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why your car shakes when you brake

if the steering wheel shudders when you brake from highway speed, or the pedal pulses under your foot, the usual cause is the brake rotors.

what is actually happening

the rotor is the disc the pads clamp. when it is no longer perfectly flat, the pads grab slightly more in some places than others as it turns, and you feel that as a shake. through the steering wheel means the fronts. through the pedal or the seat means the rears.

people call it warping, and that is usually the wrong word. rotors rarely bend. what happens more often is uneven deposits of pad material transferred onto the disc, which builds up in patches and does the same thing as a warp.

what causes it

  • heat, then sitting still. braking hard down a long hill and then holding the car on the brake pedal at the bottom parks a hot pad against one spot of a hot rotor. that is the classic way to create a deposit.
  • wheel nuts tightened unevenly or with an impact gun rather than a torque wrench, which distorts the rotor as it is clamped to the hub.
  • corrosion, particularly after a car has sat, and particularly on the rears, which do less work.
  • genuinely worn rotors, below their minimum thickness, which flex and are finished.

machining or replacing

a rotor with enough thickness left can sometimes be machined flat, which is cheaper. but rotors have got thinner as vehicles have got lighter, and on many modern cars there is not enough material to take off. machining one that is too thin gives you a rotor that overheats and warps again within months.

we measure them rather than guess, and tell you which of the two you are looking at.

do not wait on this one

a vibration under braking is a warning that the car is not stopping as well as it should. that is a different category of problem from a noise, and it is worth having looked at rather than lived with.