My Technical Website on Brake Systems
Warped Rotors
Note: DTV stands for disk thickness variation, caused by uneven rotor wear and/or pad material deposited on the rotor. Pad deposit usually happens when brakes are hot and you let them "sit cool"; you should drive around at slow speeds while your brakes cool.
Warped Rotor:
Cut an imaginary plane through the center of the disc part of the rotor that is parallel to both surfaces of the disc, this is done when rotor is new.
If the rotor is warped this imaginary plane (now part of the disc) will no longer be parallel to a reference plane, that reference plane was also parallel to both surfaces of the disc when the disc was new.
In essence a warped rotor is a rotor that is deformed throughout its thickness.
A rotor with DTV (surface imperfection) issues:
The imaginary plane above is still parallel to the reference plane mentioned in the warped rotor definition, however the two surfaces of the disc are no longer parallel to each other.
Think of 2 identical planks of wood.
1st plank you take a planer to it in a haphazard way to the surface making the surface wavy. This is a surface condition.
You can sand the surface down to get back to a smooth surface.
2nd plank you steam heat and then bend around a steel pipe. This plank is warped. You can sand all you want, but you'll never get it straight again.
You can see why it's so easy to mistake one for the other; on the surface it all looks "warped"
Of course if the rotor is too thin then it can't be "sanded" smooth, but a warped rotor can't be turned no matter how thick it is.
Lug Nut Torque
Over-torque of wheel lug nuts is one of the prime causes of brake rotor distortion. This can lead to permanent warping of the rotors, uneven wear of the rotors and pads and lots of brake chatter (NVH).
With today's very stiff alloy wheels, like from BBS, SSR, Volk, etc., when you torque down the lug nuts the wheel-mounting surface will force what ever it contacts to take its shape. Which means whatever that surface looks like will be what the rotor looks like.
Get a torque wrench and check the torques on every lug nut and make sure they are within the specs (which you should be able to find in your owner's manual). And make sure that every lug nut is torqued down exactly the same. Even if all 5 on a wheel are within specs, not having all 5 be equal will introduce distortion.
And if you think that your light alloy wheel can't possibly be that stiff, you wrong they are MUCH stiffer than the brake rotor or even the hub.
WORD OF CAUTION: Don't assume that torque the lugs to the lowest range in the spec is the best. Try and keep it nominal not at the extremes of the range.
Much of the problems with rotor warp, brake chatter, disk thickness variation can be traced back to over-torque and uneven torqued lug nuts.
After coming back from the shop, get that torque wrench and check the lug nuts yourself.