Quote:
Originally Posted by travisjb
Chris, I'm really interested in what you find out on the brakes... Figure it out and you've got some +reps coming your way !!!  Perhaps you could test combination of ducting and ABS bypass both?? I think the lack of air is a serious issue as well |
What I have learned so far is that the problem manifests itself once heat soak reaches a certain part of the brake/wheel assembly. With appropriate pads you can drive like a maniac, heat the rotors to insane temperatures and everything will work fine. Let the car sit, and the heat soaks into other parts, calipers, wheel hub, sensors etc, then things go wrong.
On Saturday I had the liberty of spreading out some auto-cross runs over a period of time. After the first run on our long 110 second course the brakes worked fine and I let the car sit and cool down for a good 20 minutes on a nice 70 degree day. At the end of that first run oil temperature exceeded 260F, and brake temperature was probably a reasonable 500-600F, calipers were cool to the touch. After that 20 minutes, the brakes rotors had cooled to below 200F (I could touch them but not hold my hand on it) and the calipers now felt quite warm to the touch (approx 120F). On the very next run after that 20 minute cool down the brakes malfunctioned on the second application of the brake pedal which was approximately 30 seconds into the run. It's hard to hear what is going on over the engine, but you can hear the ABS actuator pulsing the front brakes and you feel as though you're driving on loose sand except that there is plenty of lateral grip. You will also feel the pedal push up just slightly but you don't feel much modulation. If this was a panic stop from 70 mph the stopping distance would have been 40-50% longer than normal and you likely would have crashed, fortunately I'm only hitting cones at this point.
Other observations, if you brake gently and straight enough you can avoid the problem 90% of the time. If you suddenly lift off the gas and hit the brake pedal firmly the computer is programmed to boost the brake pressure enough to activate ABS (Brake Assist) and you will experience the problem. To avoid this you have to lift off the gas and then gradually apply the brakes, which costs time and distance.
Which part exactly is being affected by the heat I'm not sure, but I'm fairly confident that it is not the rotors. They are generating the heat which then radiates out to other parts. High friction pads make the problem worse because they are more grabby and make it easier to activate ABS and generate more heat more quickly.
Ducting may help to reduce temperatures but it won't solve the problem.
Next time out I'm going to try and bring an infrared pyrometer and get some more accurate readings. I will also try and get a fuse module that I can hack up so that I can disable ABS without killing the VVEL and the power windows (the module covers 3 slots).