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Pathlete and Pedalphile
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Discussion Starter · #1 ·
I know that there have been catastrophic wheel failures due to heat build up on descents and the tube overheating and basically exploding. Is it due to the whole rim overheating or just certain sections of the rim?
 

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Online Wheel Builder
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Hard to say.

The whole rim is heated, but its obviously one section of the tire thats failing.

I think carbon (particularly clinchers) have come a long way. The braking on the 3.4s is outstanding, and they do long descents with no problem. So much better than my old Edge rims. Those things howled like a mother on even short descents.
 

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Pathlete and Pedalphile
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Discussion Starter · #3 ·
Hard to say.

The whole rim is heated, but its obviously one section of the tire thats failing.

I think carbon (particularly clinchers) have come a long way. The braking on the 3.4s is outstanding, and they do long descents with no problem. So much better than my old Edge rims. Those things howled like a mother on even short descents.
Thanks for the reply Zen. The reason I'm asking is why couldn't you use something like Panaracer flat away instead of rim tape. It's a aramid fiber (kevlar) which is heat resistant. I know it wouldn't protect the entire rim but wondering if it may help.

Or, is it a stupid idea? :)
 

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Pathlete and Pedalphile
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Discussion Starter · #5 ·
I think thats a Band-Aid on a gunshot wound. Excessive heat build up is a sign of a poor rim design, and/or a lack of R&D.
But if you're using carbon clinchers as an everyday wheel and are doing a lot of high speed braking, wouldn't you be concerned with heat build up on any wheel?
 

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Heat is energy and in the case of braking is equal to the kinetic and potential energy dissipated by the brakes. It's the same no matter the material. What's different is the mechanisms of dissipating that heat, the resultant temperatures, and materials' tolerance of that temperature. Heat is generated between the pads and brake track of the rim, and these points will have the highest temperature. Aluminum is highly thermally conductive so heat is quickly conducted away from the brake track resulting in lower brake track temperatures and somewhat higher temps for the rest of the rim. CF is much less conductive so brake track temperatures will be higher. It's the localized high temp at the brake track that might cause the tube to fail (or the beads of the rim itself to fail).

THE DIFFERENCE BETWEEN TEMPERATURE AND HEAT

(I intentionally chose not to complicate the discussion with the concept of heat capacity (specific heat).)
 

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@looigi- Thanks for the link. I learned alot in that article.

While the material of carbon itself may not be as thermally conductive as alloy, I think carbon rims have come a long way in the past 5 years. My 3.4s are the same material as my old 65s, but they are leaps and bounds better at dissipating heat. They still get warmer than my alloy hoops, but no where near what I used to see.
 

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The only way to make carbon as thermally conductive as aluminum would be to infuse carbon rims with graphene, but that would be very expensive at the moment. Other things you could try prototyping are thin metal threads infused into the rim to draw heat from the brake track and distribute it more evenly into the rim, or you could try applying a thin film of aluminum around the rim to achieve a similar effect.
 

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Heat is energy and in the case of braking is equal to the kinetic and potential energy dissipated by the brakes.
Not to get too picky but it is only kinetic energy that is dissipated. Potential energy is just that. If you were at the top of a very steep hill doing a track stand with your brakes locked, you have lots of potential energy but will generate no heat as you are not dissipating anything. Only once you are rolling and have kinetic energy is there anything to dissipate.
 

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lots of potential energy but will generate no heat as you are not dissipating anything...
No. If you descend a hill at constant speed riding the brakes, starting and ending at the same speed, no kinetic energy has been dissipated. All the work done by the brakes was dissipating potential energy.

Of course that's an unrealistic situation. When braking down hill, like to slow for a turn, the brakes dissipating both kinetic and potential energy. The bike speed decreased which is loss of kinetic energy, and the bike is somewhat further down the hill, which is loss of potential energy. Braking on the flat or uphill, the brakes only dissipate kinetic energy.
 

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No. If you descend a hill at constant speed riding the brakes, starting and ending at the same speed, no kinetic energy has been dissipated. All the work done by the brakes was dissipating potential energy.
OK, now I'm confused. I always learned that potential energy was a static term: P = m X G x H (mass times gravity times height). But you are right that if the bike speed is held constant then there is no change in kinetic energy. And yes there is less potential energy when you reach the bottom of the hill than when you're at the top and so that energy must have been dissipated. I talked myself through it and you're right!
 

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OK, now I'm confused. I always learned that potential energy was a static term: P = m X G x H (mass times gravity times height). But you are right that if the bike speed is held constant then there is no change in kinetic energy. And yes there is less potential energy when you reach the bottom of the hill than when you're at the top and so that energy must have been dissipated. I talked myself through it and you're right!
There is also chemical potential energy, which is simply the energy stored in an object regardless of its motion state.
 

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And what on earth does chemical potential energy have to do with heat generation during downhill braking? Are you planning to do blasting with ammonium nitrate?
"I always learned that potential energy was a static term: P = m X G x H (mass times gravity times height)."

^Responding to that.
 

