GB2340568A

GB2340568A - Vehicle load-sensed braking system

Info

Publication number: GB2340568A
Application number: GB9810805A
Authority: GB
Inventors: Glenn Craig Saint; Paul A Wicks
Original assignee: Individual
Current assignee: Individual
Priority date: 1998-05-20
Filing date: 1998-05-20
Publication date: 2000-02-23
Also published as: GB9810805D0

Abstract

A load sensing and brake pressure adjusting system in a vehicle having front and rear axles 3, 4, wheels 7, 8 mounted on said axles, a body or chassis 2 resiliently mounted with respect to at least the front axle, a braking system acting on the front and rear wheels, and means 12 to accept a load substantially rearward of the rear axle, comprises a load sensing reduction valve 14 mounted between the front axle and the body or chassis. The load sensing and brake pressure adjusting system operates to reduce the braking effect on the front wheels relative to that on the rear wheels when the load sensing valve senses a reduced load on the front axle resulting from a rear overhung load and which thereby enables increased braking force to be applied to the vehicle before the point at which the front wheels lock up causing loss of steering control.

Description

2340568 VEHICLE LOAD-SENSED BRAKING SYSTEM The invention relates to

vehicle load-sensed braking systems, particularly, though not exclusively to systems for use in load bearing vehicles of the type known as "underlift" vehicles.

Commercial road vehicles which are designed to carry loads on a platform to the rear of a driving cab commonly have such a system fitted to be conscious of the load on the rear axle of the vehicle. The purpose of such systems is to 'balance' the braking forces between the 6ont and rear axles to match the loads on those axles.

Commonly, the braking systems of such load carrying vehicles are designed to be most effective and suitably "balanced' when the vehicle is My loaded. Most such vehicles have little variation in load on their front axles but have substantial variation on their rear axle loads, depending upon whether or not, and to what extent, the vehicle is loaded.

If such load sensing systems are not fitted to the rear axles of vehicles, the vehicles experience braking problems when the vehicle is unloaded. When the brakes are applied, the rear wheels may "lock up" early at relatively light brake pressure and this has the effect of reducing braking efficiency and increasing tyre wear.

The load sensing system operates, automatically, as a result of the load on the vehicle reducing the distance between the chassis and the rear axle - or between other points on, respectively, the sprung mass of the vehicle and the unsprung components attached to the rear wheel assembly(ies).

1 When the vehicle is unloaded, a load-sensing valve is caused to interfere with the fluid of the braking system applied to the rear brakes - it reduces the fluid pressure. When the valve is actuated - as a result of load on vehicle - the interference is eliminated, so permitting full pressure to the S rear brakes.

The brake fluid medium may be either oil or air or any other suitable material.

10. This common system is effective for vehicles which carry a load substantially over the rear axle or axles.

However, certain vehicles, such as some of those used in the vehicle recovery industry, may be loaded abnormally. In particular, such is vehicles may be adapted to support loads substantially rearwardly of the rear axle. Such adapted vehicles, some of which are known as underlift vehicles, are used, for example, to transport broken-down or accident damaged vehicles by lifting one axle of the damaged vehicle and semitowing it with the adapted (recovery) vehicle.

When such a load is applied it is, in effect, cantilevered out from the rear end of the underlift vehicle. This causes a high load on the rear axle and thus a load sensing valve fitted to the rear axle, as is usual, will operate in relation to the rear axle and will permit full operation of the rear brakes.

However, the cantilevered rear end load causes the front end of the vehicle to rise substantially on its front springs reducing the load on the front axle by a substantial amount. When such a load is applied therefore, there is substantially less than design load on the front axle and 2 when the brakes of such a vehicle are applied, the front wheels may lock up at relatively light braking pressure. Thus, the front wheels may skid at relatively low braking deceleration rates and with the front wheels skidding, steering control is lost. Underlift vehicles, therefore, loaded as they are adapted to be, are potentially dangerous and travel speeds have to be kept low to avoid excessive stopping distances. These vehicles are, therefore, inefficient but they are still used in large numbers because of their other benefits.

