rotors question
 

We've all heard about BMW not doing pads & rotors together anymore under maintenance. If your pads are low, they will do them, but it's tough getting them to do rotors. I had both my pads done (separate visits) but because my warranty/maintenance are about to run out, I wanted to see how close to minimum spec the rotors were so I had them measure. The statement is:

Front rotor minimum spec is 28.4MM when replacing pads but 26.0MM when not replacing pads. Rear rotor minimum spec is 18.4MM.

My measurements after 4 years: front 28.2MM and rear 19.1MM.

My first question is why would rear minimum specs be different from fronts? I know they definitely seem to wear faster (as do the rear pads). My SA also said that I shouldn't worry about the rotors unless I start feeling they're warped (brake vibration, etc.). Hard to believe - what exactly should I be looking out for, because I won't be doing pads for another 3-4 years at this rate (25k miles in 4 years)? Surprising too, with my 80% local driving. What are the measurements of brand new rotors anyway?

I just had both my fronts rotors/pads done. He said they always do both. Maybe try another dealer.

Front brakes do 70% of the stopping, so they are built heavier to start. More mass means that they can take more hard stops before getting too hot. In general, front brakes will wear faster from stopping. Brakes are also used for traction control, so faster wear on the rears can be related to the traction control intervening more often (given the rear wheel bias of power delivery).

As rotors get thinner, they will fade faster. You may not ever notice it, if you don't get them hot enough to fade.

^ so the only concern I'd have with rotors is less stopping power over time? No "catastrophic failure" scenario to worry about as they wear thinner?

The rear rotors are thinner to begin with than the front rotor, so they'll have a lower "minimum width" spec no matter what.

Factory fronts discs vary in size depending on model according to one website. My 35d has 348x30 fronts and 320x20 rears. Your '08 likely has 332x30 front and 320x20 rears.

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Not less stopping power. Each stop will be as good as with new rotors, until they get too hot and fade. Rotors act as a heat sink. When they have less mass (thinner), they will overheat sooner, because brakes produce more heat than they can radiate, and you will reach that point where they are overheated (and fade occurs).

Catastrophic failure is not likely until they are so thin that they are weakened structurally. It can happen, but that is an extreme. And they don't all fail at once.

Can anyone comment on the apparent fact that the SA is quoting two 'wear limits' based on where on not pads are being replaced? JCL, you'd know this....

In my experience the limit is the limit, and a 32mm rotor has a wear limit of 30.4; a 30mm has a limit of 28.4...

I'd NEVER run a rotor under the limit. Ever.

I suspect the two limits are for different purposes. The lower limit is the absolute safe limit. The higher limit is to do with "wastage" of useable rotor.

I checked with my SA, and I spook with some of the mechanics.

I was told, if the rotors are very close or past the wear limit they recommend change. If the rotors are in an area where it will last with the expected life of the new brakes they just replace the pads only.