Brakes: Warped rotor?

[ddub]

When I rotate the right front wheel on my Avanti I experience increased drag at some point during the cycle. I am guessing this indicates a warped rotor. If that is the case is there anything to do other than turning or replacing the rotor? Are there other causes I should be looking for?

[Gunslinger]

Do you get a shimmy when braking? If so, that would indicate a rotor needing turning or replacing.

What kind of wheel is on your car? It could be scraping the caliper...particularly if you have the Turner kit. Some wheels might not fully clear the calipers.

If you have the original Bendix/Dunlop calipers, make sure the caliper is properly shimmed and the wheel bearings are properly tightened.

if everything else checks out, then turning the rotor should be in order or replacement if it's too thin or too far out of spec otherwise.

[Dan Timberlake]

I'd say a little variation in drag might not indicate a problem. Like Gunslinger said, How does it feel when light, medium an hard braking?

[Xcalibur]

Is it a recent development or has it been that way for some time? Have you hit anything that might have bent something... just some other possibilities, but it's most likely the rotor. Unless it was replaced at some point, most "originals" (or long ago replacements) are usually waaaaaay past the specified service limit and more susceptible to distortion, especially when over-heated.

Mic 'em and you'll likely find both could do with being replaced.

[StudeRich]

Usually, when you have a warped Rotor you can feel the pulsating on the Brake Pedal when braking.

[ddub]

Thanks for the input, guys. I will do some more investigating. I have not noticed the tell-tale pulse in the pedal.

[Dan Timberlake]

There is a tolerance for thickness variation of a rotor. It's pretty tight, usually less than 0.001 inch.
Even bargain Ch*n*se calipers usually have thickness variation 0.0003 or less.
Variation in thickness would try to force the pads apart, which the stiff caliper and powerful hydraulics would not be happy about, so feed it back to the brake pedal.

Then there is separate lateral runout spec, which varies with make and model. On the order of 0.002 to 0.005" TIR.
It must be accommodated by some means.
1 piston calipers slide on well lubed pins to accommodate that runout. 2 or 4 opposing piston calipers are mounted solid, and the interconnected pistons move back and forth to track the wavy (but uniform thickness) rotor. since the fluid volume is constant (when one piston goes left, the opposing one goes right) the pressure at the pedal stays the same.
But there is only so much lateral wobble that will be invisible on the pedal.

A thou or 2 runout could cause noticeable drag when turning by hand.

[studegary]

A thought down a different path - When cars with disc brakes are not driven for a period of time, the rotors rust where the pads are not located. If you were to turn the wheels then, you would get an uneveness (rub - not rub - rub- etc.). This "period of time" varies a lot. I have had extreme brake problems where the brakes weren't used for months. I have also had problems the next morning after using some wheel cleaning chemicals. Other times, I have had cars sit for weeks/months and not noticed anything when they went back into use.

[ddub]

Thanks, Gary, that is a possibility.