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Car Care & Repair Tips - April 25, 2003

What Your Mechanic Doesn't Want You to Know


Reader Question: I bought a 2002 Camry last February and now the steering wheel is vibrating whenever I slow down or come to a stop. The tire pressure is okay so what could be causing this to happen?

Thank you, Rita

Dear Rita,

Actually I get this question a lot and it seems to bring on more questions than answers, so let me try to explain what is going on with the help of some illustrations. The steering wheel is vibrating because the front brake rotors are warped (we call this vibration "shimmy"). Ok, this is where I usually lose people…rotors? Warped? What does this have to do with the vibration in the steering wheel? So let me explain it this way.

Let's say the disc brake rotor is like the white center part of an Oreo cookie, and here is a picture of a brake rotor Click here. This disc brake rotor is what the brake pads grab to stop the car. Let's also say the brake pads are like the two dark outer pieces of the same Oreo cookie (I am starting to get hungry), and the brake caliper holds the brake pads, one on either side of the brake rotor. Here is a picture of the disc brake rotor and the disc brake caliper that holds the brake pads Click here. When you step on the brake pedal inside the car, brake fluid is sent to the brake calipers causing hydraulic pressure to squeeze the brake rotor in between the brake pads.

So what causes the rotor to warp? The brake rotor can become "warped" or have an uneven or high spot on the surface due to normal wear and tear. It can also warp because of expansion and contraction of the metal disc rotor due to driving through a puddle of water on the exit ramp after a long trip on the freeway then applying the brakes, or due to severely worn out brake pads that have been grinding metal to metal on the brake rotor, thus cutting grooves in the rotor surface.

So what can be done to correct this problem? The brake rotor can be removed from the car and "trued" where the mechanic uses a special piece of equipment to shave off a small layer of the brake rotor to make the surface smooth and "true" or even again. Here is an illustration of the truing process Click here. The brake rotor will have a limit to how much of the material can be removed or shaved off, and this minimum thickness is stamped on the rotor for the mechanic to see. If the rotor is below minimum thickness specifications, it will have to be replaced.

How does this rotor warp cause the vibration in the steering wheel? When the brakes are applied, the caliper and the brake pads squeeze the rotor which causes the car to stop. Remember the brake rotor is turning the same speed as the wheel. If the rotor surface is warped, the brake pads will pulsate inside the caliper as they come in contact with this high spot. The pulsating will cause the wheel to shimmy or vibrate, and this vibration can be felt inside the car in the steering wheel if the warped rotor is on the front of the car. The vibration is usually felt when applying the brakes in a panic type situation at freeway speeds, and can also be noticed when coming to a final stop like at a stop light.

Does rotor warp cause damage to my brake system? It can cause some damage or premature wear to the brake system, but most importantly it can cause a safety issue due to loss of control and full stopping power at freeway speeds, and can cause the anti-lock braking system to not work correctly. Your mechanic should inspect the condition of the brake rotors when he inspects the brakes, but if you are experiencing rotor warp be sure to tell him specifically that the problem exists. A visual inspection might not always determine a brake rotor is warped and in need of truing.

This valuable tidbit is just one of the many cost-saving items you will learn about when you visit our Website TrustMyMechanic.com.

Sincerely,

Austin C Davis

Austin C Davis


Interested in an e-book about everything your mechanic doesn't want you to know? Sound advice from Austin Davis. Click Here!


The above article is provided for the interest and entertainment of our visitors. The views expressed in this article are only those of the author, who is solely responsible for the content. AutoGuide.net does not endorse any of these views, and is not to be held responsible for any of the content provided in the above article.


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