THE BIG NIGGLE
Your classic problems solved
Richard’s patience is at breaking point, thanks to pedal oscillation
I’ve got an annoying fault with my MGB that’s gradually got worse over the last few months. At first, the brake pedal would pulsate a bit when I first pressed it but seemed to go away, but now the brakes judder all the time when applied. What can I do?
THEO'S JOB CARD
STOP CLASSIC BRAKE JUDDER
It’s important to first test-drive the car to see when the brake judder occurs, at what speed, how bad it is, and if it changes with multiple brake applications. We will then inspect a car with disc brakes, remove the wheels, check the hub bearings for play, the discs for run-out and general condition by using a dial-test-indicator and a micrometer, then make some rudimentary checks for ovality in brake drums, before renewing the brake discs with a fitting kit, preparing the mating surfaces to prevent run-out.
START
1 Hub bearing
With the car on stands, grasp each wheel top and bottom, and try to rock in and out. If there’s appreciable play, check it’s not other joints. Taper-roller bearings can be adjusted if OK. Inspect / renew other types.
2 Disc runout
Remove the wheels. Replace the wheel nuts/bolts with washers if the disc becomes insecure. Ideally use a DTI to check disc runout, otherwise use feeler gauges between the disc face and the fixed caliper carrier or similar.
EXPLAINER What goes wrong and why?
Discs may not run true but could wobble slightly when rotated, called run-out. Calipers should accommodate some run-out, sliding-calipers oscillating with the movement and fixed-calipers’ pistons moving out and in, effectively cancelling each other out. Too much run-out will be felt in the brake pedal. Binding will make things worse.
If the wheel bearings are loose, the run-out of the disc/ drum won’t be constrained, potentially increasing pedal travel and judder on first application. Oval drums will cause judder because the narrow cross-section of the drum will press in the wheel-cylinder pistons, raising the brake pedal; the broader cross-section will allow the pedal to sink.
Running-in new discs/drums maximises the contact areas of the pads/shoes and heat-treats the disc/drum material. Heavy braking will cause overheating, and leaving the brake applied afterwards when stationary will cause uneven cooling, potentially causing distortion. Braking lightly and often for about 200 miles is best practice.
