I'm looking for experiences or theories for what the heck happened today…
During track session #2 of day #2 of HPDE today, my left rear wheel suddenly locked. I had just done a hard slowdown (80 to 30 mph) and a 90 degree left turn (Turn 1). The locked tire skid mark shows that the lock-up began long after I was off the brakes, was accelerating, and was tracking out of Turn 1. The skid mark went about 40 yards before I turned to the right to enter Turn 2 and spun out because there was no rolling left rear tire. The rear end came around my left side and we went oversteering off-track into Turn 2's curbing and plastic posts and completing a 270 before coming to a stop in the grass; it was exciting.

Behind us, a Camaro's driver and instructor confirmed they saw the same thing.

My HPDE instructor and I had no warning anything was amiss before the car's rear very suddenly came around. As we sat off-track scratching our heads, we wondered if we hit oil or had spewed oil onto our own rear tires--until we got out later and saw the long skid mark.

Once the flatbed tow truck hooked up to my car and started pulling, all four wheels rolled--so the lock-up had resolved by then.

It is entirely possible that I bumped over the smooth concrete curbing on Turn 1.

This is a 2008 MCS R56. I had DSC turned off. The rear brake rotors are MINI (34216774987); the rear brake pads are EUR1309 and had about half thickness remaining. I was in the Green (beginner) group, but my feedback was that I had been handling braking very well--and hard--for a Green driver. (My front pads, Hawk HPS, were a little higher-performance. I was making them smoke the day before, was told I'm starting to out-drive them. They were new this week; I had been trying to make a compromise between a track weekend and daily driver use.)

The non-performance rear pads are probably going to draw a lot of, "it was those pads" responses. Any specific or other ideas? I also have trouble accepting that the pad would weld to the rotor in a split second, on the fly.

After not being able to identify the cause of the lock-up, I bailed on the last two track sessions of the day, rather than risk it happening again, in a worse location.

Sorry about the long post. I'm trying to give as much detail as possible.

 

Sounds like you came through alright, hope the car was unscathed. Good thinking to bail on the last two sessions.

I'm scratching my head about this, the only theory I have, and I'm not sure it works, is your brake fluid. Had you done a flush with high temp brake fluid before going to the track? Maybe the brake fluid reached the boiling point, so that when you were off the brakes and released the pressure from the pedal, the expansion from the boiling brake fluid caused the brake to be applied again. Then after having a chance to cool for a bit it would go back to normal. Seems a stretch that boiling brake fluid could do that because usually it just means you have poor braking , not a locked brake.

For this to happen on just the left rear probably requires some other out of normal condition, like the sliders sticking or maybe the e-brake cable not being adjusted properly so the left rear brake is dragging a little and causing it to overheat. The front brakes do most of the work, so I'm surprised that the rear would overheat like that. I'd remove the caliper and the sliders, clean everything, and reassemble with high temperature brake grease. Then do a flush with Motul RBF600 or other high temp brake fluid. Also it would be interesting to see the pads and whether there is any asymmetric wear.

Let us know what you find!

 

...and at least as important as "What happened?" is, "What should I do about it now, to regain confidence it won't happen again?" Thanks for taking that next step, squawSkiBum.

I did flush brake fluid with ATE Type 200 yellow seven days before.

 

The car seems unscathed: The red front bumper shows white evidence of mowing down two plastic corner sticks, and I have flat spots on the two rear tires. (Thub-thub all the way home.)

In the OP, I wrote I may have "bumped over the smooth concrete curbing on Turn 1." That perhaps sounds too rough, but I wanted to mention it. It is smooth curbing, a fairly gentle slope up and down. I had been rolling over the curbing many times over the weekend, on purpose. It was not a rough bump in the MCS suspension. It didn't feel like car damage--I wouldn't have been willing to trash my daily driver car for purposes of lap time fractions.

 

I would pull off the rear brake pads, check the sliders, and also pull up the rear dust boot and check for rust or corrosion under the boot. The rear calipers are known to freeze up over time from road salt that gets in past the piston dust boot.

 

You did a good job preparing for the track with the upgraded front pads and brake fluid flush. There goes my theory on the brake fluid though.

Definitely check those rear calipers! If you have to deal with winters and road salt, after 8-9 seasons they may need some TLC. Where are you located and what track?

