Brake Rotor Issue - The Unofficial BMW M5 Messageboard (m5board.com)

Joe Lapicki · 12th December 2002, 17:23

I am experiencing a lot of front end vibration upon application of brakes after only five (5) days at track driving schools this year and 23,000 miles on the car. BMW replaced the rotors at about 7500 miles for the same problem (no track days at that time). Service guy is ordering new rotors and said something about replacing "thrust rod bushings" as they had some play in them.

Should I consider upgrading from stock rotors and pads? Recommendations please. Thanks.

TCM · 13th December 2002, 02:49

Definately sounds like warped rotors to me. Before you go purchasing aftermarket brake components, make sure that your wheels are torqued correctly using either a torque stick on an air wrench or torque wrench. By over torquing the wheel, it is possible to warp the rotors due to the sheer force being applied to one specific area of the rotor. But if you continue to plan on participating in track events, a brake upgrade definately should be considered.

greg · 13th December 2002, 03:56

Rotors are made from cast iron. They do not warp. You CAN get front wheel shake under braking due to unbalanced tires or loose suspension components, but there is a far more common reason.

When you did your track days, I'll bet big money that there was at least one time you came to a full stop while the brakes were still hot, and you left your foot on the brake pedal. This causes the uneven transfer of pad material to the rotor - evidently .0001" can be felt and .001" is "severe".

Please read this article.

Paul S · 13th December 2002, 08:31

forget the new rotors. try hawk blue pads for a few days.
when you drive back into the pits after a track session, be careful not to leave your foot on the pedal at all. it's hard because there is traffic, you drop your instructor off, etc. but that's where the problem lies, as greg described. check out the new "warp-b-gone" technology stoptech recommends in this thread.
hawk blue pads

Stem · 13th December 2002, 10:48

Shell out some $$ for the floating rotors, the M-technic duct kit, and get your pad of choice. All are discussed in this forum. Your brake problems will be a thing of the past.............if you cool them down before parking your car at tha DE.

rsbj · 13th December 2002, 11:53

I'd be willing to bet your rotors are warped from the heat of running around the track. The brakes aren't that big, or have that much of a heat sink for such a powerful and heavy car.
When rotors warp from stopping a car and leaving the brakes applied, it's due to the uneven cooling resulting from the relatively cool pad clamping against the hot rotor. I can't say I've ever seen pad material deposited onto the rotor, but I've been wrong before. Generally .004" "runout" or warp is grounds for the lathe. I feel most people would lower this to .002" or so for extraordinary cars, such as ours.

If anyone knows the runout spec for the M5, I'd like to know. Need that service manual, yesterday!
Rick

Joe Lapicki · 13th December 2002, 16:42

Here's the latest......picked the car up yesterday with just the thrust rod bushings replaced (the rotors are on order) and the vibration is nearly gone. Any thoughts?

Need4Spd · 13th December 2002, 19:56

What are thrust rod bushings?

kevin034 · 13th December 2002, 20:16

I also agree with Greg and others. When coming to a complete stop at a track event (or while breaking in your new brake system), when brakes are red hot, DO NOT leave your feet on the pedal. It's one of the worst (if not THE WORST) things you can do to your rotors. Because of the heat of the rotor and pads, if you come to a complete stop the pads leaves an uneven deposit on the rotors, AT THE POINT OF STOP. So, imagine a rotor w/ a "micro speed bump" on it, plenty of viberations to be felt.

Try hawk pads, I believe that they have higher metal content and will shave away the uneven deposit on the rotors. It should smooth out the rest of your viberation problems.

greg · 14th December 2002, 06:10

Quote:

Originally posted by rsbj
I'd be willing to bet your rotors are warped from the heat of running around the track. The brakes aren't that big, or have that much of a heat sink for such a powerful and heavy car.
When rotors warp from stopping a car and leaving the brakes applied, it's due to the uneven cooling resulting from the relatively cool pad clamping against the hot rotor. I can't say I've ever seen pad material deposited onto the rotor, but I've been wrong before. Generally .004" "runout" or warp is grounds for the lathe. I feel most people would lower this to .002" or so for extraordinary cars, such as ours. If anyone knows the runout spec for the M5, I'd like to know. Need that service manual, yesterday!
Rick

rsbj - I believe you are wrong. I repeat - Please read this article.

if you want to convince me otherwise after reading the article, I'm all ears. But you'll have a tough time matching Caroll Smith's credentials (do the names Amon, Andretti, Foyt, Gurney, Hulme and Ickx ring any bells? They ALL drove Smith's cars. )

dwasifar · 14th December 2002, 06:21

Quote:

Originally posted by greg
Rotors are made from cast iron. They do not warp. You CAN get front wheel shake under braking due to unbalanced tires or loose suspension components, but there is a far more common reason.... uneven transfer of pad material to the rotor....

Please read this article.

I read the article before responding, but I'm not convinced by it, for two reasons:

1) To say the effect shown in the article's Figure 2 is not warpage is bandying semantics. The rotor shown in the diagram heated up and changed shape. I'd call that warpage.

2) I spent a fair amount of time in my earlier days running a brake lathe for an auto parts dealer, and my experience was that some rotors WERE visibly warped. When you resurface a rotor, you take off the material a little at a time. If the "bump" spot comes down to metal before the cutting tip even touches the surface elsewhere on the rotor, that rotor is warped, fancy talk about brake pad material deposited on the rotor notwithstanding. The lighter the rotor, the more likely you were to see that effect; big heavy full-size-car rotors were less likely to be out of whack than thin compact-car rotors.

Perhaps the writer is unused to seeing warped rotors because the rotors he sees are not typical. His exposure seems like it is primarily to race cars, with heavier brakes that are more likely to fail from some other cause before they will warp, rather than everyday street vehicles. I can't speak to that point; I just know what I saw when it was my job to fix it.