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I know that there have been catastrophic wheel failures due to heat build up on descents and the tube overheating and basically exploding. Is it due to the whole rim overheating or just certain sections of the rim?
First, tubes do not explode unless the rim tape fails or the tire bead comes off. Not that it matters, except I've noticed a lot of people believe that tubes explode on their own. Usually the tire bead comes off because the tube has been pinched under the tire bead.

Because carbon wheels have poor conduction compared to aluminum, the temperature at the brake track can become extremely hot during extended braking. So making the carbon able to withstand a high temperature is one way to help if the carbon itself is failing.

Another important aspect is the pad compound. If the temperature gets too high it is better to sacrifice the pad rather than the rim... via some process of melting and ablation that will dissipate heat. That is why those expensive rim-specific pads will wear down very quickly if you do extended braking.

But the other issue is what happens to the tire bead when it gets hot. We know that the beads will stretch and become looser when tension is applied over time, but there is probably a temperature aspect as well.
 

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Pathlete and Pedalphile
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Discussion Starter · #17 ·
First, tubes do not explode unless the rim tape fails or the tire bead comes off. Not that it matters, except I've noticed a lot of people believe that tubes explode on their own. Usually the tire bead comes off because the tube has been pinched under the tire bead.

Because carbon wheels have poor conduction compared to aluminum, the temperature at the brake track can become extremely hot during extended braking. So making the carbon able to withstand a high temperature is one way to help if the carbon itself is failing.

Another important aspect is the pad compound. If the temperature gets too high it is better to sacrifice the pad rather than the rim... via some process of melting and ablation that will dissipate heat. That is why those expensive rim-specific pads will wear down very quickly if you do extended braking.

But the other issue is what happens to the tire bead when it gets hot. We know that the beads will stretch and become looser when tension is applied over time, but there is probably a temperature aspect as well.
I personally haven't had a tube explode on it's own but I specifically read on this site that it appears carbon clinchers have gotten so hot that tubes have exploded. Not sure if it's true or not.

That's the reason I started this thread.
 

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The mechanism for a *tube* failure due to heat might be that it would melt and spring a leak... in theory. But it will not explode. The tube is under very little stress so long as the container is intact.

Zipp did some tests to address heat concerns with latex tubes (which do not tolerate heat as well as butyl) and determined that the rim strip would melt before the tube would have a problem.
 

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Pathlete and Pedalphile
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Discussion Starter · #19 ·
The mechanism for a *tube* failure due to heat might be that it would melt and spring a leak... in theory. But it will not explode. The tube is under very little stress so long as the container is intact.

Zipp did some tests to address heat concerns with latex tubes (which do not tolerate heat as well as butyl) and determined that the rim strip would melt before the tube would have a problem.
I guess which brings me back to my original question. If the rim strip is getting so hot that it would melt (and potentially damaging the tube), would a kevlar rim strip help protect the tube. Or, is it like putting a band-aid on a gunshot wound?

I would think any heat protection would be good but would the kevlar strip provide any needed protection?
 

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Lots going on in this thread.

First, heat dissipation is a misnomer as I see it used most often here. That would imply that some carbon wheels are better heat conductors than others. I think what most people are going after is "heat resistance" which simply means that a rim can handle more heat before it fails. Aluminum dissipates heat better than any carbon rim, but I've seen no evidence that carbon rim "a" dissipates heat notably better than carbon rim "b".

Many of the new pads prevent heat from spiking as high in the first place, and to be totally honest I can't explain that to you. All I can say is that I've been shown that rim "a" gets less hot per unit of braking force with pad "1" than it does with pad "2". This leads me to believe that pad "1" is better at dissipating heat than pad "2".

As rruff says, tubes don't explode. They'll burn/melt, but they won't just explode while they are contained by a tire and rim. They explode if the tire comes off the rim and the tube traps itself between, but butyl fundamentally won't explode. I think most (all?) reported incidents of "tube explosions" are actually failed rim strips where the tube pops in a spoke in the rim's tire bed, exposed by the failed rim strip.

A Kevlar or other such rim strip would prevent this. We are testing an industrial heat-resistant foil-based tape for use as a rim strip. It withstands more heat than even the highest-rated rim matrix can. But at some point you are just moving fuses - make the rim out of more heat-resistant material and the rims strip becomes the fuse. Stop the rim strip from failing and the problem will move downstream to the next potential fail point. Whether that is the tire disengaging from the rim or whatever, at some point if you keep introducing heat something will give.

Many of the new generation of rims are able to withstand WAY higher heat than previous generations. It's still possible to cook them, but doing so is ever easier to avoid. Use the right pads and keep your pads and rims clean. Don't ride your brakes. All braking is not the same. Dragging your brakes to maintain a constant reduced speed produces more heat than intermittent, purposeful, braking. Airflow helps rims and pads cool quickly. Don't over inflate your tires. Most rims have a limit of around 120 psi. Heavier bike/rider weights take more heat to stop than lighter ones, so be aware of that. If you are going down a huge mountain that needs a lot of braking, stop occasionally and let things cool down. Pros racing on closed roads among other pros don't brake much, the rest of us in car and/or bike traffic and with non-pro descending skills (and non-pro gossamer body weights) have it a little tougher.
 
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