Cause of blowout when heating rims?
 

Anecdotal comments of tire blowouts with hot rims are often associated with a raise in tire pressure or tire rubber weakening in some way. Statements from Wilson's Bicycling Science book suggest tire patches failing or glue weakening for tubular tires as the cause of blowouts for a hot rim. This seems reasonable especially if the patch was poor.

Focusing on clinchers with new inner tubes, heating the rim by 100C (180F, braking on step downgrade) will cause a 20 psi pressure increase, which is insufficient to blowout the tire. As the tire get hots, the kevlar bead (folding tires) has a negative expansion coefficient with temperature. Whereas for steel bead, the expansion is negligible, which suggests that a bead change does not cause a blowout. I have double-walled rims, the spoke ends do not protrude into the rim tape and cannot cause blowout due to weakened rim tape.

Are the above statements are correct? If so, it would appear that heat induced blowouts are due to improper bead seating or poor patches or spoke protrusions, rather than the limitations of tires and tubes. I have not experienced this problem, but I would like to better understand the cause so as not to have an experience.

I believe your reasoning is correct and that braking heat, in and of itself, does not cause blowouts with a properly seated and/or patched tire. You would have to get the rim tremendously hot to have that effect and then it would probably have to damage the rubber at the bead.

I have also heard reports of tires blowing out when a bike was stored in a hot car or transported in the cargo hold of an airplane at lower atmospheric pressure. I don't think either scenario would cause a blowout of, again, a properly seated tire.

Just guessing, but it may be the tyre casing, that is in direct contact with the rim, failing in some way. - the bead coming off the tyre or the reinforcement weakening from heat exposure.

Tube patch failing would only reproduce the leak, not cause a blowout.

Also, is there any reason heating caused by braking would be limited to 100C?

The issues with heating usually are for those that ride bikes at high rates of speed in the mountains and when on very steep long descents that require lots of braking.

Most of us mere mortals will continue to be just mere mortals.

But if you do happen to be on a long downhill descent that is steep and you have to ride the brakes, then maybe stop every so often and try not to be going so fast when you get to the steep parts.

I have never had a blow out at speed from braking heat. I have, however smelled brake pads on several ocassions. On rim brakes I've worn new pads down to almost nothing, experienced fade, howling, stink & rims too hot to touch. On disc, warping & blued rotors, used to happen a lot until I got bigger rotors.

Others on here have had rims self destruct in spectacular fasion. (With pictures!)

I've personally seen carbon rims, both with aluminum brake track & with out that the beadseat exploded.

All this to say that yes, yes it does happen.

Are you saying kevlar contracts when it gets hot? First time I've heard that - interesting if true. (I just checked, you're right... negative CTE)
I've had a couple of these hot braking blowouts - I can tell you for certain, the rim was FREAKING hot. And it's almost always the rear.
I know lots of tandem people that all are aware of the issue even if it hasn't happened to them
I think the airline cargo issue is not because a blowout is likely, but that if it were to happen (by complete chance) it would sound like a bomb... so they want you to let the air out before packing.

I've had lots of these discussions, and I've thought a lot about it - and the thing is, none of these factors alone is enough to explain a true blowout (tire blows off rim as opposed to a failed patch). Sometimes the bead rips away from the carcass - might be the cause of the blowout, or might be a result of the blowout. Frequently though, the tire is just fine afterwards.

1. Yes, air expands and tire pressure goes up when the rim gets hot, most people stop thinking here, but it really isn't a big effect. If you were to pump up a perfectly installed cold tire to blow-off pressure, it'd probably be double the rated pressure.
2. Aluminum rims expand when they get hot - changing the bead seating contact.
3. Aluminum extrusions may become less stiff when it gets hot - allowing for a tiny amount of elastic deformation.
4. The friction coefficient between the rim and the bead might change.
5. Steel beads expand - Kevlar not.
6. A pinched tube under the bead that isn't bad enough to present a problem under normal riding gets worse when the rim gets hot and blows the bead off.
7. Sometimes blowouts happen for no known reason - and sometimes that happens when you're braking downhill on a hot day... confirmation bias.