The invention provides a front-end vehicle load sensed braking system for a vehicle having at least one front axle and at least one rear axle, wheels mounted on said axles, a body resiliently mounted with respect to at least the front axle, a braking system acting on the wheels mounted on said front and rear axles, and means to mount a load substantially rearward of is the rear axle, comprising: a load sensing means for mounting between said front axle and said body; and means to reduce the braking effect on said front wheels relative to that on the rear wheels when said load sensing means senses a reduced load on the front axle.

Preferably the means to reduce the braking effect operates by reducing the brake fluid pressure to the front wheel brakes relative to the pressure applied to the rear wheel brakes.

Preferably the load sensing means comprises at least one valve responsive to changes in the load on the front axle. Advantageously the valve may comprise a two port or four port type or any other type or combination of valves, to suit the braking system of the vehicle. The valve comprises a 3 load-conscious reducing valve and the system of the invention further comprises the associated fixing and actuating componentry.

In the system of the invention, the load sensing means, such as a load sensing valve, operates to reduce the braking effect on the front wheels relative to the braking effect on the rear wheels when the load sensing valve senses a reduced load on the front axle resulting from a rear overhung load. This can be detected by an increase in the vertical distance between the vehicle body and the front axle. The reduced braking effect on the front axle relative to that on the rear axle enables higher braking forces to be applied to the rear axle (and therefore to the vehicle as a whole) such that higher deceleration rates can be achieved prior to the front wheels being caused to lock up.

Advantageously the "interference leveP i.e. the amount or proportion by which the front end braking is reduced (relative to the rear end braking) may be graduated. Advantageously this reduction is obtained by reduci g the front end brake fluid pressure relative to the rear end brake fluid pressure in proportion to the cantilevered load moment applied.

The front-end vehicle load sensed braking system may also be used advantageously in combination with a conventional load sensing means for mounting between the rear axle and the body which reduces the braking effect on the rear wheels when the load sensing means senses an increased load on the rear axle.

The invention further provides a vehicle comprising at least one front axle and at least one rear axle, wheels mounted on said axles, a body resiliently mounted with respect to at least the front axle, a braking system acting on 4 the wheels mounted on said front and rear axles and means to mount a load substantially rearward of the rear axle, the vehicle further comprisi 9 a front-end vehicle load sensed braking system comprising:

a load sensing means for mounting between said front axle and said 5 body; and means to reduce the braking effect on said front wheels relative to that on the rear wheels when said load sensing means senses a reduced load on the front axle.

When a vehicle fitted with the load sensed braking system of the invention carries a load which is cantilevered out from the vehicle's rear end, the front end of the vehicle is lifted up on its suspension system and the load sensing valve is actuated to interfere with the brake fluid. The valve acts to reduce the brake fluid pressure applied to the front wheel brakes which reduces the braking forces applied to the front wheels. This enables higher braking forces to be applied to the rear brakes before the front wheels reach their lock-up pressure and so enables increased vehicle deceleration without front wheel skid and hence steering control can be maintained at higher deceleration rates.

The invention will now be described, by way of example only, with reference to the drawings, of which:

Figure 1 shows a side view of a conventional load carrying vehicle; Figures 2a and 2b show a conventional rear axle load sensing system when the vehicle is unloaded and loaded, respectively; Figure 3 shows a side view of an underlift type load carrying vehicle, suitable for application of the load sensing system of the invention; Figures 4a and 4b show a front axle load sensing system in accordance with the present invention when the vehicle is unloaded and loaded, respectively; and Figure 5 shows a vehicle in accordance with one embodiment of the S invention.

In prior art systems, as shown in Figures 1, 2a and 2b, a vehicle 1 comprises a chassis 2 resiliently mounted with respect to a front axle 3 and a rear axle 4. On the chassis 2 is a cab 5 and a load bearing bed 6.

When a load is applied to bed 6, the bed 6 depresses, as shown by the dashed lines 6a. This increases the load on the rear axle 4, decreasing the distance between the bed 6 and the axle 4, but has little effect on the loading on the front axle 3.

When the vehicle 1 is driven, the brakes (not shown) act on the front wheels 7 and the rear wheels 8. When a load acts on the rear axle 4, the brakes are fully effective but, if there is no load-sensed rear brake pressure reducing system fitted, when there is no load the wheels may "lock-up" at a relatively light brake pressure. This would reduce braking efficiency and increase tyre wear.

