A friend put my bike (roadie 700C) on their car rack (towbar one) and told me to let the tyres down a bit in case they blow out due to the hot temperature (30 deg C +). I laughed and said what difference is there to riding on a hot road, that should be hotter and I've never had a blowout because of heat. Then today I had the bike in the back of my car because I was planning to go racing after work and it was a hot day (32 deg C +) and when I came out to the car at 5pm the rear tyre of the bike was flat. This has actually happened to me at least twice before but I just put it down to a piece of glass or other debris still in the tyre from a previous puncture. But this being the third time I'm thinking that maybe my friend is right and the heat is causing a blowout. The latest 'puncture' seemed to involve the valve splitting away from where it joins the tube (Conti brand tubes). I'm thinking too that Conti tubes might be a bit thinner and lighter than the other cheapo tubes I have previously been using therefore exasperating the problem. Anyone else experienced this?

I have had 2 blowouts in the past 4 or 5 years... and they both happened in my apartment at night.

My guess is that any blowout not caused by road hazards is at a random time, caused by tube or rim issues. For example one of mine was related to a worn rim strip, the other was on an older tube at the base of the stem (probably due to topping off the pressure many times). Lowering pressure may ensure that this doesn't happen while the bike is idle... but think of the alternative.

Thinking about it just now, I would leave the tires inflated to increase the odds of a tire blowing during the 23 hours a day I am not riding, since it probably increases the chances that a weakness in the tube fails while riding it if I reduce the chance of it blowing while it is just sitting there...

My thoughts may not prove out by facts, but I would rather have a blowout when off the bike than when riding...

I seem to recall reading here in another thread that the pressure increase associated with a temperature increase is only a few percent. So, I will consider any minimal increases in pressure based on heat a stress test.

Urban myth. The only possible source of heat that might cause tube failure is excessive braking, usually associated with long downhill/mountain descents.

Like Little Darwin, I have had flats in the middle of the night. I woke up once just before it happened. Cause, bad/cheap rim. The tube went out with hiss, not a bang.

I have seen tires damaged while on trunk mounted racks, due to proximity to exhaust pipe. Yeah the tube blew, but the tire would have been unrideable anyway, due to melting!!!!!

The rule of thumb is 2 psi increase in pressure with each 10 deg F rise in temp. So even a dramatic change from something like freezing to a an extremely hot day is only going to cause around a 15 psi rise. Highly unlikely the temp rise is the cause of your problems.

- Mark

I see the logic in the 2psi for every degree, but I have also touch some metal things that have been in direct sunlight and almost been burned. Mostly speculation, but I would assume that black tires in the sun would get a lot hotter than the ambient temp. I had a tube blow out while I was doing farm work and it was parked in the sun for several hours in the middle of the day.

Have Fun,
Travis

I second this. I've seen a tire blow when left out in direct, intense sunlight, but the heat from a trunk or enclosed car is not going to be enough to blow out a tire.

...not only that, but I've seen it happen firsthand.

We returned from a group MTB ride in Riverside CA one summer day, and while sitting there heard a guy's road bike left in his car go "BLAM!". It had been sitting there for hours, the only explanation was the heat.

The heat from the exhaust pipe is enough to weaken the tire and allow a blowout. Trunk mount bike racks should have large warning notices on them to keep the wheels out of the exhaust stream.

Warning notices get people to think? The only way I see that helping is if the sign is large enough to block the exhaust.

Keep in mind that the heat does TWO things. The air presure goes up for sure but as mentioned that won't be enough to burst the tire. However the heat also affects the rubber working to soften it. In this more pliable state it could concievably slip over a rim bead if the fit is loose enough or a valve stem can loose adhesion with the rubber or if there's a spot where some cords in the casing have been cut from a previous puncture the surrounding cords may move within the rubber of the tire and let the tube herniate out and pop.

get some air-free tires and never worry about this kind of crap again.

His tire was already air-free...hence the issue.

Tubes torn at the base of the valve stem can be due to a problem with the rim, like a sharp edge. Or from having the stem rock back and forth from using a frame pump. Another possibility is a tube made defective during the process of molding the stem with the tube.
IMO tube thickness has little if anything to do with tube failure. In fact I've always had better luck with ultra light thin tubes, maybe because the quality control is better.

Al

Add to that list the tendency of a tire to slip and rotate on the rim, putting excess pressure on the valve stem. The tendency increases with heat and hard braking as the tire becomes more flexible.

I keep meaning to suggest to Myth Busters that they check that out.

Good point. I've seen tires rotate on rims and that always seems to be due to under-inflation.