 

Scratching my head as I read this I have to agree with the previous posts on things to check and then I wrote this... We know the Pistons only apply pressure when brake pressure is applied, this is either done at the pedal or the DSC, unless your instructor had somehow subconsciously grabbed the hand brake and was using it to hold onto but that would have more than likely resulted in a event causing both rear brakes to lock. So since your foot was off the brake and on the gas I would have to point my finger at a .... DSC. Yea, I know it sounds ridicules, but with my experience working with computer control modules and other various types of control electronics I am quick to turn my gaze at the magic that is Electronics. So why the DSC, well it has to do with the ABS and the combination of the two, again we know that when DSC is engaged it has a function of monitoring the wheel spin speed and if one or more wheels are spinning faster then the others then the brakes are applied via the use of the Isolation valve and the dump values per each wheel as appropriate stop /slow the faster wheels down, inversely when stopping when the wheels are not spinning as fast as or have stopped spinning the ABS again steps in and opens and closes the appropriate valves to release brake pressure to allow the locked wheel to again spin at the same rate as the others, generally the sensors that read all this information checks wheel speed around 30 times a second. SO... this is what I think happened... with the DSC off as you were under heavy braking setting up for the turn, everything was fine then as you accelerated the wheel sensors caused a reading glitch and the ABS valves for the Left Rear act up , my guess is the isolation valve closed and the pressure that was applied during the heavy braking was maintained in the left rear and the dump valve failed to allow the system to release. and then it wasn't until the system had a chance to relax momentarily that the ABS valves returned to normal and normal function was restored.


So what does all this mean... well you can check to see if any error codes had been thrown when this happened if you have the SCAN tool to look at your computer, this could give some additional insight to what/why it happened. But because my suspicions lay intertwined in the magic that is the electronics aspect of the controllers this even could happen again, But then again... it might not. But before the next HPDE event you may want to really take a good look at your ECS/ABS system (stem to stern) just to make sure it is all operating at peak performance.


Best of luck.


Motor On!

 

This car has always lived in North Carolina, where our roads only get a light salting about four days a year. This track was Carolina Motorsports Park in Kershaw, SC. The air temp was about 90 F.

I usually hear that overheated brake fluid causes loss of braking--can it also cause a brake to lock up? All four wheels were running pretty hot, the air temp was hot, and the rear brake pads may have been marginally inappropriate.

When I get the codes checked and calipers taken apart tomorrow (by a local performance shop), I'll let y'all know what we find.

 

I thought the overheated brake fluid was improbable, since you had flushed with ATE 200 I'm confident that wasn't the problem. I have tracked my car multiple times and never seen anything like that.

Let us know what the shop finds, I'm curious.

 

UPDATES
I have some updates to offer, but I still don't have a great answer for the puzzle, and I might never solve it.

MECHANICAL
The calipers were taken apart and reassembled, and nothing seemed awry.

DIAGNOSTIC CODES
Several DSC codes were stored in the computer, though I don't think we have any way to know when they got stored, over the past nine years.

Here are the codes, and what I think they mean. I would welcome corrections:
1. D35DH: No idea what this code means.
2. 5DFB: Maybe Interface Signal Fault. There is a service bulletin that says this code can get tripped due to the steering column switch cluster fogging due to out-gassing of the steering wheel components.
3. 5D8C: Wheel speed sensor, front left, short circuit to earth. This one is interesting to see, but it's not related to the wheel that misbehaved. Maybe someone briefly unplugged this sensor during service, years ago.
4. 5DF0: I have conflicting information about this one:
a. DSC sensor voltage fault (maybe could arise from dead battery).
b. Failed abs module and steering wheel sensor.
c. BMW 3 series forums say ABS hydraulic pump motor failure--sometimes its the ABS computer and sometimes the pump.

It's no surprise that these codes were also stored:
5. 5E5C Front brake pad wear sensor
6. 5E5D Rear brake pad wear sensor
7. 5E31 DSC CONTROL UNIT, VOLTAGE FAULT, UNDERVOLTAGE
8. 5E30 DSC CONTROL UNIT, VOLTAGE FAULT, UNDERVOLTAGE

I have had draining and dead batteries, so I'm not surprised about an undervoltage fault code being stored.

We did clear these codes, and I'll check again after a month or two of city driving, to see whether any come back.


THEORIES
1. May never know what happened, but it's related to the rear brakes being marginally inappropriate for the track, and things just got too hot. This might mean we reached the higher boiling temperature of the high-temp brake fluid, or somehow the heat caused the ABS wheel speed sensor to be fooled, or....
I do promise that I will have higher-performance brakes all around, if I ever track again.

2. A tripod corner made ABS freak out. I hear MINIs sometimes lift the inside rear wheel when cornering hard, and I've heard of a BMW ABS system that locked the rear tires during a jump*. The tripod scenario roughly matches what my car did, except this should be happening to a lot of track and autocross MINIs if that's the reason. Also, I'm still pretty certain I was clear of Turn 1 before the skid mark began.

*In this anecdote story, the ABS let the tires roll again after they dragged on the ground for a short spell, like 10 yards.

3. There might actually be an ABS hydraulic pump or electronic controller problem. It's an odd coincidence that it happened during the short time I'm on the track. On the other hand, the extreme conditions of being on-track could have exacerbated an underlying problem. Code 5DF0 is a little alarming, but I'm not sure I'm ready to throw a thousand dollars at a shot in the dark.