Brake heating
04-08-24, 01:29 AM
#101
Newbie
Thanks for the link. I've never raced and we don't attempt to beat other riders in a descent. This article focuses on the modulation of disc brakes and describes rim brakes as all or nothing. That has not been my experience with rim brakes. Most of them have seemed to me to be able to modulate - but maybe not as well as a disc. I've never thought of rim brakes as binary. The disc brakes on my mountain bike work quite well. I use them much more than I do on a road bike. However, they do tend to make noise after repeated application.
I do appreciate all of the thoughts on this. However, all of this does not answer my original query: If you dump an equivalent amount of energy into a set of disc brakes and a set of rim brakes (until at least one begins to fail), which one fails first and what is the nature of that failure?
I was just wondering out loud. I am too lazy to set up and run this experiment myself.
I do appreciate all of the thoughts on this. However, all of this does not answer my original query: If you dump an equivalent amount of energy into a set of disc brakes and a set of rim brakes (until at least one begins to fail), which one fails first and what is the nature of that failure?
I was just wondering out loud. I am too lazy to set up and run this experiment myself.
04-08-24, 10:25 AM
#102
Junior Member
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!
Last edited by TobyGadd; 04-08-24 at 10:41 AM. Reason: e
04-08-24, 11:14 AM
#103
Junior Member
Join Date: Mar 2024
Location: San Diego
Posts: 153
Bikes: Paramount Track Bike, Colnago Super, Santana Tandems (1995 & 2007), Gary Fisher Piranha, Trek Wahoo, Bianchi Track Bike, a couple of Honda mountain bikes
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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!
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!
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Cheers, Mike
Cheers, Mike
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