[david in germany]

Originally Posted by kring

Depends on age/miles of the rotors, they key is being in spec. BMW Recommends in their technician manual to resurface if there is enough rotor to do and keep in spec.

However, it ALWAYS best to replace both at the same time, but as you pointed out it cost a bit more. This will give you the best performance, life and quiet stable braking for another pad lifetime.

Rotors develop a resonant frequency and that marries the pads as they break in together. Quite often a resurfaced rotor and a pad will not develop into a harmonic match and you will forever have brake squealing, the degree depends on how harmonically they are out of sync. Or you may get lucky and they align. I have turned maybe 10 sets of rotors in my life and I’d say only twice it worked out perfectly… the rest either had squeal, relatively quickly they developed uneven stopping or accelerated wear.

If you can get them turned for nothing, you could try it.. but keep in mind that id its a bust, you may now be out another set of pads if you have to go for the full rotors the second time. You wouldn’t want new rotors with used bedded pads. Could you? Of course you “could” do that… but you might as well trade the S4 in for a prius at that point.

Give your car the love that it gives you and do a proper brake job on it.

Just lol…
Can you provide a copy and paste of said tech manual?
I have never EVER seen ANY BMW dealer / official garage that would resurface a rotor. It is not cost effective for them to take the time to do it. A BMW mechanic would simply check the minimum thickness and slap pads on the car if they were at or thicker than the minimum unless there is any damage visible to the rotor surface. It is the service advisors job to sell more parts therefore they will always try to upsell the parts once out of their cost.

Squealing comment… uh, ok.. addressed correctly a few times above.

Harmonic mismatch…. if you have surface contact you have friction, friction means stopping ability.