What are hot spots on brake discs, and why does my steering wheel shake when braking?
The vibration people call a “warped disc” is almost never a warped disc. It is usually disc thickness variation caused by uneven pad deposits, or hard spots where the cast iron has changed structure after extreme local heat.
Disc thickness variation (DTV)
When the pads clamp a disc whose thickness varies around its circumference, the clamping force rises and falls once per revolution. That pulse travels back through the caliper into the pedal and, on the front axle, into the steering. A variation of around 0.01–0.02 mm can already be felt.
DTV develops in two ways. Lateral run-out (the disc wobbling because of a dirty hub face or a bent hub) makes the pad touch one area more than the rest; over thousands of kilometres that area wears differently. And uneven transfer: pad material is deposited unevenly on the disc, so the friction surface becomes a patchwork of thicker and thinner deposits.
How deposits and hot spots form
The classic cause is stopping hard and then holding the brake with the car stationary. The pads sit hot on one spot of the disc and leave an imprint of pad material there. The next stops run over a disc that is no longer uniform.
If the local temperature goes high enough, the grey cast iron of the disc changes its microstructure at that spot (formation of cementite/martensite). These hard spots look like bluish or dark patches, wear more slowly than the surrounding iron and create thickness variation of their own. Typical triggers: track use without a cool-down lap, a sticking caliper or slide pin, pads that were never bedded, or repeated hard stops on a downhill.
How we measure it
Run-out is measured with a dial gauge on the disc face, with the wheel bolts or spacers torqued. Thickness variation is measured with a micrometer at several points around the disc. Visual inspection shows deposits (dull, patchy bands) and hard spots (bluish islands). Caliper slides and pistons are checked for free movement.
What can be fixed
Light, even deposits can sometimes be cleaned up by re-bedding the pads. A disc with real thickness variation and enough material left can be machined on the car, which removes run-out from the hub as well. Hard spots are a different case: they are harder than the iron around them, so machining often leaves them standing proud again after a few hundred kilometres. In that case the disc is replaced, and the cause (caliper, hub face, bedding, driving pattern) is corrected first.