When Should BMW and MINI Brake Rotors, Disc, be Replaced?
Automotive disc brake systems have an air of mystery and some fear for many vehicle owners, BMW and MINI owners included. While the job of our vehicle braking systems is certainly important, critical even, the system itself is actually rather simple. With this said, we certainly don’t want to blindly go at a “servicing” of the system without an understanding of the system, what it does, how it can fail, how it wears and how to actually determine what and when to service.
In this article, we will address the brake rotors, or discs. Specifically; why, when and how they should be replaced. We see and hear DIYers, internet experts and claimed service experts offering (often while pounding on the table) their input on when disc brake rotors should, and should not, be replaced. Brake discs do wear and fail in other modes and do need to be replaced periodically, but lets take a look at the actual reasons and modes that should prompt replacement.
Expert #1 claims that the brake rotors need to be replaced whenever the pads are replaced, due to the pads and rotors having become a matched set and running new pads on previously worn rotors will cause symptoms and issues such as; braking vibrations, braking noise and accelerated pad wear or even braking failure. In fact, the worn pads and the rotors will indeed have some wear pattern and the rotors may need to be replaced. However, if there has been no noise or vibrations from the brakes and the pad/rotor wear surfaces are basically smooth and free of rust, the wear pattern will typically not be an issue that “requires” rotor replacement. This is assuming the pads are being replaced BEFORE they have fully worn out and the metal backing plates are contacting the rotors (which does indeed require that the rotors be replaced) and the minimum rotor thickness has not been reached.
Expert #2 claims that the brake rotors need to be replaced whenever the pads are replaced, due to the rotors wearing out with the pads. This claim may note that the rotors ares simply “worn out” or that they will be worn too thin for further use causing imminent brake failure if new pads are installed without rotor replacement. This claim has some truth to it as the rotors do indeed wear and become thinner. There is a specific minimum thickness specification for each rotor and this is stamped on the rotor. With this noted, similar to the notes in #1, above, if the rotors have not shown vibration or noise symptoms, the wear surfaces are smooth and not overly ridged and the rotors are not rusty on the wear surfaces, they may be usable with a new set of pads. The final test for rotor useability is to measure the rotors’ thickness and compare these measurements to the minimum thickness specs. The minimum thickness specification will be stamped on the rotor, commonly on the outer circumference or the rotor hat. If the rotors have not reached the minimum thickness and it is determined that there is enough material to run through another set of pads, the rotors can be reused.
On the other side of these two replacement claims, we have those who would re-use any brake rotor that does not exhibit warpage (causing vibrations while braking), to include overly rusty, worn past the minimum thickness, extreme ridges (scoring) in the wear pattern and heavy edge lips (due to heavy wear). All of these conditions can result in a minimum of noise or vibration, to a reduction of braking power to actual brake failure. In the comparison of these schools of thought on brake rotor service, we’ve gone from the extreme of replacing every brake rotor regardless of condition to attempting to never replace them (regardless of condition).
Click image below for replacement rear BMW and MINI Brake rotors and Brake renewal parts and kits:
Examples of common brake rotor wear and faults:
When does a disc brake rotor NOT specifically need to be replaced? A brake rotor may be considered reusable if ALL of the points below are met:
NOTE: Even if all of the points below are satisfactory, reusing a brake rotor that has wear from a prior set of pads, increases the potential for brake noise when new pads are installed.
* The brake pads have not worn fully through the friction material, i.e., the brake pad backing plates have not made contact with the rotor.
* There is no vibration or pulsation in the suspension during brake application.
* There is no squeal or squeak noise generated during brake application.
* The pad wear surface is relatively smooth and without obvious scoring.
* The rotor is not at the minimum thickness specification (or less).
* The wear surface width is equal to the brake pad’s friction material width, i.e., the pad is making full contact with the rotor and wearing the surface evenly.
* There is no rust within the pad wear area on the rotor.
* There are no cracks in the rotor’s wear surface, or anywhere else.
* The external, non-wearing, surfaces of the rotor do not exhibit excessive flaky rust
What are the indicators that a disc brake rotor SHOULD be replaced? A brake rotor should (may we argue … MUST?) be replaced if ANY of the following conditions is evident:
* The brake pad friction material has worn fully away and the metal pad backing plates are contacting the rotor.
* The vehicle exhibits a pulsation or vibration when applying the brakes. While this could be due to worm control arm bushings (as the most common alternative cause), if the pulsation or vibration only occurs while applying the brakes, it may likely be due to a warpage, pad material transfer or hard spots on the rotor …. all of which, require rotor replacement. Actual warpage should be no more than 0.003″ of run-out in the rotor’s surface.
* The rotor’s pad wear surface exhibits noticeable scoring.
* The rotor is at or very near the minimum wear thickness.
* The rotor surface has noticeable lips at the edge of the wear surface transitioning to the unworn surface.
* The rotor’s wear surface shows uneven pad contact across it’s face.
* The rotor’s wear surface shows rusty areas where the pads should be making contact.
* Inspection shows micro-cracks in the rotor’s wear surface.
* The external non-wear surfaces of the rotor are heavily rusted and flaky, especially the cooling vent vanes between the inner and outer rotor wear surfaces.