Spectacular rear brake rotor near-failure

MikeWhitney · **Joined:** Wed Aug 20, 2003 9:15 pm **Posts:** 4724

Had a BIG surprise when I looked at the rear rotors on the M3 right after the timetrial yesterday. Never felt a thing -- only noticed when I pulled the car on the trailer and was strapping it down. I dodged a big bullet.

Happened to both rear rotors with slightly different cracking pattern. It's through the dics (see the light picture below). These are BALO rotors sourced from Black Forest in September 2006. They have about 5k street miles, maybe 6 autocrosses, and 3 total days on track on them. Only been driven in the rain a few times -- that is surface rust. I was running Axxis Ultimates on street and PFC 97 pads on track.

I have some theories about how this happened - any comments?

clinehall · **Posted:** Mon Feb 11, 2008 10:49 am

Wow!!! You are a lucky man!

Bernie Baake · **Posted:** Mon Feb 11, 2008 10:53 am

I've been told that the cracks occur when the rotor is cooling down not on the heating portion of the cycle. So your next session would have been spectacular, if you didn't feel the rotor imbalance. Makes you wonder what would have happened if the 30% or so of the rotor let go when you were rolling.

MikeWhitney · **Joined:** Wed Aug 20, 2003 9:15 pm **Posts:** 4724

I just sent this to info@balo.de:

Please forward this message to the head of your Quality Assurance department:

Hello,

I recently experienced failure of two BALO rear brake rotors on a 1997 BMW M3 street car, and I wanted to send you some information for your awareness, and if possible to receive some explanation of what may have gone wrong. I have used and trusted BALO rotors for many years.

The two rear rotors were purchased new in September 2006 from a local distributor, and I have driven 5000 miles and participaed in two high-performance driver's education events since. Two different sets of brake pads have been used with them -- Axxis/PBR Ultimates and Performance Friction PFC97 pads. While track use does thermally stress brake components much more than street use, I have been an instructor in these driver's schools for 10 years and I have never seen or experienced such a failure.

Following are pictures of the brake rotors:
http://lh5.google.com/whit32/R6_QIOvJ_G ... C01428.JPG
http://lh6.google.com/whit32/R6_PlevJ-0 ... C01448.JPG
http://lh3.google.com/whit32/R6_PouvJ-2 ... C01452.JPG
http://lh6.google.com/whit32/R6_PkevJ-z ... C01445.JPG
http://lh5.google.com/whit32/R6_PsOvJ-5 ... C01461.JPG
http://lh5.google.com/whit32/R6_P1OvJ-- ... C01412.JPG
http://lh6.google.com/whit32/R6_P4evJ-_ ... C01413.JPG

I am bringing this to your attention because total failure of the components at speed would have resulted in a very dangerous condition, with a back wheel most likely skidding, possibly at very high speed.

I would be happy to package these rotors and send them back to BALO for your failure analysis. I have studied materials engineering and have a BS and MS in Materials Engineering and have worked in Quality and Qualification for 10 years in electronics manufacturing -- so I am quite curious to see a detailed Failure Analysis report if it is at all possible to share that information with me.

Please respond with any questions, information, or futher directions.

Regards,
Michael Whitney
whit32@gmail.com
112 Pictureque LN
Cary, NC 27519 USA
919-387-4949

Scott Johnson · **Posted:** Mon Feb 11, 2008 11:20 am

turn off traction control next time.

MarcusMcRae · **Posted:** Mon Feb 11, 2008 11:23 am

Bernie Baake wrote:

I've been told that the cracks occur when the rotor is cooling down not on the heating portion of the cycle. So your next session would have been spectacular, if you didn't feel the rotor imbalance. Makes you wonder what would have happened if the 30% or so of the rotor let go when you were rolling.

If you look at the wear pattern near the cracks it appears the rotors were cracked while they were being used.

I always thought it was the front rotors that were prone to failure during track days.

Chuck Branscomb · **Posted:** Mon Feb 11, 2008 11:40 am

MikeWhitney wrote:

I have some theories about how this happened - any comments?

What's your thinking Mike? Can you tell where the crack originated on either rotor? Unless they are using thinner cross-sections in their design, I'm stuck on the material properties or a manufacturing error.

Richard Casto · **Posted:** Mon Feb 11, 2008 11:59 am

It is amazing that the rotor(s) are still in one piece with that large of a crack.

