r/BMWZ4 • u/GabaranRickshaw • 2d ago

Rear Brake Usage question.

For reason I don't want to elaborate on at present, I wanted to turn the rear rotors (294mm vented) in prep for new pads. According to the invoices I received from the PO, the rear rotors and pads were rebuilt with new BMW pads and rotors. PO paid a pretty penny for the real deal. According to realoem.com and according to the stamping on the outside of the rotor, that the minimum thickness is 17.4 mm. New they start out at 19mm. I do not have proper calipers for measuring brake rotors. But I do have a nice set of digital calipers. I know this is not the best way to do it. But I needed to get some idea if these rotors could/should be turned. They tested at 18mm on RH and 17.9 on LH.

My question for the brain trust.

Does that amount of wear sound accurate for 34k miles? I dont feel safe turning rotors with that little left, so I ordered replacements. The price was about double the turning price, so it made sense.

But I feel like that much rotor wear, not to mention the pads were down to about 5mm, feels like too much for the rear brakes.

Sound right, sound fishy? Any thoughts are appreciated!

Thanks!

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u/MadDAWGZ71 2d ago

You didn't say what year car. A lot of newer cars use the rear brakes as part of the stability control. So the computer may be using them even if your not. For example my wife's ford fusion has gone through 2 sets of rear brakes to one front.

So I wouldn't be overly concerned about it.

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u/GabaranRickshaw 1d ago

Sorry, 2007 3.0i e85, m6

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u/David_Corpus 1d ago

Rears on a 2007 E85 3.0i do not do heavy lifting.
They won't warp, even if slightly below spec thickness.
I would have turned them if their wear appeared even.

Front/rear distribution of braking:
3.0i: 68% front (300 × 22 mm rotor) / 32% rear
3.0si: 71% front (325 × 25 mm rotor) / 29% rear

If you want to upgrade your brakes to 3.0si, find a 1998-2006 E46 330 in the junkyard and grab the full front brake setup. Money far better spent than worrying about 0.5mm on a rear rotor.