Did DSC warp my rotors at the track?

[mhrir]

I went to Road America with the Z4MC this weekend for the Badger Bimmers DE event. I signed up for Friday in addition to Saturday/Sunday. Since it was much colder than when I was last at RA in July (where the car performed flawlessly) and the track was green for the first session I put the car in Sport mode, but left the stability control on. By the end of the first session the left front rotor (and possibly the right front to a lesser extent) had warped. As I continued through the day, the warping got worse until it seemed the whole car was violently shaking everytime I got on the brakes. The car stopped fine but I stopped having fun. Interestingly, for the first lap each sessiong the warping wasn't too bad but as soon as the brakes warmed up it was much worse. This was the fifth DE event on the stock rotors. I looked at them before the event when changing the pads/fluid and they had no visible wear or micro-cracks.

Other than the ambient air temperature (which should have helped with cooling) I have have come up with two variables. First are new brake pads. I used the same compound (Raybestos ST43) as the last time at RA. Second, is the stability control which I did not use the last time.

I don't think it was the pads which leaves the stability control. Could DSC have warped the front rotors?

[jragan]

Are you sure they're warped and don't just have pad material bonded to them from the heat?

[Finnegan]

Originally Posted by jragan

Are you sure they're warped and don't just have pad material bonded to them from the heat?

+1

[mhrir]

The surface of the rotors are clean, if that is what you mean. The pads look good as well. I did notice that all the vent holes were filled with pad dust and many were completely blocked, which I have never seen before (even the last time at Road America).

Here is a picture of the right front rotor. You can see the vent holes are almost completely blocked. Additionally, surface cracks have appeared. As I said, I did not notice them when I was putting the track pads on for this event.





I am in the process of switching back to my street pads. I will use my pressure washer to make sure the interior of the rotors is cleaned out.

[dekaliber]

After Putnam Park a week ago with Windy City I experienced similar brake dust clogging the drilled holes in the rotors. I had DSC off for 5 1/2 sessions (in one session I had forgotten to take it off until partway through). I don't think that abnormal.

The cracks on the rotor surface do not look good though.

[DerekSelmanRacing]

Can't really address the DSC issue; it may be entirely possible if the car was using brake differential force to stabilize. I always run all off but, 5-6 weekends on ft brake discs is not bad. Obviously, yours are toast...you don't need to bother cleaning them!

[mhrir]

I figured they were done. I was just surprised they suddenly warped at the end of the first session at Road America (which I know is a brake intensive track).

Next question... Should I get new OEM rotors from Tischer? Or is there a better option that will work with the OEM calipers?

[Serious]

Originally Posted by mhrir

I figured they were done. I was just surprised they suddenly warped at the end of the first session at Road America (which I know is a brake intensive track).

Next question... Should I get new OEM rotors from Tischer? Or is there a better option that will work with the OEM calipers?

Only other real option IMO are the PFC direct drive rotors.

http://store.bimmerworld.com/perform...scs-p1455.aspx

[Shipkiller]

My first track event after I picked up the MC earlier this year, I ran the whole first day with DSC on with no detrimental effects. DSC off for the second day and every event after... No issues so far.

I checked the run-out three weeks ago just before my last event and they are fine. Oh, checking run-out on drilled rotors is a PAIN...

[mhrir]

Originally Posted by Serious

Only other real option IMO are the PFC direct drive rotors.

The link says these rotors are for their big brake kit.

[silversprint]

change to a different track pad.

[The HACK]

Those rotors are fine. The fact that they were not "vibrating" when cold-ish means they're not warped. There's pad deposit on the rotors that leads to high spots, and the more you use the brakes, the higher and bigger those high pad deposit spots get. You only need to drive on them for about a week on the street for them to go away, or leave the track pads on and do a couple of hard, cold "stops" to swipe most of that deposit off.

I'd be more concerned with the rotor thickness (looks like it's got a nice lip on it) than the actual cracks on the rotors. Cracks on tracked rotors is a sign of life. Unless the cracks run all the way from the edge of the rotor I wouldn't worry about it.

If you don't want to pony up for the PFC Direct Drives, these are what I have on my front:

http://www.racingbrake.com/M3_E46_Co...p/2102-311.htm

I've had the same rotor on my MZ4 Coupe for about 3.5 season now and they're just now barely starting to show signs of use. I could probably go full 5 seasons before I need to replace the rings.

