Change of brake pads = change of rotors? Fact or myth?

[Lotus Man]

So I was told that with BMWs, once the pads wear out, the rotors are pretty much done, too. Is this true? Of the numerous vehicles I've owned, I've never had one that worked this way. For once, I'd like to get a least a couple of brake pad changes in on the same set of rotors, before shelling out big bucks for complete pads/rotors replacement with every brake service!

My three theories on why rotors have to be changed before getting too thin:

1. The walls wear down so much that they risk exposing the ventilated channel veins (inside the walls). Once exposed, the pads will catch on these exposed veins and shatter the entire rotor. But seriously, that wall is so thick, that it would take a lifetime of pad changes before even disappearing to expose the center ventilated channels.

2. Too thin of rotor walls present too much clearance for pads to make contact. Brand new pads at full caliper piston extension can only reach so far towards the rotors, so having the rotors too thin would imply the pads and brake pistons reaching their maximum distance of travel, and still not make contact with the rotors. But seriously, are the tolerances on the piston/pad travel that small that slight change in rotor thickness would affect grip?

3. Brake piston/pad barely makes contact the thin rotor. But they do the job in stopping the vehicle. But the brake piston is extruded past normal service distance from the caliper housing, just to assist the pad in making contact with the rotor, that the surface around the piston (outside the o-ring seal) is constantly being exposed to environmental elements/brake dust, resulting in premature wear and tear. But am I thinking too much?

In essence, I'm starting to sense a conspiracy theory here -- when I just want to replace brake pads in my X5, I'm always told that the rotors pretty much have worn down to minimum thickness requirements and that I need to replace them, too (despite them being brand new when installed with the same set of pads!). Somebody please verify this. Thank you!

[Tim]

If you don't have brake pulsing (warped disk) and the rotor measures within tolerance there is no need to replace a rotor.

[smarttask]

In a word, BULLsh-t

[Boston X5 4.4]

BMW is renowned for soft brake rotors, they give much better stopping performance, but tend to wear out fairly quickly. Despite this, it is not always necessary to replace the rotors with every brake pad change - if the rotors are within tolerance.

The major reason for replacing rotors is due to excessive lip on the outer circumference of the contact area. A large lip will cause the pads to sit on an angle, reducing contact between pad and rotor to less than half of the pads width. This results in very low pedals, and diminished braking capacity.

[Wagner]

Soft rotors = GREAT stopping power.

Yes BMW builds its cars to have the rotors replaced, any BMW tech will tell you that turning rotors on a BMW is really not an option and they should be replaced every other pad change.

[X5 Meister]

I'm not sure about the soft rotors theory, but after having changed pads myself at least 40 times in my life and dealt with numerous dealers here what I can say on the topic.

1. The bottom line is that once the rotors are below minimum spec they are no longer recommended to be used safely. The thinner they get the hotter they will become when stopping the car. You eventually run the risk of overheated brakes. Plain and simple.


2. The calipers have plenty of space to push thin pads onto thin rotors. Just make 100% certain that you have enough brake fluid in the system/reservoir for this to happen!

3. Under the free maintenance plan BMW always changed the pads and rotors together in my experience.

4. Front pads/rotors obviously wear faster than rears. I've measured very accurately the wear rate on my front rotors and extrapolated the mileage I would get out of a set of rotors. With mostly highway driving I can usually get away with changing rotors on every other pad change, with one very big exception below.

5. Because the pads eventually 'bed' into the rotors they seem to make a channel or groove in the rotors that gets deeper as they wear down. From my own experience, YMMV, the outside edge of the rotors always develop a 'lip' that is significantly higher than the surface that the pad has been contacting during braking. After having discussed this issue with BMW NA and several BMW Master Technicians this scenario is not recommended for a simple pad change because the new pads will not simply pop into the pre-existing wear channel and end up making less than 100% contact with the rotors. BMW feels this would significantly increase braking distances and so they do not recommend a simple brake change for reasons of safety. If your rotors, as measured in the channel, having plenty of wear left on them then you could take them off and have them machined to be perfectly even (you can get this done at a local Pep Boys for example) and then you are good to go. Because it's a huge pain to put the car on stands, drive dirty rotors to Pep Boys, wait for them to be machined, bring them home, install them again I have generally opted with simply putting on new rotors. One alternative is to put new rotors on and sometime later have the old set machined to be used for the next change, but this is more work than I have time for these days.

Anyway hope this helps.

[The Cleaner]

Always replace rotors or expect problems with noise, grinding and pulse. You will be back to do the job again if you cut corners and try to get another 30k out of your rotors.

[smarttask]

Never done 30k in any car, always change it before the guarantee
has runout.

[Lotus Man]

Thanks for all the great responses! I get a better feeling now that I know the rotors were designed to wear somewhat quicker than other vehicle make/models.

[///MB]

My front pads and rotors, but ONLY rear pads were replaced at around 25k under free maintenance. I've had terrible brake squeal ever since and have taken it in several times for this issue. Most of the time they say some squeal is normal! I don't see how my car had no brake squeal before and now it does and it's normal!?! Also, it only squeals when I'm driving slow, like in a parking lot, which I think is very embarrassing. I'm at 46k now and took it back in and they finally replaced my rear pads and rotors but not the fronts. I still have terrible brake squeal though and they said it's from the fronts, which is normal!

I had 7mm left on the front pads at that time. The brake noise seems to be getting worse. Does anyone know how thin the pads have to be before they replace them under warranty? One dealer told me 5mm, another told me 2mm. I wanna try and get new front pads and rotors before 50k. Also, how thick are pads when new?

Thanks.