Originally Posted by
PromptCritical
Thanks! It is an interesting thread as is the one about the descent of Mt. Ventoux in France. The failures surprised me.
How did you test the cooling of the disc (I'm a recovering engineer) brake? I've thought about making an air scoop to cool the disc.
The failure on Mt. Ventoux seems to be credited to the leader telling people to drag their rear brakes. If that's what actually what happened, then that clearly demonstrates why that's terrible advice!
Here's what I did to test how quickly rotors cool down:
Test 1: I dragged one brake until it chattered--and then let it up and coasted for about 30-45 seconds before stopping with the other brake. The dragged rotor was still somewhat hot, but cool enough to touch! The other rotor was too hot to touch.
Test 2: I dragged one brake until it chattered, and then I quickly stopped with the other brake. Both were too hot to touch, and they both took a long time to cool down while we were stopped.
Conclusion: Based on these tests, I believe that spinning rotors cool down surprisingly quickly as long as friction isn't being applied, and that stationary rotors take a long time to cool down. My real-world riding experience seems to correlate nicely with these findings as well. I'm guessing that all of the cutouts on the rotors are designed to increase airflow and cooling.
Limitations: This was a very small sample size, and the tests weren't done on exactly the same stretch of road. I used my fingers intermittently, not a real-time thermometer, to evaluate temperature. It was done on a cool day, probably around 50F. I didn't use a stopwatch, but rather did ballpark estimations of time. "Chattering" isn't a terribly objective way of evaluating temperature or speed. Many other variables weren't accounted for. Basically, a limited test that could be used as a starting hypothesis for a much more rigorous study. Anyone want to write a grant proposal?
One other thought: I've descended some long and steep hills on a MTB tandem. Wow, do the brakes get hot! On technical terrain, the "stab and release" technique isn't terribly feasible. Nor is aerodynamic braking. While I've never melted anything, or lost the brakes, I have certainly had massive chattering and a bit of fade before deciding to stop for a bit to let things cool down. My Magura MT4 MTB tandem rotors look like they've been hit with a blowtorch--but they still work!