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Broken Rotor
What happens when you wear down a brake rotor

Report by Rich Merritt

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{ Click the thumbnail photo to see a LARGER version. }

Here's the warrior, after three track days at two events over a five day period.  We ran it one session too many.

At 100 mph, entering Turn 1 at Blackhawk Farms Raceway, I hit the brakes and heard a CLUNK.  Here's what we saw upon returning to the pits.
There used to be a rotor in there.

Here is it, with the wheel off.  Note the small piece of rotor still clamped in the caliper.

Marks on the inside of the wheel show the impact of rotor pieces flying around in there before emerging as a
"flaming mass."

The rotor broke into to four pieces and nearly set the grass on fire at Turn One.

Note that there are no visible stress cracks.  The other rotor looks just like this, with three or four deep cracks.
It would have been next.

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The Report

Well, gang, I've done it now. 

I broke a Porterfield cryogenically treated rotor into four pieces at Blackhawk Farms Raceway (South Beloit, Ill) today. 

I was coming into Turn 1 at the end of a straight, pumped the brakes once to see if I had pedal, and then hit the brakes at about 100 mph.  I heard a big CLUNK and then no brakes.  Well, severely diminished capacity, anyway.  I went sailing off the end of the straight into the grass, did a little offroading, and managed to escape without hitting anything. 

I heard later that the start/finish line crew reported seeing "a blazing mass" come off my front wheel.  The corner worker at 1 said he thought I had broken off some pieces of carbon fiber bodywork, until he saw them almost set the grass on fire.  "Good thing it isn't July, or the grass would be dry, and it woulda caught fire," he said.  "The only thing I have here to put out a grass fire is my cooler full of beer."  (For after the event, of course).

Anyway, I got the pieces.  The rotor broke off at the hat, and then broke into three pieces.  I'm missing the third piece of the rotor puzzle, but I think maybe it's still clamped in the caliper.  We put the car on my buddy's trailer and brought it home.  I'll take it to Denny's Mufflers to have it taken apart by experts.  I don't know if there is any damage to the caliper, or if the caliper is frozen (melted?) to the rotor, or what.  I'll let you know, and I'll take pictures.

Amazingly, I still have brakes!  I can actually drive the car, and it stops.  Not very well, of course, but it stops.

As some of you may recall, I have broken rotors before, but they were PowerSlot piece-of-shit rotors.  When those rotors broke at the hat, I had NO BRAKES!  This time, I had enough brake to keep out of the trees.

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The Verdict

Here are the results of my recent researches into the mysterious exploding rotor:

It was simply a matter of a too-thin rotor.  Thickness on the rotor is:

New:      1.185
Discard: 1.118
Mine:     0.991

Its partner in crime, on the other side of the car, is showing deep, 3/4 in. long cracks, and would have been the next to go.

When we turned the rotors last week, the shop said they were getting close to discard depth, so we just trimmed off the sharp edge on the outside, where the pads cut into the rotor, and did not turn the rotor.  I took a spare rotor along, figuring that if a rotor got too thin, it would warp.  Alas, it didn't warp, so I didn't replace it trackside, and Bang!  It broke.

I guess putting three track days on a marginal rotor was too much.

The lesson here is:  Don't let your rotors (stock or Porterfield) get below discard depth.  This may require taking a ruler along to the track.  Also, look for cracks before going out.

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Other Observations

1.  Carbotech pads must be a little wider than others, because they did not cut into the rotor.  They completely cover the rotor, so there was no sharp edge on the outside.

2.  Cryogenic treating really works:  Up until the bitter end, thin as it was, the rotor did not warp.  It's warped now, of course.

3.  The four pieces of the broken rotor do not show any cracking, surface or otherwise, except where it broke.  Its partner has deep cracks, but there are only a few of them (of course, that's all it takes).  It looks as if the other rotor would have broken the same way into three or four pieces.

4.  It apparently had nothing to do with water injection.  It was simply a too-thin rotor.

5.  The missing piece of the broken rotor was captured by the caliper.  If it had not captured the piece, there is an excellent chance the flying metal would have seriously damaged the caliper.  I have to rebuild them, because a seal is leaking.

6.  I have interesting shiny gouges on the inside of the wheel where the rotor pieces bounced around before escaping.  No serious damage, though.

7.  I drove the car to the brake shop this morning.  Strangely enough, the brakes still worked on only three wheels.  I probably could not stand an emergency stop, but I managed to get through stop and go traffic just fine.  The last time I broke a rotor, I had no brakes.

8.  All three times that I broke rotors (two Powerslots, one Porterfield), it was on the right front.

Rich/Slow old Poop

'94 3000GT VR4 with:

Big Red Porsche calipers
Porterfield cyrogenically treated rotors
Carbotech Panther Plus pads
BFG SS lines
Motul 600 fluid
Air ducts
Water cooling to the brakes

The brakes were working perfectly up until the very second the rotor exploded.

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Article and Rotor Images 2003-2004 Rich Merritt, All Rights Reserved.
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