GB2138086A

GB2138086A - Brake force distribution in motor vehicles

Info

Publication number: GB2138086A
Application number: GB08408187A
Authority: GB
Inventors: Werner Stumpe
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1983-03-30
Filing date: 1984-03-29
Publication date: 1984-10-17
Also published as: DE3311556A1; GB8408187D0; FR2543500A1; FR2543500B1; GB2138086B

Abstract

To ensure that front wheels lock before rear wheels in a motor vehicle with or without ALB (automatic load-dependent brake force regulation), the rear axle brake force is held back in the lower range of brake pressure (range of small and minimum braking operations), so that the braking action at the rear axle is reduced or entirely eliminated. This corresponds to parallel displacement of the brake force distribution curve I to I1 in the brake force graph (Fig. 2) with the simultaneous retention of effective participation of the rear axle in the braking operation at average and high brake pressures. Fig. 5 shows a form of pneumatic relay valve having a spring 17 to hold back opening of a triple valve until the control pressure reaches a set value. <IMAGE>

Description

SPECIFICATION Brake force distribution in motor vehicles The invention relates to the distribution of the brake force between front and rear axles of motor vehicles.

It is common knowledge that, although motor vehicles whose front wheels lock as a result of a braking operation are no longer steerable, they at least continue to travel straight ahead on a homogeneous roadway in contrast to motor vehicles whose rear wheels are the first to lock.

Hence, by using specific methods of regulation, such as automatic load-dependent brake force regulation known under the designation "ALB", endeavours are being made to distribute the brake forces to the vehicle axles such that (partial) over-braking always first occurs on the front axle for any state of vehicle load.

In any event, endeavours are being made jn this direction and it is already known from, for example, EC Directive EGRR (71/320/EEC) and its amendments.

However, irrespective of the factors involved in distributing the brake forces to the respective vehicle axles, the braking operations are always effected on the front and rear axles simultaneously, even if the rear axle is braked to a greater or lesser extent under the action of the ALB in dependence upon the prevailing state of load of the vehicle. Of course, this known brake force distribution cannot avoid frequently observed, excessive wear on the rear axle brakes, since, as tests have shown, the greater portion of the braking operations takes place with an extremely low pressure and in this range the slope of the brake force distribution curve (normally a straight line), which possibly has a greater slope towards the front axle, only has a slight effect and then over-braking of the rear axle nevertheless occurs.Existing regulating means can become particularly critical when only slight adhesion exists between the tyres and the road, for example when a vehicle has to be driven on slippery, wintry roadways which in any event make it necessary to drive carefully and correspondingly to initiate braking operations very carefully. Hence, it is precisely under such conditions that a vehicle can become unstable during braking, both in the absence of automatic load-dependent brake force regulation or despite the existence of such regulation. It will be appreciated that these comments only apply to the case in which the motor vehicle is not equipped with a so-calied ABS (anti-skid brake system) which is designed in a known manner such that wheels are in any case prevented from locking as a result of a braking operation.

The present invention resides in a method of distributing brake force between the front and rear axles in motor vehicles with or without ALB (automatic load-dependent brake force regulation), in which, in the lower range of brake pressure (range of small and minimum braking operations) the rear axle brake force is reduced or is brought to zero by holding back the brake pressure introduced for accurate or approximate parallel displacement of the brake force distribution curve in the brake force graph, such that the front axle is the first to show a tendency to lock in the lower range of brake pressure with the simultaneous retension of corresponding participation of the rear axle in the braking operation at high and maximum brake pressures.

The invention includes a device for distributing brake force between the front and rear axles in motor vehicles with or without ALB (automatic load-dependent brake force regulation), in which brake pressure hold-back means responding at low or minimum brake pressures are provided for the region of the rear axle.

Thus it can be ensured that there is the greater probability that the front axle will lock first even in the case of the substantially more frequent, light braking operations, and hence, with appropriate braking, the stability of direction is ensured, this applying particularly to a slippery roadway and a careful style of driving.

Hence, the holding-back of the action of the brake force at the rear axle in favour of the front axle with corresponding reduction, achieved by parallel displacement of the brake force distribution curve in the brake force graph, improves the braking behaviour in the case of low brake pressure values and, in the case of possible heavy braking operations, at the same time prevents the rear axle from being under-braked to too great an extent, both in the case of an unladen vehicle and also in the case of a fully laden vehicle.

The structural means for realising the present invention are negligible and possibly only require the use of stronger compression springs in the rear axle brake cylinder or the provision of additional springs or of an additional valve in the rear axle brake circuit.

Hence, the present invention can be realised in a particularly inexpensive manner.

It is also advantageous that an advance in the behaviour of the rear axle brakes, provided for other reasons in the service brake valve, is compensated for by the pressure hold-back provided in accordance with the invention, so that the front axle and the rear axle can, in any case, respond with the same pressure. Hence, no intervention has to be made in the operating behaviour of the service brake valve, so that, when the vehicle is towing a trailer, there are no disadvantageous consequences for the time behaviour of the trailer control. Although the invention continues to ensure the desired advance in the rear axle brake circuit for the rapid response of the trailer control valve, the invention prevents the advance from taking effect on the effective braking range of the rear axle of the tractor vehicle.

