunger and the other end being connected by a fixed length link to a plate pivotally mounted on one of the sub-frame or arm, the link being connected to the plate at a position which is a fixed length from its connection to the lever throughout the range of movement of the arm, and spring means acting between the plate and the other of the subframe or arm to apply a load to the plunger, the spring means being arranged such that as the arm moves relative to the sub-frame the load on the plunger will vary increasing as the load on the wheel increases.

9. A vehicle braking system according to Claim 8 in which the plate is pivotally attached to the subframe and the connection of the link thereto is coincident with the pivot axis of the arm.

10. A vehicle braking system according to Claim 9 in which the lever and plate are interconnected by a rigid rod means being provided to permit tilting of the rod relative to the lever and plate as the vehicle wheel moves relative to the sub-frame.

11. A vehicle braking system according to any one of Claims 8 to 10 in which the spring means is a tension spring.

12. A vehicle braking system according to any one of Claims 8 to 11 in which the spring means is connected to the arm by means of a setting plate adjustably clamped with respect to the arm, so that extension of the spring may be adjusted to provide a predetermined load on the plunger at a standard wheel loading.

13. A vehicle braking system according to Claim 1 in which a first lever is pivotally mounted at one end to the vehicle sub-frame and at a point intermediate of its ends to a mounting bracket which moves with the brake and wheel associated therewith, so that upon movement of the wheel relative to the sub-frame the lever will pivot about its intermediate point, a second lever pivotally mounted in fixed relationship to the brake, one end of the second lever acting upon the plunger and the other end of the second lever being interconnected to the end of the first lever remote from the sub-frame by sring means, such that as the wheel moves relative to the sub-frame the first lever will pivot to vary the load applied to the plunger, the load on the plunger increasing with the load applied to the wheel.

14. A vehicle braking system according to Claim 13 in which the first lever is pivoted to the sub-frame at the centre of movement of the wheel relative to the sub-frame.

15. A vehicle braking system according to Claim 13 in which means is provided to permit axial movement of the first lever relative to its pivot to the sub-frame in order to accommodate variation in the distance between its pivot to the sub-frame and its pivot to the mounting bracket.

16. A vehicle braking system according to any one of Claims 13 to 15 in which the second lever includes an arm adjustably clamped with respect to the main body portion of the lever, by means of which the load applied by the spring means to the plunger may be adjusted to provide a predetermined load at a standard wheel loading.

17. A vehicle braking system in which brake actuators with integral valve means are provided in brakes associated with a pair of wheels mounted on a common axle of the vehicle, means as claimed in any one of Claims 1 to 16 being provided for each of the brake actuators to independently apply a load to the plunger, the load applied to each plunger being varied in response to the load applied to the wheel with which each actuator is associated.

18. A vehicle braking system in which brake actuators with integral valve means are provided in brakes assocated with a pair of wheels mounted on a common axle of the vehicle, a common means being provided in accordance with any one of Claims 1 to 16 to vary the load to the plungers associated with both b.rake actuators, the loads applied to each plunger being proportional to the mean load applied to wheels on the the common axle.

19. A vehicle braking system according to Claim 18 in which a lever is pivotally connected to a sub-frame, spring means acts between the lever at a point remote from its pivot and a component of the vehicle suspension which moves with the wheels of the vehicle relative to the sub-frame, the lever being interconnected with levers associated with each of the brake actuators by cable means, by means of which the load applied by the spring means is transmitted to the plungers.

20. A vehicle braking system according to Claim 19 in which means is provided for equalising the load applied to the plungers of the actuators associated with the two wheels.

21. A vehicle braking system according to Claim 20 in which the levers associated one with each of the plungers of each brake actuator are connected by Bowden cables to a pivoted swivel plate at diametrically opposed points, a third Bowden cable being connected between the swivel plate and the lever pivotally attached to the sub-frame.

22. A vehicle braking system according to Claim 20 in which the levers associated with the plungers of each brake actuator are connected by Bowden cable to the pivoted swivel plate at diametrically opposed points, the lever pivotally attached to the sub-frame being interconnected with the swivel plate for rotation therepf, by means of complementary formations.

23. A vehicle braking system according to any one of Claims 19 to 22 in which the spring means is connected to a setting plate, the setting plate being adjustably clamped with respect to the lever pivoted to the sub-frame, so that extension of the spring means may be adjusted to apply a predetermined load to the plungers at a standard wheel loading.

24. A vehicle braking system according to Claim 18 in which a lever is pivotally connected to a sub-frame, spring means acts between the lever at a point remote from its pivot and a component of the vehicle suspension which moves with the wheels of the vehicle relative to the sub-frame, a pulley being pivotally mounted to the lever and a cable being mounted over the pulley, the opposite ends of the cable being connected to levers associated with each of the brake actuators.

25. A vehicle braking system according to Claim 24 in which the cable is provided with outer casings adjacent each end, each casing extending between a stop adjacent the lever associated with the brake actuator to which that end is connected and a lug mounted in fixed relationship with respect to the sub-frame, adjustment means being provided to adjust the length of the outer casing at one end of the cable so that extension of the spring means may be adjusted to apply a predetermined load to the plungers at a standard wheel loading.

26. A vehicle braking system substantially as described herein with reference to, and as shown in, Figures 1 to 21 of the accompanying drawings.