Warped Rotors? Are You Sure? How to Diagnose BMW and MINI Brake Rotor Problems
I have a 2008 535i. My shop says I need to replace my brake rotors because they are warped. I am frustrated because this is the third set of rotors that they have procured and installed in a few months (they install new pads each time too … it is starting to get expensive). I can’t believe that this is normal for a BMW. Is there any way you can help?
Thanks, [BMW Owner] Geoff
Here at BAVauto, we get a lot of contacts from BMW and MINI owners who wish to replace their brake rotors due to apparent warpage. Often, the replacement rotors (regardless of brand) will fix the problem for a bit and then it’s back. Alternately, the replacement rotors do not change anything (except the bank account). If the replacement rotors do not fix the issue at all or the issue returns in short order, it’s likely that the fault lies someplace other than with the rotors.
If you experience a vibration or shake while braking, there are a number of potential causes for this. It may, or may not be, directly due to damaged brake rotors.
1) The brake rotors may be damaged. Brake rotors can indeed be the cause of a shake or vibration under braking. In these cases, the rotors may actually be “warped” with the friction surface not running true to the rotor’s hub mounting face Additionally, damage that requires rotor replacement may be due to rust (from longer periods of sitting plus high moisture in the air or there may be pad material transferred to the rotor’s friction surface causing a change in friction at that spot or the rotor may have been over-heated and the friction surfaces have hardened spots which offer differing friction (similar to pad transfer).
2) The brake caliper(s) may be at fault. The caliper piston may be sticking in the bore and may not be fully releasing pressure against the brake pads (which are pressing against the rotor), the caliper guide pin bushings may be sticking on the guide pins or the pads may be stuck in the caliper and/or bracket guide tracks. Any of these cases will cause the brake pads to remain in contact with the brake rotor, with some amount of pressure (as if you had your foot on the brake pedal to some degree).
3) The front control arm bushings may be weak. BMWs and MINIs are very susceptible to front-end vibrations due to weak thrust arm (also known as strut arm, stress arm, compression arm, tension arm, upper arm and others) bushings. As the bushing material ages and becomes effectively softer, the arm can oscillate against the bushing in compression or tension, worsening as the brakes are applied. Basically, this is like pushing or pulling against a spring (in very small movements). When the brakes are applied, at speed, this compresses or pulls on the bushing, which then allows more movement of the control arm and the whole arm starts to oscillate …. which is felt as a front-end vibration.
4) Wheel lug bolt torque may be uneven. Improperly and/or unevenly torqued lug bolts can cause the rotors to stress unevenly, resulting in brake pulsations.
5) Brake caliper hoses may be deteriorated. The interior of the brake fluid hoses will deteriorate over time. The ultimate result is that the inner diameter of the hose is pinched closed due to swelling of the rubber. In this case, the high pressure fluid can push past the closure, to the caliper, when the brakes are applied, but the pressure will only partially release through the closure when the brakes are released, resulting in the caliper still applying pressure to the pads and rotor, overheating both.
How To Diagnose Braking Vibrations or Shake
Before disassembling anything, perform the following road test:
Find a smooth parking lot or section of road (it is important that the pavement be smooth, so that you can feel what the chassis is doing), and slow the car from about 20 to 30 MPH, using varying light to moderate pedal pressures (however, keep the pressure the same for each test run). If you can feel the car surging as it comes to a stop, at least one rotor is warped or otherwise damaged (pad transfer, etc.). If no surging can be detected, the shake or vibration problem may very well be related to the control arm bushings.
* Brake pad material transfer -
Pad material transfer can happen if the pads and rotors are very hot and then the vehicle is stopped before the pads and rotors can cool down a bit. A close inspection of the brake rotor friction surface can usually spot the signs of pad transfer. There may be a clearly defined or a ghosted area that looks like an imprint of the brake pad. Pad material transfer can often (but not always) be eliminated by re-bedding the pads to the rotors. With the brakes fully warmed up, from 60-mph, use the brakes to slow the vehicle in a moderately aggressive manner, down to about 20-mph (no need to “throw the anchor out”, but more aggressive that your normal braking). Accelerate back to 60-mph and repeat. Do this 3 to 4 times, then drive at about 60-mph for a minimum of 1-mile to cool the pads and rotors. Be sure to ONLY perform this procedure in a safe place and a safe manner, with no other traffic. If the issue remains, replace the brake rotors and pads.
* Sticking brake caliper pistons -
The caliper piston(s) may be sticking in the bore(s). In this case, the piston does not fully retract when braking pressure is released and it keeps some amount of pressure on the brake pads, pushing them against the rotor. This creates extra heat and overheats the rotor, causing it to warp and/or also transfer pad material (as noted above). A quick test can be performed as follows; 1) Lift the wheels off the ground (using an appropriate floor jack and jack-stands), 2) Apply the brakes firmly, then release the brake pedal, 3) Spin the wheels, they should spin freely. If one or both show noticeable drag, the caliper piston is likely sticking (or caliper guide pins, as in #3, below). Note that the rear drive wheels, or the front on 4wd vehicles, will not just freely spin due to the drag of the axles and drivetrain. However, if one side has more drag than the other, it may be a sticking caliper. In this case, the cure is to rebuild or replace the sticking caliper(s) and replace the pads.
* Sticking caliper guide pins -
Most BMWs and MINIs use a single-piston brake caliper. In these designs, the caliper floats (or slides) on cylindrical rods (guide pins) so that the force of the piston extending from the bore causes both sides of the caliper (and the brake pads) to press against, or pinch, the brake rotor. The guide pins are actually bolts with polished shanks (these are the Allen-head bolts that you remove in order to replace the pads). The caliper has rubber bushings pressed into it. The guide pins fit through these bushings. The caliper then floats, or moves side to side, on the guide pins via the rubber bushings. With age, the bushings can begin to stick on the guide pins. This causes the outer brake pads to remain in contact with the brake rotor after the brake pedal is released, overheating the pad and the rotor and potentially warping the rotor or causing pad material transfer. Performing the wheel spin test, as noted above, may show a dragging brake in this case. Additionally, the outer brake pad will typically show more wear than the inner pad. Finally, if only one of the two guide pin bushings is sticking, the brake pads may show a tapered wear. In other words, one end of the brake pad is worn more than the other end. The cure for sticking guide pins is to either remove the pins and clean all rubber off them, lubricate with the Sta-Lube synthetic brake grease and re-install …. or …. install new rubber bushings and new guide pins (with the Sta-Lube lubricant). We typically prefer to replace the bushings and the pins, as the bushings do soften with age (allowing excessive caliper movement and tapered pad wear) and the inner bores of the bushings may be rough and torn up (due to the transfer of the material to the guide pins).
* Sticking Brake Pad Backing Plate Mounting Ears -
Similar to the sticking guide pins, above, if the backing plate mounting tabs or ears are sticking to the caliper and/or mounting bracket, the pads(s) will not freely float. This will result in either the inner or outer pad showing more wear that the other pad. The pad(s) may also show tapered wear (more wear on the end of the pad that is sticking). In this case, the cure is to disassemble in preparation for new pads. Fully clean the brake pad backing plate guide/slide areas on the caliper and mounting bracket (using the Wurth Brake Parts Cleaning Wire Brush and Brake Parts Cleaner). Apply Sta-Lube Caliper Grease to the guide/slide areas on the caliper and mounting bracket (an old toothbrush works great for this). Clean and grease the guide pins and bushings, as in the notes above. Reassemble with new brake pads.