Steering vibration most likely caused by bad tire, wheel

Hey there, time traveller!
This article was published 29/08/2003 (8103 days ago), so information in it may no longer be current.

QUESTION — My 1984 Pontiac has developed a vibration in the steering when I drive around town.

On the highway, the shake in the steering wheel goes away. I have had the tires balanced and the shake is still there. Now they tell me I need a wheel alignment to fix the problem. I had the alignment checked a few months ago and everything was fine. Why should the alignment suddenly be wrong, and will doing one fix the shake in my car?

ANSWER — The vibration or shake you feel in the steering wheel is most likely being caused by a bad tire or a bent wheel. When a tire is out of balance, the vibration levels increase as vehicle speed increases. Your vibration disappears at highway speed so tire balance shouldn’t be the cause of your problem.

A tire on the front end of your Pontiac may have had a belt deform inside the rubber. When this happens, the tire’s tread doesn’t follow a true path around the tire. If you look at the tread as the tire is turning, you may see the tread suddenly move sideways in one spot. As you drive the car, this sudden sideways movement of the tread pushes the steering back and forth and is noticed as a shake in the steering wheel. The shake disappears at higher speeds because there is less time for the steering to move as the tire rotates faster.

Moving a tire with a shifted belt to the rear of the vehicle will lessen the effect of the shake and this is sometimes used to diagnose which tire is faulty. Replacing the tire is the recommended repair.

Other causes of a shake at low speeds are a bent wheel, incorrect mounting of the wheel on the hub or a bent axle. These conditions are usually caused by a slow-speed accident such as sliding the car into the curb on an icy street. There may be no obvious damage to the car and it may work fine on slippery road surfaces but when the tires have traction again, then the shake is noticed. Locating a bent part is done by rotating the wheels or axles and measuring the side-to-side movement with a dial indicator. Typical maximum sideways movement is .045 inch (1.143 mm).

A wheel alignment may be required on your car because of wear or damage to steering components but should not fix the car’s shake!

QUESTION — I installed a new set of brake pads on my 1992 Cougar. The car seems to stop fine but a friend tells me I should have had the brake rotors machined to provide a smooth break in the surface for the new brake pads. I have driven the car a couple thousand kilometres already and wonder if it is too late to have the rotors machined or do I really need to do it?

ANSWER — Brake rotors are machined (turned, in the trade language) to correct two problems: score marks on the rotor and warped rotors. Slight scoring on the brake rotor is acceptable. If the grooves are worn deeper than the raised edge of a dime, then the rotors should be machined. The equipment used to machine the rotors must produce a very smooth non-directional finish to enable the new pads to bed in. If your rotors are scored excessively, it is not too late to have them machined but the pads will take a little while to bed in because they may be worn to the shape of the rotor.

There is a minimum thickness the rotor can be machined to. If it becomes thinner than the specifications, then the rotor cannot dissipate heat quickly enough under braking and brake performance would decrease. The minimum thickness is stamped or cast into the rotor. Avoid machining rotors unless there is a problem.

Warped rotors also need machining to make the rotors surfaces parallel. A warped rotor will cause brake pedal pulsations and may even be noticed by passengers as a vibration during stops. Your car doesn’t have these symptoms so I would suggest they are not warped enough to require machining.

Jim Kerr is an experienced mechanic, instructor of automotive technology and freelance journalist. You can e-mail questions to Jim at the address below.

kerr.jim@sk.sympatico.ca