Vehicle brake judder is a vibration phenomenon responsible for
an expressive number of customer complaints. In order to prevent
judder from occurring, new vehicle developments are putting in
practice dynamometer and vehicle brake tests to assess the DTV
growth and the effectiveness of applied countermeasures, when
necessary.
The measurement of DTV is very sensitive and requires high
precision sensors. Due to these facts, incorrect DTV measurements
are not uncommon, since the rig or vehicle setup, the
assembly/disassembly of the sensors or brakes and even the
vibration of the dynamometer itself may figure as sources for
measurement errors. In the other hand, if the test vehicle or
dynamometer is equipped to acquire brake torque and brake pressure
variations (BTV and BPV, respectively), those measurements may
suffer less interference from external parameters and present good
relationship with DTV and judder.
As a study to verify this correlation, discs in different levels
of DTV were prepared and assembled on a vehicle, instrumented to
vibration measurements. A test procedure with snubs at different
pressures was carried out with each disc, as a way to evaluate the
condition for critical judder. After that, the same procedure was
simulated in an inertia dynamometer, were the BTV and BPV were
measured against the different levels of DTV.
As a result it was observed that the
BTVrelative
measurement, as well as the
BPVmaximum
measurement, in snubs with pressure between 30 and 50 bar, present
well-defined levels which allow identifying if a brake disc DTV is
above the acceptable limit for non-generation of vibrations
classified as uncomfortable. From this observation, the conclusion
is that brake test procedures can incorporate additional snubs with
pressure in the range of 30 to 50 bar, in strategically defined
stages, where the
BTVrelative
and/or
BPVmaximum
measurements may figure as part of the criteria for brake
approval.