Warped brake rotors question

Hey all, Occasionally, I experience pedal vibration when braking which I'm guessing is due to slightly warped brake rotors. My question is, why does it only occur in some instances and not others? I figure it should happen all the time. (especially applying same pressure etc) I've had the same thing happen on other cars so I guess it's more of a curiousity thing more than anything else. Thanks! -Moto

You don't mention if there are any certain conditions when the vibration happens. IE: amount of pedal pressure, brakes cold or warmed up, etc.

I have seen varying amounts of brake pad deposits on rotors. There may be an area on the rotor with more pad residue on it than on other areas. Remove your wheels and inspect both sides of the rotors. If you see darker and lighter area, then sand down the darker areas to remove the excess pad material on the rotor. The rotors may be a bit warped, to cause this, or it can be caused by using the parking brake when to rotors and pads are hot, or by keeping your foot on the brake when the rotors and pads are hot. Sanding down the pad residue spots is just a first step, and is no way a cure-all. It may solve the problem for you.

A lot of times it can actually be pad deposits on the rotor. It feels like a warp, but it can go away with another heat cycle. Basically, the rotors and pads got so hot at one point that pad material started to transfer surfaces. If your rotors are fairly new and you've only been doing some "spirited" driving, you probably just have pad deposits.

Here's a link to a pretty interesting article from the StopTech site on The Myth of Warped Rotors . Here's a quote from the article: "In fact every case of "warped brake disc" that I have investigated, whether on a racing car or a street car, has turned out to be friction pad material transferred unevenly to the surface of the disc. This uneven deposition results in thickness variation (TV) or run-out due to hot spotting that occurred at elevated temperatures."

To answer your question about why it seems worse some times than others, if more than one rotor is warped (which is usually the case) then the pulsation will vary depending on the allignement of the warped rotors. For example, if the right rear wheel hits it's high spot followed by the left rear, followed by the right front, followed by the left front, there may appear to be no pulsations what so ever. But if all wheels hit their high spot at the same time, the pulsation would be severe. Rotors don't actually warp. As pointed out this pulsation is usually caused by pad material transfer or uneven hardening of the rotor caused by overheating. The parking brake uses different shoes (not pads) so using it when the brakes are hot will not cause a problem.

Slight contradiction to Gator (sorry), but using the parking brake when the brake system is hot *can* warp the rotors. The pressure of the parking brake on the inside of the rotor (the parking brake is a drum brake inside the rotor hub) can be enough to cause a deformation. However, your brake system has to be pretty damn hot for that to happen. If you were to have an incredibly hard run on the track, don't forget to do a cool-down drive AND don't use the parking break when you park. My former car, Subaru STi, had horribly hot brakes. Even with ducting, 2-piece slotted rotors, etc., I could never seem to keep the brakes cool enough. I thought it was just me, until I bought a 944 track car and the Cayman . I've not had brake problems on either car since.

As a footnote to what usa35 said, applying the parking brake after a track session can also melt the brake shoes onto the inside of the hub/drum.

Agree with Gator Bite. When the brakes are hot do not apply the brakes while the car is stationary. When stationary, brake pad will transfer its imprint to a hot rotor, causing pulsation. A few warp speed to 20MPH drop with a good amount of brake force will cure the problem.

As several others here have said, the pulsations that you may feel may be secondary to deposits on your rotors from the brake pads. Re bedding brake pads will often resolve the issue. Here's a good link on the subject: StopTech : Balanced Brake Upgrades

Also curious if you have aftermarket spacers. Your photo looks like you might. Out of round spacers can do some damage.

Thanks for the responses everyone. Very informative. Let me see if I can answer some of the questions: I would say I think it happens more often when warmed up but not positive. The amount of pedal pressure, I will say it varies from slow stops to hard stops. Sometimes it happens, sometimes it doesn't. Very hard to pin down. Car has never been tracked but has been on a few spirited drives. Also, it only has ~8000 miles. I have never used any spacers. And it seems the problem actually started before switching to HRE rims and I torque everything down every time I remove/install rims. Thanks for the links and the suggestions. I guess I'll be doing some reading. -Moto