Plate brake tester

A plate brake tester for testing the brakes of a motor vehicle, comprises two pair of tread plates one beside the other, each of said tread plates having a measuring device for measuring the force exerted on the tread plate, a processing unit for processing measuring signals provided by the measuring devices and a display for displaying the measuring results. The plate brake tester comprises a weighing device for determining the weight of the front axle and the rear axle of the vehicle, respectively. The processing unit is adapted to compute the braking retardation from the measured brake forces and the total weight of the vehicle. The plate brake tester computes the brake force ratio front axle/rear axle from the measured brake forces and the dynamic weight ratio front axle/rear axle from the computed braking retardation, the wheelbase of the vehicle, the height of the center of gravity of the vehicle and the measured weight of the front axle and the rear axle, respectively. Further, the processing unit provides an indication through the display if said brake force ratio deviates from said dynamic weight ratio and this deviation exceeds predetermined threshold values.

BACKGROUND OF THE INVENTION 
The invention relates to a plate brake tester for testing the brakes of a 
motor vehicle, comprising two pair of tread plates one beside the other, 
each of said tread plates having a measuring device for measuring the 
force exerted on the tread plate, a processing unit for processing 
measuring signals provided by the measuring devices and a display for 
displaying the measuring results, further comprising a weighing device for 
determining the weight of the front axle and the rear axle of the vehicle, 
respectively, said processing unit being adapted to compute the braking 
retardation from the measured brake forces and the total weight of the 
vehicle. 
Such an apparatus is very suitable for testing the brakes of a motor 
vehicle, because the brakes are tested under circumstances as will also 
occur during braking on the road. An important criterion for evaluating 
the operation of the brakes of a vehicle is the ratio of the braking 
forces exerted by the front wheels and the braking forces exerted by the 
rear wheels, which ratio can be indicated as brake force ratio front 
axle/rear axle. In many countries the government requires that this brake 
force ratio is in any case such that during braking on the road the front 
wheels will always be the first to skid. For these reasons car 
manufacturers choose a brake force ratio with such a safety margin that 
rear wheel skid cannot occur. However this results in a lower maximum 
obtainable braking retardation because the brake force which can be 
transfered by the rear wheels on the road is lower than the dynamic weight 
on the rear wheels. Up to the present there is no brake tester available 
by means of which a test of the brakes of a motor vehicle can be done in 
simple and for the garage industry economic manner in order to check 
whether the brake force ratio meets certain requirements like for example 
the requirements of the government. 
SUMMARY OF THE INVENTION 
The invention aims to provide a plate brake tester which meets the want for 
such a brake tester and by means of which the brake force ratio can be 
tested in a simple manner. 
To this end the plate brake tester of the invention is characterised by 
means for computing the brake force ratio front axle/rear axle from the 
measured brake forces and means for computing the dynamic weight ratio 
front axle/rear axle from the computed braking retardation, the wheelbase 
of the vehicle, the height of the center of gravity of the vehicle and the 
measured weight of the front axle and the rear axle, respectively, wherein 
the processing unit provides an indication through the display if said 
brake force ratio deviates from said dynamic weight ratio and this 
deviation exceeds predetermined threshold values. 
In this manner a plate brake tester is obtained, by means of which the 
brake force ratio can be compared quickly with the optimum brake force 
ratio for this vehicle, i.e. the dynamic weight ratio. For the maximum 
brake force which can be transfered by a wheel to the road equals the 
vertical force (the weight), exerted by this wheel on the road. Due to the 
diving tendency of the motor vehicle during braking the maximum vertical 
forces which can be exerted by the wheels, are therefore determined by the 
dynamic weight ratio of the motor vehicle. 
According to a favourable embodiment the processing unit provides an 
indication through the display if the deviation between said brake force 
ratio and the dynamic weight ratio showes that the rear wheels lock before 
the front wheels. 
According to a prefered embodiment the processing unit indicates said brake 
force ratio and, said dynamic weight ratio as optimum brake force ratio on 
the display.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT 
The plate brake tester 1 shown comprises two pair of tread plates 2 and 3, 
respectively. These tread plates 2,3 are provided with measuring devices 
2', 3' schematically shown and by means of which the force exerted on the 
tread plates can be measured and converted into an electrical signal. 
These measuring devices 2', 3' can for example be made as stretching or 
bending bar with strain gauges, as electromagnetic transducer or the like. 
As schematically indicated by connection lines the measuring devices are 
connected to a processing unit 4 for processing the measuring signals 
provided by the measuring devices. This processing unit 4 further 
comprises a display 5 and a keybord 6. 