William Gravely · **Posted:** Mon Feb 11, 2008 2:01 pm

Mike, if you had broken these earlier, I could have done a write-up for you in my failure analysis class! I'm interested in what went wrong with these. Glad you're okay.

Vincent Keene · **Posted:** Mon Feb 11, 2008 2:25 pm

Damn. I've been running my same rear rotors for two years now. I have about 15,000 street miles and 15 track events on them. Not one crack or even a heat check.

My fronts only last about 3 track events, and for $59 each at NAPA, I can live with that.

Donnie Barnes · **Posted:** Mon Feb 11, 2008 3:39 pm

Step 1: Install fresh rear rotors on Spec Miata.
Step 2: Race said SM in 13 hour enduro with 8 pit stops and a 20 minute off track excursion.
Step 3: Run just fast enough to win anyway.
Step 4: Inspect rear rotors.

Our experience with step four was "damn, good thing we were off track that 20 minutes or so....we may have broken rotors before the race ended and not won!" They looked worse than yours except for most of the cracking was radial instead of having those big scary cracks running along the center section. A couple cracks looked like the grand canyon on ours.

Several folks did have rear brake failures, though they seemed to mostly be loss of pad, not rotor (which still makes you lose all your brakes, so it's terminal anyway). That's pretty bad for no more use than the rotors had, though, IMHO.

--Donnie

Chris Landi · **Joined:** Mon Mar 08, 2004 6:31 pm **Posts:** 579

Good thing that you trailer your car. Seems like a 'must' for the high speed stuff. Second good thing is that you're observant. The what-ifs are scary.

MikeWhitney · **Joined:** Wed Aug 20, 2003 9:15 pm **Posts:** 4724

Chris Landi wrote:

Good thing that you trailer your car. Seems like a 'must' for the high speed stuff. Second good thing is that you're observant. The what-ifs are scary.

I'm getting really, really hard on equipment the more I do this. The first year or two of track running nothing went wrong, pads last for ever, tires too. Now I'm going through hundreds of dollars in consumables. I have yet to get more than 4 days out of a set of pads or more than about 8 days out of tires.

There's a big big difference in equipment stress between running "fast" and "super holy fricking try to kill yourself fast".

Graham Jagger · **Joined:** Fri Aug 29, 2003 12:53 am **Posts:** 1718

You did dodge a bullet.

I'm surprised you didn't feel it. I would expect pulsation in the brake pedal even from the rear. From the front I'm sure you would have felt it and probably heard it coasting down pit road to grid.

My theory:

Look at the 1st picture. Read the crack from the right side of the picture to
the left side. More or less how the crack grew.

The crack on the right running across the surface. During the cool down cycle after a session. That started from the usual spider crack growing. Then it starts splitting. It grows in both directions moving to the outer and inner edge. The outer edge is the first exit for the crack. The outer edge has more temperature drop at a faster rate during cooling. Now there is only one place left to go. Towards the inner edge. This is a rear rotor and appears it has the emergency/parking brake inner ring. This means more material. Not the path of least resistance. So the weakest link is the thinnest material. That is to follow around that inner ring. At some point another weak spot on the rotor surface, from another spider crack, splits. The crack now starts moving through the weak spot towards the outer edge. The only thing left holding that big chunk of rotor surface in place appears to be the vanes.

A front rotor wouldn't do this as there is no path for a crack to chase its way around. It would just split the inner edge too and be done.

I don't consider this to be a defect. It's a plain old rotor. Never designed or intended for track use or race pad use. It did what they do. Failed due to heat stress over X thermal cycles. The cracks appeared at the places where the metal was weakest.

MikeWhitney · **Joined:** Wed Aug 20, 2003 9:15 pm **Posts:** 4724

Actually the vanes are not holding the rotor together -- only the outer 1cm or so of rotor surface. The crack didn't make it to the edge anywhere.

On the other side (don't have a good pic) the cracks are only through the hub/vane surface and hadn't yet made it "out" to the rotor working surface.

Edit - pic #3 might be misleading. This is the inside of the parking brake surface -- not the actual rotor brake surface. One of the cracks appeared to start there.

Here - pic #3 is a closeup of the top crack in this picture. It doesn't connect to the "main" crack anywhere:

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Spectacular rear brake rotor near-failure

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