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Brake heating

Old 04-08-24, 01:29 AM
  #101  
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Originally Posted by oldacura
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.
When we bought our tandem it came with 180mm basic steel rotors and cable Avid bb7 cable discs with a rim drag brake operated by the stoker. We ride Audax through the winter and even using the drag brake sparingly found that the rim was wearing quite quickly. I switched up to a Hope 225mm front disc and a 200mm rear with hydraulic calipers. The feed back of the brakes is much better. And my hands are much less fatigued at the end of a 100mile ride. I am sure that if we kept with rim brakes I would be looking to change the wheels far more often. And the supply of suitable tandem rim brake wheels will only get smaller as the majority of bikes go to Disc.
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Old 04-08-24, 10:25 AM
  #102  
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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!

Last edited by TobyGadd; 04-08-24 at 10:41 AM. Reason: e
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Old 04-08-24, 11:14 AM
  #103  
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Originally Posted by TobyGadd
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!
That seems like a pretty good test to me. Yeah, Iíve had more brake challenges on downhill MTB stuff than tandem rides, but Iím way more aggressive on a MTB than a tandem.
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