Blame that shimmy on a warped brake rotor

Question: On a recent drive through the San Gabriel Mountains, my steering wheel started to shimmy violently when going down hill. The problem went away shortly after. Can you tell me what caused this problem? And now that it has gone away, can I forget about it?

S.T.

Answer: You can forget about it, but only at your own peril.

The shimmy you experienced was almost certainly caused by warping of the brake rotors, which caused the steering wheel to vibrate when you applied the brakes going down hill, said Steven Mazor, safety and engineering chief at the Automobile Club of Southern California.

Mountain driving subjects braking systems to the most extreme service, and the mountains of Southern California are particularly tough because of grades that involve more than 5,000 feet of elevation gain.

The work of slowing a car against the pull of gravity occurs at the brake rotors, finely machined cast-iron disks about the size of a dinner plate. The rotors spin along with the wheel. When you apply the brakes, brake pads squeeze against the rotors to stop the car.

To avoid vibrations through the steering system and brake pedal, the rotors must be almost perfectly flat. If the rotors are warped by even several thousandths of an inch you will begin to experience braking problems.

As you brake while driving down hill, the rotors heat up. High heat can cause the rotors to warp, and any existing warping will get worse.

Mazor advises that you have the entire braking system inspected. “The rotors will not go back to flat when they cool down,” he said.

One reason rotors often get warped is that automakers are trying to save weight, and the rotors are particularly heavy.

Although automakers can save a significant amount of weight by making rotors less beefy, there also are residual problems. That’s because rotors are part of the “unsprung” weight of the car, along with the tires and wheels. Reducing the weight of the rotors allows engineers to reduce the weight of the springs and the shock absorbers. The weight reductions improve fuel economy and trim manufacturing costs. But a less beefy rotor is more likely to warp.

Sam Memmolo, a mechanic and nationally recognized brake expert, said almost any rotor can warp if it gets hot enough.

Warp also can occur if mechanics do not evenly tighten the lug nuts or lug bolts that hold the wheels on the hubs. The uneven bolt pressures cause stress that warp the disks when they get hot, he said.

It is possible to machine the warp out of the rotors, though it is possible they will warp again.

Memmolo recommends that you have your tires inspected to make sure the steering wheel shimmy is not coming from a damaged tire belt.

Another recommendation when driving in the mountains is to use the engine to help brake the car. By setting the transmission in a lower gear, which is possible on most automatics, you can relieve the brakes of a tremendous amount of work going down hill.

Consult your owner’s manual to determine how much you can safely rev the engine in a low gear, but almost all engines can withstand about 3,500 revolutions per minute and most can withstand 5,000 rpms.

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Ralph Vartabedian cannot answer mail personally but responds in this column to automotive questions of general interest. Please do not telephone. Write to Your Wheels, Business Section, Los Angeles Times, 202 W. 1st St., Los Angeles, CA 90012. E-mail: ralph.vartabedian@latimes.com.