[mfanatic325]

^ I think you mean sessions, not seasons? But great info here guys, thanks! Good stuff to know

[The HACK]

Originally Posted by mfanatic325

^ I think you mean sessions, not seasons? But great info here guys, thanks! Good stuff to know

SEASONS, as in I've had these rotors on my car for the last 3.5 years with on average 15-20 days on track/auto-cross each year. For track use I've been exclusively on track pads (Cobalt XR3s and now Hawk DTC-60s). They will likely last me 5 years before I need new rotor rings.

Before anyone says "well you don't brake hard," well, I don't. The less you have to brake the faster you will go.

I will say this. The two "home tracks" for me, California Speedway in Fontana and Buttonwillow Raceway in Buttonwillow are two very brake intensive tracks. Cal Speedway has a 120mph -> 40mph HEAVY braking zone, a 90mph -> 30mph HEAVY braking zone followed by a 180 degrees decreasing radius hairpin, and a 115mph -> 35mph HEAVY braking zone followed by a combination of fairly slow turns. Buttonwillow has 3 back to back to back 100+mph braking zones. These brakes gets put through the torture chamber at least once every other month throughout the year.

[DerekSelmanRacing]

I can't believe your rotors last that long; I'm going to see if they have my application.

I use Hawk DTC60 on the front (will be changing to DTC70) and cryo treated discs. When I get moderate cracking, I don't get pedal pulsation but they do get wavy enough that there's a fair amount of drag.

[The HACK]

Originally Posted by DerekSelmanRacing

I can't believe your rotors last that long; I'm going to see if they have my application.

If they don't, you can probably see if the hub dimensions on the Z4M is the same as the regular Z4, and if the dimensions are the same you can potentially have a custom sized ring made to bolt onto the Z4M rotor hub and use that.

[mhrir]

Originally Posted by The HACK

Those rotors are fine. The fact that they were not "vibrating" when cold-ish means they're not warped. There's pad deposit on the rotors that leads to high spots, and the more you use the brakes, the higher and bigger those high pad deposit spots get. You only need to drive on them for about a week on the street for them to go away, or leave the track pads on and do a couple of hard, cold "stops" to swipe most of that deposit off.

I'd be more concerned with the rotor thickness (looks like it's got a nice lip on it) than the actual cracks on the rotors. Cracks on tracked rotors is a sign of life. Unless the cracks run all the way from the edge of the rotor I wouldn't worry about it.

It is hard to get excited about a link to rotors that are backordered...

I have since switched back to the OEM pads. From cold there is a slight pulse in the pedal when stopping. As the brakes heat up that pulsing gets progressively stronger, although nothing like what I was experiencing at Road America. There are no detectable deposits on any of the rotors (unless it is invisible) so I don't think that is the issue. While changing the pads, I spun the rotors and could not detect any warp with my eyes for what that is worth.

The best my butt sensor can tell is either the left front or left rear rotor is the culprit. I've already purchased new OEM rotors for all four corners to have as spares, so I am going to replace one at a time until it goes away.

The only other clue that I did not mention was that the left rear caliper piston was pushed all the way in when changing to the street pads. When I was at the track I could hear a slight rattle coming from the left rear when driving slowly over bumps (which I did not relate to my warped rotor issue). I duplicated this rattle at home by jiggling the inboard pad since it had plenty of space between the rotor and piston. It did not rattle when braking. I pushed on the brake pedal and the piston came back out and stayed. This is what makes me think it is the left rear rotor that is warped. As it gets hot it warps more which in turn pushes the caliper piston in.

Or maybe I should have my junior detective badge revoked...

[The HACK]

Pad deposit CAN be invisible to the naked eyes.

[mhrir]

Originally Posted by The HACK

Pad deposit CAN be invisible to the naked eyes.

Fair enough.

Would that explain why the warping gets worse (even with street pads) as the brakes heat up?

[The HACK]

Originally Posted by mhrir

Fair enough.

Would that explain why the warping gets worse (even with street pads) as the brakes heat up?

It's why I think it's pad deposit. Because if the rotor is really warped, it'll vibrate when hot or cold.

[mhrir]

Originally Posted by The HACK

It's why I think it's pad deposit. Because if the rotor is really warped, it'll vibrate when hot or cold.

So I replace the left rear rotor since I suspected it was the culprit. Sure enough, the pulsing pedal was gone. Yay!

Looking at the old rotor, I am wondering if what I am seeing is pad deposit. If so, how to remove it. Driving around normally with the OEM pads did not seem to change it. I have concluded that DSC was the cause. My guess is that it was frequently applying the left rear brake. That was the first and last time I will leave DSC switched on at the track.

[The HACK]

The slightly darker hues are pad deposit. You can either lightly sand down the area, or put race pads back on the car and while cold, do a couple of hard stops (don't be surprised if sparks flew) while cold.