This problem is overcome by fitting load sensing valves 9 as shown in Figures 2a and 2b. The valve 9 is connected to a brake fluid pipe 10 and acts to interfere with the fluid pressure to the rear axle brakes when there is little or no load on the vehicle 1, thus reducing braking pressure to the rear wheels.

6 When a load is applied to bed 6 the rear suspension system (not shown) is compressed and the distance between the chassis 2 and the rear axle 4 is reduced. This causes the valve 9 to be actuated so that the interference on the brake fluid pressure in the pipe 10 is reduced and ultimately removed, thus permitting full pressure to the rear brakes.

An additional problem arises, however, with vehicles which are adapted to support loads substantially rearward of the rear axle, as shown in Figures 3 and 5 (in which the same reference numbers are used as in Figure 1 for corresponding features). A vehicle 11 of the type common in the vehicle recovery industry in which one or more axles of a broken down or accident damaged vehicle is lifted and semi-towed by the recovery vehicle is shown in Figure 3. Such a semi-towed load may be in addition to another vehicle carded on the bed 6.

When a semi-towed load 12 is applied to vehicle 11, it is, in effect, cantilevered out from the rear end 13 of the vehicle 11. This causes a high load on the rear axle 4 but the cantilever effect on the rear end 13 of the vehicle 11 causes the front end of the vehicle 11 to rise substantially on its front suspension system (not shown) reducing the load on the front axle 3 by a substantial amount. The commonly fitted load sensing valve 9 shown in Figures 2a and 2b can operate in relation to the rear axle 4 and function as with a flat bed load, as shown in Figure 1, so that full operation of the rear brakes is possible but with substantially less than design load on the front axle 3, when the brakes of the vehicle 11 are applied the front wheels may lock up at relatively light braking system pressure making the vehicle potentially dangerous.

7 The present invention provides a means of improving the braking system of underlift vehicles to make such vehicles safer and more efficient.

As shown in Figures 4a and 4b, a vehicle load sensing system in 5 accordance with the invention comprises a valve 14 and connected actuating mechanism mounted between the front axle 3 and the chassis 2 of a vehicle 11 as shown in Figures 3 and 5. Referring to Figure 4a, when the front axle 3 is normally loaded, i.e. when the vehicle 11 is unloaded or loaded only on the bed 6, the chassis 2 and the axle 3 are relatively close together and the valve 14 does not interfere with the brake fluid pressure in the brake pipe 15, enabling the front brakes to operate to full pressure. However, referring to Figure 4b, when a cantilevered load is applied to the rear of the vehicle 11 the load on the front axle 3 is reduced and the front end of the vehicle 11 becomes raised up from its is normal position on its suspension system. This increases the distance between the axle 3 and the chassis 2. In this position, the valve 14 interferes with the fluid pressure in the brake pipe 15 to reduce the pressure and thus reduce the pressure applied to the front wheel brakes relative to that being applied to the rear wheel brakes.

When the system of the invention is in use on a vehicle, when the driver of the vehicle brakes, the fluid pressure to the front brakes is reduced but, as the driver presses harder on the brake pedal, the pressure at the rear brakes is increased. If the driver continues to press hard on the brake pedal, he may eventually achieve the same pressure at the front brakes as he would have done without the system of the invention but there is now much more braking on the vehicle overall because of the higher pressure on the rear brakes.

8 Figure 5 shows a vehicle 11 in accordance with a preferred embodiment of the invention. In this embodiment, the vehicle 11 is fitted with a conventional load sensing system on the rear axle 4 and with a load sensing system in accordance with the invention on the front axle 3.

When a load is carried to the rear of a vehicle 11, the front end of the vehicle 11 is lifted up on its suspension system and the load sensing valve 14 is actuated to interfere with the brake fluid in the pipe 15 to reduce the brake fluid pressure applied to the front wheel brakes. This enables higher braking forces to be applied to the rear axle(s) 4 before the front wheels reach their lock up pressure and so enables increased vehicle deceleration without front wheel skid i.e. steering control can be maintained at higher deceleration rates. As the vehicle 11 decelerates, the increased deceleration rate throws more of the vehicle weight onto the front axle 3, the effect of the valve 14 may then be reduced and further braking can be achieved with the front brakes without loss of steering control. Because of the additional load on the front axle resulting from the forward "throw", the brake system pressure may be further increased. This is applicable to both the front and rear axles 3,4 even if the interference effect of valve 14 is not reduced by extra load on the front axle 3.