The invention is further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a conventional brake force distribution graph; Figures 2 and 3 are corresponding brake force distribution graphs in the realised invention for a laden and an unladen vehicle respectively; Figure 4 is a circuit diagram of a brake system in which the holding-back, in accordance with the invention, of the brake pressure in the region of the rear axle, is achieved; and Figure 5 is a cross section of a device for achieving brake pressure hold-back in accordance with the invention.

Better comprehension of the present invention can be provided by the brake force distribution graph shown in Fig. 1 and which shows the installed sequence of locking (front axle first). The braking forces Bv and Bh, related to the weight G of the vehicle, of the front axle and of the rear are plotted along the abscissa and ordinate respectively of the graph of Fig. 1. The connecting lines between equal numerical values of the abscissa and ordinate show brake force distributions with the same obtainable retardation Z, this retardation of the vehicle resulting from the following formula: Bv Bh Z= + G G In a practical embodiment the brake forces are then distributed to the axles along the (idealised) curve P, such that the two axles are subjected to the same brake force. The front axle is over-braked below the curve P, and the rear axle is over-braked above curve P.Furthermore, the limits admissible in accordance with the EC guidelines, corresponding to the limiting curves G1 and G2, are also plotted in the graph.

A brake force distribution means installed in a motor vehicle, and which follows the curve of the straight line I, is oriented on the curve P, the curves P, G1 and G2 being located in different positions according to the load on the vehicle when ALB is used. The brake force distribution means installed is then influenced in a known manner by the load-dependent regulating device of the ALB in accordance with curve I. Of course, it will be appreciated that the basic idea of the present invention can also be used on those motor vehicles which are not equipped with automatically operating load-dependent brake force regulation (ALB), since the advantages presented by the invention, namely avoiding over-braking at the rear axle, particularly in the case of slippery roadways and minor braking operations, can also be obtained when ALB is not installed.

The brake force distribution curve I between the front axle and the rear axle is varied in a known manner by the ALB regulator in that the scope of the said curve in the brake force graph is correspondingly varied, following the double arrow A. Limitation of the rear axle pressure can then be effected in the upper range of pressure, hence leading to better adaptation to the P curve in the upper range of braking. A margin of safety is observed in the brake force distribution of the brake system, as installed, to achieve over-braking on the front axle in order to maintin the stability of direction of the vehicle during braking.

However, since this cannot solve the problem during braking operations with extremely low pressure, the present invention uses a different method which, in particular, resides in subjecting the installed brake force distribution to a more or less accurate, possibly only approximate parallel shift, corresponding to Figs. 2 and 3, to the effect that the curve I becomes the curve 11 realising the invention, with the result that, owing to the distinct formation of a break at low and minimum brake pressures on the rear axle, pressure hold-back is obtained and, at any event in the first instance, takes effect such that no pressure, or no appreciable pressure, is fed to the rear axle brake cylinders, or such a pressure does not take effect, so that the rear axle brake cylinders also cannot build up any braking action on the rear axle (Figs. 2 and 3).It will be appreciated that the aim of the invention is also achieved when the above-rnen- tioned parallel displacement is effected with small angular differences.

Figs. 2 and 3 show, by dash-dot lines, the curve I of the installed brake force distribution of the case of a laden and an empty vehicle, and show at 12 a conceivable installed brake force distribution achieved by a considerably greater slope towards the ordinate, whereby the front axle is greatly preferred in the brake force distribution, at any rate in the range of average and high brake pressure values. However, Fig. 2 clearly shows that, even when including a considerable change in the slope of the curve 12 relative to the initial brake force distribution I with low and extremely low brake pressures such as predominantly occur, this slope can have virtually no effects. In fact, the action of the brake force for the rear axle, corresponding to the ordinate values for mininum brake pressure, is not appreciably cancelled by this slope.

This is where the solution in accordance with the invention has a great effect. In the case of minimum retardation, it in the first instance reduces the action of the brake force at the rear axle completely to zero up to a predetermined pressure value at the front axle, then enables a gradual rise, and has the great advantage that, on the other hand, in the case of greater retardation, by virtue of the curve 1, of the brake force distribution, the rear axle also participates in the braking action to a substantially greater extend, and more realistically, than is possible in the case of brake force distribution with a greater slope corresponding to curve 12.Hence, both with respect to braking operations with extremely low pressure as well as in the case of braking operations with average and high pressure, the solution in accordance with the invention is substantially more effective and is optimally adapted to the actual road conditions with respect to the invention of brake force effectively obtained between the front and rear axles.

The characteristic of the brake force distribution, corresponding to curve It, realised by the present invention, avoids the "scissors" like opening between I and 12 in a dangerous manner precisely at higher brake pressures and prevents the rear axle from participating in the braking operation to a lesser extent than would even be necessary, particularly in cases of emergency for example, and also at the same time avoids dangerous over-braking of the rear axle during braking operations with a low and a minimum pressure.

