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Description IW4/IW7 WB/WBV
4 D1 0412BA1-GB04
As described above, brake tests on 4-wheel drive vehicles are feasible if both wheels on the
same axle counter-rotate at the same speed. As, in practice, the circumference of the left and
right wheel is not exactly the same due to different tire tread depths and uneven tire pressure,
the RPM of the two drive motors is normally different. Therefore the drive motors must control
their speed in order to obtain the same RPM for both wheels.
Approximately the same speed of the roller set drive motors will be sufficient for a 4-wheel drive
vehicle having a soft VISCO clutch in the drive shaft as no brake torque is transferred by the
VISCO clutch when the drive shaft rotation is low. Therefore the speed control of the drive
motors alone will be sufficient in this case.
Contrary to the above, when testing the brakes of 4-wheel drive vehicle having a hard VISCO
clutch in the drive shaft both wheels on the same axle must rotate synchronously during the
brake test, as the clutch viscosity is so low that even the slowest rotation of the drive shaft will
transfer brake torque to the other wheels through the VISCO clutch.
c) Rigid drive shaft between the differentials
To perform a brake test on a 4-wheel drive vehicle having a rigid drive shaft, the wheel rotations
must be exactly controlled in such a way that no brake torque can be transferred by the drive
shaft.
Controlling the synchroneous rotation of the wheels
In order to control synchronous rotation of the wheels, reflector strips are fastened to the sides
of the wheels which trigger a signal via two photo-electric cells, mounted on the side of the roller
set.
On vehicles equipped with a rigid drive shaft or with a hard Visco clutch the wheels cannot be
individually turned. If one vehicle wheel is turned forward in the roller set, the other wheel (on the
same axle) will be turning backwards in a synchronous manner. If one wheel is turned slowly in
forward or backward direction, one will note that the other wheel starts turning with a little delay.
This slight delay in following the other wheel to turn is caused by the gear play (backslash of
teeth) inside of the differential.
To perform a brake test on a 4-wheel drive vehicle having a rigid drive shaft, the wheel rotation
must be controlled in such a way that no brake torque can be transferred by the drive shaft. This
is achieved by staying within the gear play of the differential during the brake test. The
differential will be in a "balance" state.