Rotors
 

how do i know i have to change my rotors ? does it necessary for me to replace them with my brake pads also?

Use a caliper (a thickness measuring instrument - not the brake assembly that straddles the disc ;) ) to measure its thickness. The minimum thickness is stamped on the rotor. Note that you should factor in the rotor wear with your new set of pads. I've found that the fronts won't safely go past 2 sets of pads, but the rears will be fine with 2 sets of pads. In other words, replace the front rotors with every pad changes and replace the rear rotor every other pad changes. That's my recommendation.

BTW...my observations are with regards to OEM pads and rotors. Other brands or types may varies.

I always replace rotors with pads. Call me wasteful but hey....its what I do.

so i guess i really need to replace it .my car now has 117k miles on it. still what would happen if i keep the rotors with the new pads

I can only tell you what can happen (not what will happen): rotor cracks, breaks, and you are :wow: ! I've only heard of this happening on race cars, but the X5 is a pretty heavy vehicle and I'd want my brakes operational at all times, but that's just me.

You do not need to put new pads on when you put on new rotors, but a lot of people do since they are 'in there' anyways.

Measure your rotor thickness against the minimum spec stamped on the hat of the rotor. FWIW, my old E30 325iS in stock class club racing (way under braked, imho) used to wear about 0.001 inches of rotor for a race weekend with race compound brakes.

If they are near or below the minimum spec, change them out.

If your rotors have developed a 'lip' on the outer edge from the pad wearing away the rotor significantly then new pads will never properly bed against the rotor and you will have sub-optimal braking and reduce the life of your new pads as well. IMHO, it is not necessary to change rotors with every pad change and there are 2 criteria I take in account for a rotor change:

1. The thickness of the rotor relative to minimum thickness.
2. The presence of a significant outer edge lip.

I can say that you DON"T have to factor in the wear with the new pads when measuring the rotors... BMW accounts for that. What we do at the dealership (for cars under warranty as it is controlled by BMW) is if the rotors are within specs we put a stop squeal solution on the new pads (which is merely graphite powder suspended in an alcohol solution). What that does is help keep the pads cool as you set them in as the pads are a fresh/flat surface and the rotors have small grooves cut in them. But usually (about 30-40% of the time) the cars will come in later with a brake squeal caused by the lip on the rotor and then we are allowed to change the rotors...

But on customer pay cars that the rotors are within specs I at least recommend rotors. Bottom line is, if you have the extra funds, go for the rotors. If you are tight due to the current economy, you can go without with the expectation of some future slight squealing as a possibility.

Great info. Does BMW have any spec on the size of the lip?

Good thread, I also need to have my rear brakes replaced, but I plan on having some friends who work at a good year do the job. I plan to get the parts myself though. This will be my second set of rear pads with the current rotors, but my car has 76k, and the brakes were last replaced at 28k. I guess they lasted a long time huh? Anyway, does anyone have a good link to parts I can use? What are OE pads and rotors?


Thanks!

No, just overall thickness.