Once fitted, the actuation, and therefore correct functioning, of the load sensing valve 14 is automatic. The valve 14 reduces the pressure applied to the front wheel brakes (relative to the rear wheel brakes) when the adapted vehicle is carrying a rear-cantilevered load, but does not "interfere" with the front end braking when there is no rear-end cantilevered load. The "interference level", i.e. the amount or proportion by which the front end braking is reduced relative to the rear end braking 9 may be graduated such that the pressure reduction is proportional to the cantilevered load - The use of the "active" load sensing system of the invention, which 5 operates to reduce front wheel braking levels relative to the braking levels on the rear wheels when a vehicle is laden with a rear overhung cantilevered load, in addition to the known rear axle load sensing system, which operates to reduce rear wheel braking when the vehicle is unladen, enables a flexible system of improved braking to be applied. The invention provides the advantage over the known rear axle load sensing system alone of enabling safe braking to be achieved under a wider range of loading conditions.

Claims

1. A front-end vehicle load sensed braking system for a vehicle having at least one front axle and at least one rear axle, wheels mounted on said axles, a body resiliently mounted with respect to at least the front axle, a braking system acting on the wheels mounted on said front and rear axles, and means to mount a load substantially rearward of the rear axle, comprising: a load sensing means for mounting between said front axle and said body; and means to reduce the braking effect on said front wheels relative to that on the rear wheels when said load sensing means senses a reduced load on the front axle.

is

2. A vehicle load sensed braking system according to claim 1 wherein the means to reduce the braking effect on said front wheels relative to that on the rear wheels reduces the brake fluid pressure to the front wheel brakes relative to the pressure applied to the rear wheel brakes when said load sensing means senses a reduced load on the front axle.

3. A vehicle load sensed braking system according to claim 1 or claim 2 wherein the load sensing means comprises at least one valve responsive to changes in the load on the front axle.

4. A vehicle load sensed braking system according to claim 3 wherein the load sensing valve comprises a two port valve or a four port valve or any other suitable valve or combination of valves.

11

5. A vehicle load sensed braking system according to claim 3 or claim 4 wherein the load sensing valve operates to reduce the braking effect on the front wheels relative to that on the rear wheels when the vertical distance between the vehicle body and the front axle is increased.

6. A vehicle load sensed braking system according to any one of claims 3 to 5 wherein the load sensing valve acts to reduce the brake fluid pressure to the front brakes relative to the pressure applied to the rear brakes.

7. A vehicle load sensed braking system according to any one of the preceding claims wherein the amount by which the front end braking is reduced relative to the rear end braking is graduated.

8. A vehicle load sensed braking system according to clahn 7 wherein the front end braking effect is reduced by reducing the front end brake fluid pressure relative to the rear end brake fluid pressure in proportion to the cantilevered load moment applied.

9. A vehicle load sensed braking system according to any one of the preceding claims in combination with a conventional rear end load sensed braking system mounted between the rear axle and the body.

10. A vehicle comprising at least one front axle and at least one rear axle, wheels mounted on said axles, a body resiliently mounted with respect to at least the front axle, a braking system acting on the wheels mounted on said front and rear axles and means to mount a load substantially rearward of the rear axle, the vehicle further comprising a front-end vehicle load sensed braking system comprising:

12 a load sensing means for mounting between said front axle and said body; and means to reduce the braking effect on said front wheels relative to that on the rear wheels when said load sensing means senses a reduced 5 - load on the front axle.

11. A vehicle load sensing system as hereinbefore described with reference to Figures 3 to 5 of the accompanying drawings.

12. A vehicle as hereinbefore described with reference to Figures 3 and 4 of the accompanying drawings.

13. A vehicle as hereinbefore described with reference to Figure 5 of the accompanying drawings. is 13