The basic idea of the present invention which corresponds to an initial hold-back of pressure upon actuation of the rear axle brakes, can be realised by disposing an additional blocking valve 10 in the rear axle brake circuit HAK, as is shown only diagrammatically in Fig. 4. In the present case, the blocking valve 10 is disposed upstream of the ALBA device 11. A rear axle brake cylinder illustrated diagrammatically is designated 1 2.

The brake system which is only illustrated highly diagrammatically in Fig. 4, is completed by compressed air reservoirs 13, a front axle cylinder 14 and a service brake valve 1 5. If the vehicle is equipped with an ALB device, and the additional valve 10 is disposed between ALB device and the rear axle brake cylinder 12, it may be advantageous to raise the actuation point, that is to say, the commencement of pressure reduction for the rear axle region by the ALB device, thus to counter the effect of a resultant loaddependence on the function in accordance with the invention. The mode of operation of the additional valve is such that it only opens at a limiting value of the pressure corresponding to the kink or break pressures designated x and x' respectively in the graphs of Figs. 2 and 3.The brake pressure introduced by the service brake valve is transmitted to the rear axle only after this kink pressure or blocking pressure is exceeded, so that the desired pressure hold-back for the rear axle is obtained.

A further embodiment resides in providing strengthened compression springs as brake return springs for the rear axle brake cylinders, a measure which is particularly inexpensive and which also acts in the sense of pressure hold-back and parallel displacement of the brake force distribution curve and which, advantageously, is also combined with the raising of the actuation point by ALB device.

A third and last embodiment, which is also advantageous with respect to cost, resides in ensuring the desired pressure hold-back by fitting an additional spring 1 7 in a relay valve 1 6 (Fig. 5) which is provided downstream of the ALB device and which, alternatively, can be integrated in the ALB device. Hence, the initial stress of the spring has to be dimensioned such that the relay valve only opens while the kink or break pressure corresponding to the graphs of Figs. 2 and 3 is reached.

The invention is also advantageous if the service brake valve in motor vehicles is designed such that an advance in the rear axle brake circuit results in rapid response of a trailer control valve. This advance does not affect the rear axle of the tractor vehicle by virtue of the pressure hold-back measures in accordance with the invention.

The invention renders it possible to reduce the brake force at the rear axle in favour of the front axle, or to set it to zero, such that, when braking with a low or extremely low pressure, such as occurs particularly frequently in the case of low coefficients of friction on the road, a stable braking behaviour is obtained without under-braking the rear axle to too great an extent in the event of violent braking operations.

Claims

1. A method of distributing brake force between the front and rear axles in motor vehicles with or without ALB (automatic loaddependent brake force regulation), in which, in the lower range of brake pressure (range of small and minimum braking operations) the rear axle brake force is reduced or is brought to zero by holding back the brake pressure introduced for accurate or approximate parallel displacement of the brake force distribution curve in the brake force graph, such that the front axle is the first to show a tendency to lock in the lower range of brake pressure with the simultaneous retension of corresponding participation of the rear axle in the braking operation at high and maximum brake pressures.

2. A method as claimed in claim 1, in which the brake force distribution curve displacement from the zero point of the brake force graph commences at a predetermined front axle kink or break pressure value with a slope determined by the load-dependent regu lating behaviour of the ALB.

3. A device for distributing brake force between the front and rear axles in motor vehicles with or without ALB (automatic loaddependent brake force regulation), in which brake pressure hold-back means responding at low or minimum brake pressures are provided for the region of the rear axle.

4. A device as claimed in claim 3, in which, for the purpose of holding back the brake pressure for the region of the rear axle, there is disposed, in the brake pressure line leading from a service brake valve to the region of the rear axle, a shut-off valve which assumes its open position only when a predetermined pressure value is exceeded.

5. A device as claimed in claim 4, in which the shut-off valve is disposed upstream of an ALB device in the rear axle brake circuit.

6. A device as claimed in claim 3, in which strengthened compression springs are disposed in the rear axle brake cylinder, such that a rear axle brake force action is initiated only when a predetermined kink or brake pressure value of the brake pressure introduced by the service brake valve is reached.

7. A device as claimed in claim 3, in which a relay valve is disposed in the brake line downstream of an ALB device or is integrated therein and has additional spring which releases the supply of brake pressure, introduced by the service brake valve, to the rear axle only when a predetermined kink or brake pressure has been exceeded.

8. A device as claimed in any of claims 3 to 7, in which an ALB actuation point (commencement of pressure reduction) is additionally raised for the purpose of holding back the pressure.

9. A method of distributing brake force between front and rear axles of motor vehicles, substantially as herein described with reference to Figs. 2 to 5 of the accompanying drawings.

10. A brake force distribution device substantially as herein described with reference to and as illustrated in Figs. 4 and 5 of the accompanying drawings.