That's partially correct. They don't warp again; they haven't finished warping. There are two common things that can accelerate warping. One is to get the brakes unusually hot then splash water on them from driving though a puddle. A more common cause is failure to use a click-type torque wrench when tightening the lug nuts. The uneven clamping forces promotes warping during the heating and cooling cycles.
There have been huge problems with Chinese replacement rotors too. There is nothing wrong with the parts but when parts are made from cast iron here, they are set aside for 90 days to "age" before the final machining. Chinese parts are cast, machined, packed, and shipped right away, then they age on the car. Once they warp and are machined, they generally hold up just fine.
Sometimes the warped spot in the rotor puts extra pressure on the brake pads as it passes through them. That makes that spot get hotter than the rest of the rotor which promotes further warping. In that case, machining can be a permanent fix. What you are referring to can result in severe cases where a lot of metal is removed from the high spot leaving that section thinner than the rest of the braking surface. That could cause additional warping if the rotor sees a sudden temperature increase such as braking down a long or steep hill. If the temperature goes up over a longer period of time, such as prolonged city driving, the heat has time to migrate around the rotor and heat it up evenly.
When there is an excessively thin spot, it will expand and contract at a different rate leading to thickness variation. As the thicker part slides through the brake pads it will push them apart. That pushes the piston into the caliper which pushes brake fluid back up to the master cylinder and pushes the pedal up against your foot pressure. You feel that as a pulsation in the pedal.
Even without thickness variation, the entire braking surface can twist relative to the center hub. You won't feel that in the pedal unless it's really bad, but it will shake the caliper and tug on the steering linkage. You'll feel that in the steering wheel and / or in the seat.
As you can see, there are a lot of variables that affect repeated warping. In my experience with older heavier cars, warping was less of a problem and machining solved it with very few repeat failures. Newer cars have thinner rotors to save weight so very often they will already be worn down to the legal minimum thickness and can not be machined. The trade-off is they are very inexpensive now. Many shops automatically just replace them because the cost of cutting bits for the brake lathe and the additional labor time is more than the cost of the rotors.
My guess is your rotors have been replaced once or twice already based on the age and mileage. I have no opinion on whether it's better to replace them or machine them. Being frugal, I machine my own when they are still thick enough. As far as warping, they are likely to last just as long as new rotors. The least expensive route for you would be to remove them yourself then take them to a shop or auto parts store that has a machine shop. If you pay someone for the entire service, they will give you an estimate both ways. This is a good time for a brake inspection too. It might be time for new brake pads. Then, machining or new rotor installation will be included in the service.
Sunday, June 5th, 2011 AT 12:25 AM