Distorted brake rotors can produce shudder

Published 5:00 am Sunday, March 27, 2011

Q: I noticed my steering wheel shudders slightly when I firmly press on the brakes. I’ve never noticed this before and wonder if it could be a result of my kids using the car for a snowboarding trip the previous weekend. They tell me they used the chains and didn’t run into anything other than pushing through some deep powder. What do I do? Is this serious?

A: Your steering wheel shudder is likely caused by distorted front brake rotors, due to heavy brake use. My assumption is the kids didn’t select a lower gear when descending one or more steep grades, using only the brakes to maintain speed, which can lead to excessive brake temperature. Brake rotors (also called discs) are typically constructed of cast iron and resemble a thick dinner plate. A caliper uses hydraulic force to clamp two brake pads containing friction material against the rotor, converting the vehicle’s kinetic energy into heat. The more stopping one does, the greater the heat produced.

Excessive brake heat can cause the brake rotors to become distorted, more specifically lateral run-out (warping) and thickness variations. In both cases a vibration can result, felt in the steering wheel, brake pedal or seat of the pants as the brakes are used. I should add that in severe cases, other troubling issues can develop related to the rotors and brake pads, but we’re likely not in that territory.

A slight amount of brake shudder may be considered harmless but can be annoying. In more severe cases, a loss of braking force can occur, making prompt corrective action important.

The first step for a fix is a brake inspection. The rotors are checked for thickness and variations in thickness, as well as for cracking, scoring and hot spots. If found to be in good condition and there’s sufficient thickness remaining, rotors can be resurfaced, either on or off the vehicle. Resurfacing the rotors involves using a lathe to remove a small amount of metal from each side, restoring a true, smooth surface. Rotors must have sufficient thickness to properly dissipate heat.

Wear and previous resurfacing, or machining, can result in a rotor not being thick enough to machine or reuse. In this case, replacement is the only option. Depending on the vehicle, replacement rotors can run about $100-400 per pair — avoid the cheap ones.

At the time of rotor correction or replacement, it makes sense to also renew the brake pads, regardless of their remaining thickness. If the rotors became hot enough to become distorted, the brake pads have likely seen better days. Also, since the calipers need to come off to access the rotors, it would only take a couple of extra minutes to swap in new pads.

Here’s my rule of thumb for preserving brakes on descending grades: Select a lower gear if more than three brake applications, lasting for longer than three seconds each, are needed within a span of a few minutes to maintain a safe speed. In a lower gear, the engine will provide hold-back, reducing or even eliminating the need to use the brakes. Third gear provides helpful hold-back for most vehicles in the 30- to 50-mph range, and second gear works well in the 20- to 40-mph range.