Brakes - Vibrations While Braking
SI No. 510-1919PI No.
Date June 1998
Distr. all
900
Vibrations while braking
Cars affected
All Saab 900 M94-
Background
If a customer experiences vibrations during braking, it can be due to two possible causes:
- Excessive irregularity of the wheel hub.
- Excessive thickness variations of the brake disc.
If the total run-out of the brake disc/hub exceeds 0.08 mm, both the brake disc and wheel hub must be replaced.
If only the brake disc is replaced when a significantly uneven wheel hub has been confirmed, front assembly vibrations will reappear after 650 - 1000 miles (1000-2000 kilometres) of driving.
If only thickness variations of the brake disc have been confirmed, that is, the wheel hub is not significantly uneven, replace only the brake disc.
It is important that both the hub and brake disc run- outs are measured in order to avoid the problem with reappearing vibrations.
Checking brake discs and wheel hubs
Refer to Service Manual 5:1 Brakes for checking the brake discs.
See below for checking wheel hubs.
1 Remove the wheel, brake caliper and brake disc.
2 Make sure the hub is clean and free from impurities. Only one dial is used. Make a recess in the tool as illustrated so that the wheel hub will not make contact with the tool.
3 Rotate the hub and check the run-out (the value between the greatest and the smallest readings).
If run-out exceeds 0.03 mm, the hub must be replaced.
Important:
The dial will jump about 0.03-0.05 mm at each bolt hole. This is normal.
4 Fit the brake disc, brake caliper and wheel.
Tighten the wheel bolts with a torque wrench.
Tightening torque 120 Nm (89 lbf ft).