Brake problem! need advice!

[JCL]

There isn't much mention here of the pads being properly bedded in, both when the vehicle was new, and when the rotors were replaced. Rayxi mentioned it, and I agree with those comments, but I think it should be emphasized.

The sense of having less braking power is likely most related to the lack of bedding in, and is not related to the surface area of the pad that is in contact with the rotor. Braking torque is a function of clamping force and rotor diameter, not of brake pad contact area. Clamping force is a calculation based on the hydraulic system specifications. This all assumes that the pads have been bedded in, and that the brake rotor surface has had some pad material transferred to it.

The variable I left out of the previous example is pad coefficient of friction. It varies greatly with pad selection, and can vary with temperature. But, given that there is no mention of changing to a different pad composition, I would look to the bedding in step.

The 'warping' may have to do with the brakes being 'ridden hard and put away wet' to use an old horse analogy. The driving habits being described may include use that crosses over into abuse for the composition of the standard OEM pads, and the vehicle may have been parked with the brakes very hot; that would result in pad material adhering to the rotor as the pads remain in contact with the hot rotor. This results in a problem that is often referred to as rotors being warped. This can also be related to water hitting the hot disks, as Powers1 mentioned, causing uneven cooling and resulting transfer of pad material.

DinanM3, the X5 is much heavier than your M cars. Have you considered whether the brakes may be being overheated and that this may be part of the problem? It seems to me that the rotors aren't the problem, as they were replaced and the same problem re-occurred. That makes the rotors sound like a symptom, not a cause.

I also don't agree that you always must change pads when rotors are changed. Often, yes, But if the pads are near new, and are worn evenly, but are glazed, deglazing them can be sufficient. (glazing is usually heat related) Then, they need to be bedded in.

Hope this helps.

[Mackskibum]

Dinan, does your dealer properly torque the lug bolts when they perform the service? Aside from all the other issues with old pads/new rotors mentioned above, this is one of the most common issues with alloy wheel/rotor combinations. I always retorque the lugs after the delaer has my car- whether its the X or anything else. Typical technicians will attach the lugs with an impact and some use a "wobble stick" The only correct way to attach the wheels is with a click or beam type torque wrench.

I generally have no problems with rotor warpage and haven't for a long time. I have had some "sensitive rotor vehicles" most notably a 1995 and 2000 Jeep grand cherokee ltd. and was able to get 80000+ miles from a set of rotors without the dreaded brake pulsation. I actually got into an argument at a delaership (Pontiac) with the service manager about this issue and he stated that they don't have time to hand torque wheels. I then asked him if they use wobble sticks for their rods, mains and head gaskets. He said no- they use a torque wrench- why? because its more accurate. I left and never went back. I have also seen techs guess by how the impact sounds whether they have the torque correct.