chaadster

I don’t agree with that, and the statement from Vittoria which I posted upthread supports the idea that if high temp induced tube failure is the problem, then eliminating the tube by going tubeless will eliminate the problem and greatly reduce the risk of tire blow-off.

In that statement, Vittoria say their tires are tested to 200% of maximum pressure, so if we assume a 100psi max tire inflated to 90psi at 70ºF cold, then brake heated to 284ºF— the top end of the range which Vittoria said would “destroy” the tube— then the Ideal Gas law tells us the pressure at 284º would be 132.3psi, well below the tire failure pressure.

Also, because tubeless supports lower pressure, it provides an additional level of overheat protection.

phrantic09

We’re clearly just shills for big tubeless.

Anecdotally I’ve never blown out (not off) a TL tire on a descent but I have a regular tuned clincher. I also heat failed a TUBE back in the day on the trainer (the tire never unseated)

Polaris OBark

If the increase in air pressure inside the tire due to nR(delta)T/V is what is causing the tire to blow off the rim, I guess I would agree with them. But the OP calls it "heat related deflation", which suggests something less dramatic, and the opposite of what one expects for increased pressure due to temperature increase. In any case, rubber/bead deformation due to a heated rim, rather than expanding gas, is likely the culprit (if indeed the tire is becoming unseated, which isn't clear.)

Whatever the mechanism, disc brakes will be the safest option.

FWIW, Vittoria tires (tubed) are the only ones I have ever experienced blowing off rims, and the pressure was well under the stated max (it was about 70 psi for 28mm). Thankfully, this happened in my garage, and it did not involve a temperature change.

Also, FWIW, P(2) = 125 psi (assuming the volume does not change).

Lombard

To be clear, I have never melted a tube and I've gone down plenty of steep hills on bikes with both rim and disc brakes. Granted I use standard butyl tubes. If you are melting tubes, you don't know how to brake. Some riders, especially beginners, are afraid to use their front brake. The front brake has more braking power and should be where most of your braking occurs. Also, if you need to brake down a long steep hill, you should do short alternate "bursts" of both front and rear rather than "riding" either of your brakes.

Furthermore, if your tube does melt, your tire will not "blow", but rather will lose air slowly. It surely will not be a pop. It will not blow out or blow off the rim unless there is an issue with the tire itself.

chaadster

Hmm, I’m not so sure about that…

I think a tube needn’t melt (i.e. change state), but merely to soften, in order to go into failure.

Then, I also think the failure can be a sudden, total loss of pressure. I also think it can be heard. I’ve definitely heard tubes burst without any de-beading of the tire.

Further evidence:
https://readconsulting.com/rubber-fa...-tube-failure/

Polaris OBark

Any chance the carbon fiber rims themselves become distorted when they heat up from braking?

Racing Dan

What makes you believe the tyre can withstand more heat than the tube? The tyre is in direct contact with the backside of the brake track. The tube is not. Imo the likely failure mode is the the bead coming off the tyre or the tyre blowing off of the rim from heat and pressure, not the tube melting. - assuming its a normal butyl tube.

chaadster

I took it from the mouth of those who make tires and tubes, and posted it for everyone to read upthread. It may be fortunate that it also makes sense to me that the the thicker and differently compounded rubber of the casing, even if heat affected, has less impact than the bead core in retaining the shape and position of the tire than the tube has on retaining air, and commonly used bead materials, like aramid, are quite heat-reistant and tough.

Racing Dan

Seems to me you are applying a Q and A that is clearly directed at heat issues relating to latex tubes, to any tube.

terrymorse 

Yes, certainly the earlier carbon rims that didn't use high temperature resin could deform permanently under heavy braking.

In the 2010s, we used to rent out Reynolds Attack carbon wheels, until they started to come back with destroyed side walls. The hilly Levi's Gran Fondo ate a pair once, front and back. Reynolds replaced them under warranty.

chaadster

Q:“What is the heat tolerance of the average lightweight butyl tube and latex tube?”
A: “If the tube reaches the level of 100-140 degrees Celsius, all kinds of inner tubes will be destroyed.“

terrymorse 

As will any tire. The design upper limit for tires is typically 90C. At 120C, tire rubber is soft enough to deform plastically.

Racing Dan

Sure, I knew you would throw that at me. However both the initial question and subsequent Q and A is obviously directed at Latex tubes. Still you are trying to contort it as if latex and butyl is all the same. Its not. A latex tube fail long before a butyl even if both and the tyre and the carbon rim will all fail given a high enough temperature.

At this point its just speculation and conjecture what happened and the OP isnt playing anymore, so no clarification either.

Kontact

Front braking is more efficient, so it should control speed with less friction heat.

Polaris OBark

I think the reason it is more efficient is that about 80% of the rider weight is effectively loaded onto the front brake, so I would guess that it would proportionally heat up more. You have a fixed amount of kinetic energy to dissipate as heat, and that scales linearly with mass. So if the rear brake is doing half the work of slowing the rider, it should absorb half of the heat. If you only use the rear (or front) brake, it will be the only one that heats up.

Kontact

My original point was that maybe, like many cyclists do, the OP was relying overly on his rear brake due to the "I'm afraid I'll flip over the bars" phobia. Distributing and alternating between front and rear brakes will distribute the thermal load from braking better.

I'm no expert, but I thought the point of using rubber compound brake pads is that they wear off instead of concentrating heat in the rim. Which is why no one talks about brake fade with rim brakes.


Anyway, we still don't know if the OP had a blow off or a melted his tube.

Polaris OBark

I know, and agree.

But I was just making light of that by pointing out that if one had to heat up a rim to the point of tire blow-out, it would enhance survival odds to do this to the rear wheel.

Kontact

If this was a normal part of riding a bike, I'd take up hiking.

terrymorse 

Then again, if you are braking so much that a blow off is likely, you are probably going rather slowly, and any crash will be a slow one.

chaadster

It is not true that innertube rubber, IIR (aka butyl) has the same working temp range as the butadiene and natural rubbers used in sidewalls and treads, and is in fact lower for butyl than it is for BR or NR, with natural rubber’s critical temp reaching upwards of 200ºC. The 140ºC mentioned by the Vittoria guy as the upper limit at which “all kinds of tubes will be destroyed” is still below the critical temp for BR, too.

It should be clear that when it comes to temperature induced failure in bike tire/wheel systems, the butyl innertube is the weakest link.

Lombard

It doesn't work that way. Try it sometime and you will see it requires much more lever pressure (equates to more friction) to get the rear to slow than the front. If you do this with disc brakes, you can easily feel that the rear brake will get much hotter - be careful with this test as you can easily burn yourself.

terrymorse 

It might be clear, if if were not for all the real world evidence to the contrary.

As experienced by countless cyclists over multiple decades, the dominant temperature-induced failure mode of a clincher tire is blow-off.

bblair

I would think that carbon rims would conduct heat less than alloy. Sure, braking not as good, but perhaps not heat up as much? Either way, at speed, I would think that surface would cool very quickly when you ease up on the brakes.

Perhaps the OP was running clinchers on a nonhooked rim.

I have a hard time believing that the tube temps could reach 100ºC and double max rated pressure.

Lombard

Then again, since carbon conducts heat less than alloy, it would be slower to dissipate heat as well.

terrymorse 

People have reported steam from spoke holes after a descent.