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I'm having my car inspected (VT); its a 2014 BMW 328xi. The dealer is inspecting it and said there is a "heavy amount of rust on the inside of the rear brake rotors," and that they would have to turn them to pass inspection. The car brakes fine, there's no unusual noise or vibration or anything. This seems a bit suspicious to me; I've never had a car not pass inspection due to rust on the rotors, and the car isn't even that old. Does this seem legitimate, or are they trying to get some extra money out of me?

I had a chance to talk to the dealer; there's no pitting or anything else wrong with the rotor, and the brake pads themselves are fine, just what they are claiming is "excessive" rust. They said 2 hours labor, ~$200 to turn them.

left right

Resolution: Took it to a mechanic I trust, and it passed inspection fine. Said there was rust, but nothing where the brake pad actually hits, and no pits / grooves to worry about.

Andy
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It would suggest to me that the rear brakes aren't contacting the faces of the inside of the rear rotors efficiently. The car is approaching three years old and bear in mind that brake rotors are usually just mild steel. Being under the car they are subjected to all kinds of road salt, water, mud, slush and grime. If you leave a car parked up for a few days you'll start to see the rotors rusting. This is swept away when the car is driven but if the pads aren't hitting the rear rotors properly, I could well believe they'd get fairly corroded.

Also, most braking is done with the front wheels. As much as 85% of braking is done by the front of the car which is why the front brakes are typically larger with beefier calipers and pads. If the rear brakes were below optimal efficiency, there is a chance you may never notice.

I'd suggest going and taking a look yourself. It should be very easy to see either shining a light under the car or using your phone with the flash on to take a video with your arm extended if you can't fit your head under the rear of the car.

If they are corroded, I'd suggest having the rear caliper slides lubricated and adjusted as appropriate at the same time the rotors are turned, otherwise they may corrode again fairly quickly.

Steve Matthews
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im a nys inspector and i fail for rusted contact surfaces all the time. in your case from the pics i would not fail that rotor. BUT i haven't seen the inside of the rotor either. 99% of the time the outside looks alright its not until take a look at the inside you find they are hammered. my rule of thumb is if 50% of 1 side of the rotor is hammered i will fail it. but every inspector is different. its their license and their discretion.

Ryan Rohauer
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All brake rotors always rusting because they must be made from high carbon steel. Some coating or painted help but only cosmetic matters. The main point is hardness, and deformation resistance to heat and pressure. Nothing else matters. Brake pads should wear the rotors evenly, no deep scorrings, 1 or 3 thin scoring lines are normal from trapped stones or road debrees.

Bob Cross
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user21846
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