Re: Airplane, if you took the tire into the vacuum of space (the worst case altitude) the differential pressure would increase 14.7psi.

if you do the math, a tire at 100psi at room temp (say, 20C) would be at 127psi at 100C - not enough to blow a well-seated tire off a rim. If you do actually get heat-induced tire failure, I imagine we’re talking about temperatures way higher than the 100C example cited in the OP

With double wall rims and plastic rim strips, I'd be concerned about the plastic deforming into the holes too far, cutting the tire. Not a blowout though.

I witnessed two blowouts in one day due to thermal expansion. I cycled to work on a 10F morning, after pumping the tires up to rated pressure (about 100 psi) in the cold garage. I brought the bike into the 70F office. Around mid-morning, we heard a muffled BANG from somewhere in the building. Then an hour later, another one. When I got the bike out of the storage room for the commute home, I found both tires blown out.

Some thoughts - rims can get really hot. (Pumping and alternating brakes can make a large difference. A short, hard braking, then a cooling period is far better than a long, steady drag on the brakes. Not hard for me to imagine that really hot rim might soften glues in the tire, maybe weaken rubber. Also, as Diablo Scott pointed out - that kevlar bead that tightens? Well the rim expands. Kevlar is a very high modulus material. So, as the rim heats up, the kevlar is forced to stretch as it wants to shrink. A neat formula for raising the stress level in the kevlar. Now, I doubt this stress would be enough to break the bead but any weak points? (from manufacturing, poor use of tire irons, life as a bicycle tire, ... I don't know if they make the bead as a continuous loop or weave strands among themselves. If the latter, maybe that weave slips a little under that high load.)

Tires see a lot. We don't get to know all the little impacts, small sharp stuff, etc. they see. Get a tire hot and up the pressure 25%? Well, if that tire was close, now it may be past its limit.

I blew out my front tubular descending Mt. Washington. (Tubular, so flat but still ridable.) That rim was VERY hot.

In short, get that rim hot and you are doing the search for the weak link. The hotter the rim and faster the speed, the more likely the tire failure and the more serious the consequences. Starting pressure, rims, tires, tubes, tapes and glues all factor in. (Dropping pressure at the mountaintop has been done forever. Not by most people but it really works.) Unzip your jacket, sit up, make yourself big.

Hot trunk
 

I had also done the math and discounted the possibility of heat-related blowouts. In my case it was about storage in a car, after inflation to riding pressure (110 on 23s?).

Yet, once when taking my bike out of the trunk, one of the tube/tires was flat. And upon changing the tube, it had failed along a split, not a hole as if punctured.

I now prefer to let air out of tires and re-pump when a bike will be in a hot car, rather than shout "Math! Physics!" at the snickering universe.
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I suspect that we tend to overestimate the margin available for pressure beyond the specified maximum on tire sidewalls, many of us hypothesizing that tires are designed to take upwards of twice the maximum specified pressure to blow a tire off the rim. Post #10 would indicate that the margin is far less than that, more in the 20 psi range. The only time I had a tire come off the rim was when I used a pump with a defective pressure guage that read about 20 psi lower than actual. I can easily believe that a 20 psi increase on a tire running at high speed over rough surfaces or a road hazard could very well blow off the rim.

It is possible to pump up a tire on a cold morning, then leave the bike in the sun as the day gets hot, and have a blowout.

On a current thread people are discussing how worn their tires are when they replace them - so my question is how worn were your tires when you had a blow out? Were they new and potentially had a manufacturing/installation defect? Were they heading for the exposed thread, throw it away, end-of-life phase? Or were they prime-of-life?

I think a more relevant question is: how close do people run tires to their maximum pressure? I don't even ride bikes with skinny tires now, as I am heavy, and even at maximum pressure, the tire can compress all the way to the rim when I hit a bump. So pumping a tire up to the maximum pressure on a cold morning, and leaving it exposed to the hot sun can result in a blow out even with a good tire with no defects. I think some brands of tires are tougher than others. Some would be able to handle pressures significantly above their rated maximum pressure without problems.