The plate brake tester 1 shown further is provided with a so-called 
side-slip measuring device 7, including a measuring plate 8 at the left 
and a preceding compensation plate 9, which plates 8,9 are preceded and 
followed, respectively, by a coupling plate 10. At the right the side-slip 
measuring device 7 includes an auxiliary plate 11. 
Further the plate brake tester 1 is provided with a weighing device 12 for 
measuring the weight at the front axle and rear axle, respectively, of a 
vehicle 13. The weighing device 12 is preceded by ramp plates 14 and 
followed by coupling plates 10. At the other end of the plate brake tester 
1 ramp plates 15 are provided. 
The plate brake tester 1 has the important advantage that a dynamic test 
can be made, i.e. a test under circumstances comparable with the 
circumstances on the road. 
In normal use of the plate brake tester a measurement is started in that 
the force exerted on the tread plates 2 intended for the front wheels 
exceeds a certain threshold value whereafter the forces exerted on the 
tread plates 2, 3 are measured by the processing unit during a 
predetermined measuring period, for example 750 ms. The processing unit 4 
is adapted to check if the forces exerted meet the minimum braking 
retardation legally required and entered as a parameter through the 
keyboard 6. 
The braking retardation is computed by the processing unit 4 from the total 
of the measured brake forces divided by the weight of the motor vehicle 
13. Further it is now possible to compute the dynamic weight ratio by 
means of the weight at the front axle and the rear axle, respectively, of 
the vehicle 13 measured by the weighing device 12, from the braking 
retardation, the wheelbase of the vehicle 13 and the height of the center 
of gravity of the vehicle 13. For the weight displacement .DELTA.Q it 
applies: 
##EQU1## 
in which D is the braking retardation in g, P is the total vehicle weight, 
h is the height of the center of gravity of the vehicle and e is the 
wheelbase. The dynamic weight at the front axle and the rear axle, 
respectively, is then P+.DELTA.Q and P-.DELTA.Q, respectively. 
Further the processing unit 4 computes the brake force ratio front 
axle/rear axle from the measured brake forces. Thereby this brake force 
ratio can be compared with the dynamic weight ratio front axle/rear axle. 
As the maximum brake force which can be transfered by a wheel to the road, 
equals the vertical force, i.e. the weight of the respective wheel, the 
dynamic weight ratio actually corresponds with the optimum brake force 
ratio. Therefore the processing unit 4 showes through the display 5 
besides the brake force ratio, the dynamic weight ratio as optimum brake 
force ratio so that the user can determine in which manner the measured 
brake force ratio deviates from the optimum brake force ratio. 
The government usually requires that the rear wheels will not skid before 
the front wheels. When a comparison of the brake force ratio and the 
dynamic weight ratio showes that the rear wheels would lock before the 
front wheels, the processing unit 4 will then indicate this through the 
display 5. 
Further the processing unit 4 can indicate through the display 5 if the 
measured brake force ratio deviates more from the dynamic weight ratio 
than a predetermined threshold value. This threshold value may for example 
be 15 points. Further the processing unit 4 can indicate through the 
display 5 if the rear wheels provide less than 10% of the total brake 
force. 
In order to provide a better explanation the following example can be 
given: 
weight motor vehicle: 1000 kg 
weight front axle: 600 kg 
weight rear axle: 400 kg 
During the brake test by means of the plate brake tester 1 a total brake 
force of 6000 N is measured, wherein the front axle provides a brake force 
of 5100 N and the rear axle a brake force of 900 N. From this measurement 
it follows for the brake force ratio front axle/rear axle in per cents 
85/15. The braking retardation amounts 6 m/s.sup.2. 
With this braking retardation a dynamic weight ratio of 72/28 can be 
computed from the static weight ratio 60/40 by means of the wheelbase and 
the height of the gravity center. This means that in the example the 
contribution of 85% by the front wheels in the total brake force is 13 
points higher than the part of the weight at the front axle of 72%. As the 
brake force provided by the front wheels is higher than the vertical force 
exerted by the front wheels, the front wheels will block first. 
The processing unit 4 reads the wheelbase in a memory by means of the 
determined total weight of the motor vehicle. Further a fixed value of 
0,55 m is taken for the height of the center of gravity. If desired the 
wheelbase and the real height of the center of gravity can be entered 
through the keyboard 6. Further the Cw-value of the motor vehicle 13 can 
be entered through the keyboard 6 so that the dynamic weight ratio can be 
computed at a speed of for example 90 km/hour, wherein the effect of the 
wind caused by driving is taken into account. 
The invention is not restricted to the abovedescribed embodiment which can 
be varied in a number of ways within the scope of the following claims.