In Reply to: Jim Cash - Brake Rotors posted by Zach A. on August 13, 1999 at 16:14:36:
: Dealer recommended replacing front pads and rotors on my 98 528ia with 20,500 miles. Took car to a local respected private BMW/MB mechanic who says pads only. Rotors have very slight lip - less than 1/16" @ top & bottom - and are not warped. No warning lights. Any thoughts?
If you hadn't asked me I would not have responded to this issue, because I am NOT a brake expert.
Having said that, I will state my own "opinions" on this.
I have 47,000 kms on my 97 540i/A and no real sign of problems. My driving is about half city and half highway. I do take my pads off about once a year and clean/inspect them, put antiseize on the calliper pins and check the rotors for any scoring, etc.
I have never changed my own rotors(on previous cars), only because you have to remove the carriers and I have never been able to exert enough torque on the carrier bolts to loosen them ( I don't have an impact wrench).
It surprises me a bit that your pads are down enough to change at 20 K miles. And that lip on the rotors is very small.
In theory you are safe till the brake pad warning light comes on - but by then them you have a fair bit of travel in the brake pedal, and not as much pad and rotor mass to soak up heat should you have to do a lot of braking.
The indicator that your disks need attention/replacement is scoring marks, a wide lip on the edge, or warping/uneven thickness that causes pulsing in the brakes, or vibration in the steering when braking.
Normally the rotors will last longer than the pads. Having said that, there is a school of believers that say change your rotors every time you change your pads. They have good reasoning for this, and I "sort of agree" but it depends on how you normally use your brakes, and the type of pads you use.
I "suspect" your dealer is advocating this approach. While it may not be absolutely necessary, it is erring on the side of ensuring your brakes are in top condition.
Personally, I would be inclined to go with just the pads since that is the recommendation of your independent expert.
What ever you do when you do replace the rotors always do both sides at the same time (I would even do the rears at the same time as the front if they were getting close). Also I would not recommend having the existing rotors turned. I did this on my E28 once and it only lasted about a year before they warped again.
I don't know if this is a factor or not, but after I wash the car I run it around the block using the brakes a lot to heat the rotors and dry everyting. Also, when it is raining, I drag the brakes for the last 50 yards before putting the car in the garge.
I am sure other people will chime in with even better information.