That is an issue that has long confused me - why do we inflate tires to a set pressure independent of our weight? One of my lecturers at college would apply some colored chalk to an area of his tires, ride over a sheet of newspaper and measure the width. Surely the optimum tire pressure must relate to the actual loads on the tire.

Yes. The optimum pressure depends on the load the tire is carrying. That is until the optimum pressure exceeds the pressure the tire is designed to hold. A small rider may inflate a tire to well below the maximum pressure. A heavy rider may may inflate a tire to it's maximum pressure, and then the pressure is really still too low for the rider.

The solution. Use wider tires. Pressure is pounds per square inch. If you have more square inches, you need less pounds per square inch.

The elasticity & pliability correlating to how likely a burst might occur might have a very narrow window of measurability imo.
Personally, I've been to both, older & newer "bangs" from thermal impacted psi expansion.
In no particular order: Old tires with new tubes, New tires with one old & one new tube, New tires with new tubes. Had the PSI just under the sidewall MAX rating, put the bicycle in the back of the vehicle with temperatures being under ambient, & with the enclosed area trapping in heat of unknown degree, "BANG!" happened. One of the times was inside of a car's cargo area, then the other time stowed in a SUV, & another time under a tonneau cover of a truck's bed. After the first event, I knew what to do thereon after, but with age comes forgetfulness. I now make it a note of many to "drop the psi 30ish & pack the higher leverage pump, cordless pump, or mini Cali Air compressor.

Sounds like a lot of people need to go out and start collecting data by riding their bikes. Too much what if'ing going on.

Yabbut...

Don't drag your brakes down a 4 mile, 10% grade. Or a 6 mile, 6% grade. (I didn't, either time, but I saw the results. Pretty nifty handling by the lady who blew her front tire!)

Look at it the other way. The big unknown here is what the actual blow-off pressure would be.

If we were to assume for argument, that we tested a particular tire and rim combo and found that it would blow off at 160 psig ( road bike wheels, average over several tests at room temperature conditions, and I think that's a reasonable number), then we could determine what temperature would give that pressure (from 100 psig at 68F) and that's 340F... and I don't think that's a realistic temperature (could be wrong, should be an easy measurement).

Therefore I conclude, that hot rim tire blow-offs are not solely attributable to increase in pressure - that other effects come in to play as well.

Regardless though, the answer is the same - feather your brakes on long descents, and let some air out when you park in a hot place.

I had a plastic rim strip fail on a 8-mile descent with 8 to 14% grade. It basically melted, allowing the tube to "blow in" to the spoke holes in the rim. Frustrating, since it was essentially unfixable and I had to get sagged in. I swore by the holy Velox now.
That day had been one long climb. Beautiful scenery though: Day 10: Lillooet to Pemberton - A bike ride in Lillooet, British Columbia (ridewithgps.com)

I got to town under similar conditions once by rotating the plastic strip about 20 mm and hogging out a new valve stem hole with my multitool. Back to Velox for me, too. Well, that and disc brakes.

Thank you for the comments. Assuming that most "weakest link" items have been serviced well...good vulcanized patches, tires not very old, decent rim tape, proper bedding of bead (can I call this beading?), then it appears from the comments that unexpected tire blow-offs could be attributed to a high variability in tire specifications from manufacturers.

The 100 C temperature rise I originally mentioned was not without some thought. Sustained braking of a tandem on a 10% slope at 45 mph will raise rim temperatures ~35C (Wilson). Braking a tandem from 45 mph to a quick full stop will raise rim temperature another 50 C. Of course, this does depend on the Al mass of rims for its heat capacity. Light weight rims or carbon hybrids with little Al mass would go higher. A 100C increase would feel very hot to the touch but should not be enough to blow off a tire.

The comments regarding blow-offs from ambient heating suggest that the margin of error in stated tire manufacture pressure limits are highly variable. Certain manufacturers may have lower error margins that other, even with lawyers breathing down their necks. This is not wholly unexpected. There are certain tires I tend to trust more than others.

Thanks, and yes...don't drag brakes at 20 mph for miles down hill, do stop to let things cool, and do use very cool French Velox rim tape :)