Al

Using the combined gas law:



Due to the nature of the tire we can assume the volume of the tire will be very close to the same with a slight increase in pressure. In other words it may stretch slightly but the tire cords prevent much change. So we will set V1=V2. Even if the volume did increase it would only lower the pressure change with temperature.

Assuming it was a nice brisk morning at 40°F when the tire was filled or 278°Kelvin. Also assuming that the tire is left in a nice hot car at 125°F or 325°Kelvin with an initial round number inflation pressure of 50PSI. The pressure could be converted to pascals but it does not matter here.

This yields a pressure increase in the tire of 8PSI to 58PSI, clearly not enough to blow a tire. Or roughly about 1PSI for every 6°C or 10°F. Please punch thru the calculations yourself.

Pressure or temp conversions can be found here:
https://www.unitsconverter.net/

The reason a tire would blow would be road debris, tire/rim damage, faulty rim, tire/tube mismatch, tire/tube shift due to under-pressure or braking, or UV damage to the tube at the valve stem.

No, the only explanation you could think of was the heat. Simple pressure increases due to increases in ambient temperature are not sufficient to blow a tire. BCRider brings up a good point - it is conceivable that high enough heat can cause damage to the tube, or more likely, increase suppleness enough that a less-than-perfectly mounted tire allows the tube to slip out from under the bead. But even that's a bit of a stretch. The former in particular is difficult to believe in the case of leaving a bike in a hot car, although it is almost certainly the major cause of blowouts on long descents with excessive braking. Less than excessive braking would be more than enough to raise the temperature of the air inside the tire to beyond 125 degrees F. And yet, we don't get blowouts going down any old steep hill on a hot day.

The truth is that we don't actually know that tires are more likely to blow out in high ambient heat. Cognitive bias is a powerful thing. We might be more prone to remembering it when it happens, since it fits with our expectations and we can readily come up with an explanation, unlike other flats that occur for no apparent reason.

If it was in the trunk, I wouldn't have expected it, but if it was exposed to the sun, i.e. on the folded down rear seat, then the temp inside would be much higher than 32C, and if the tire was indirect sun, much higher still. Then, yes, it could be heat.

Ah, OK...so considering that the ambient heat increase during our ride and also the anecdotal evidence that the temperature inside a car can be much more than the ambient and was the only factor that had changed, it is better to assume that it is inexplicable rather than that the heat increase was in at least part a factor in the tire blowing because it can't be proven conclusively. Gotcha. Musta been gremlins.

Look, several people have already stated that pressure does increase with temperature. Any tire/tube/rim combo has a finite limit to how much pressure it can take before failure. If you disbelieve this, please feel free to get a high capacity compressor and fill your tire until it explodes, then examine all you will what exactly failed and at what pressure. Whether it was the bead seat failing, a flaw in the tire or weakness in the tube, the trigger that fires the gun is the increased pressure. All I stated and all the OP asked is if heat can in some way increase the chances of a tire blowing, and it can. I never said it will always be the sole cause of failure.

yes, this happens to people all of the time and they dont even know it was heat. Made me think of a Funny story though... I had the NYPD knocking on my door at 3am once with their guns out demanding me to put my hands up and for me to come outside the apt. so they could search it. A neighbor had called 911 claiming they heard gun shots from my apt. I had leaned my bike in it's regular spot but earlier at work i changed my tires to high pressure ones pumping them up to 145 thinking nothing of it. Well the radiator was on and i had closed the window earlier that morning so the combo of all of those things made both my tires explode. After they searched the apt. they laughed and one of the cops actually came to my shop a month later for a tune up on his kids bike.

Where was the blowout? At the stem? I've had tubes blow when the sun was shining on them through the back of the car windows/sunroof. Every time the blowout has been at the stem. I assume direct sunlight in an enclosed space on the metal it is possible? The metal gets hot, the rubber gets soft... boom.

It might not have been gremlins, but I can state with absolute certainty that a mere increase in pressure due to temperature did not blow the tire off. Try your own experiment. You will require somewheres around 200% or MORE of the maximum recommended pressure to blow a tire off a rim from excessive pressure alone. While a 130 degree car may seem incredibly hot to you, it isn't much in terms of the effect it will have on tire pressure (compared to, say, an ambient temp of 80), the structural integrity of the tube or the position of the tire bead within the rim. The point we're trying to make is that while, yes, pressure increases with heat, it does not increase enough, given the temperature difference between the extremely hot interior of a car left in the sun and a warm day, to blow a tire. Even when a tire DOES blow due to heat, it's because of heat damage to the tube or tire and has practically nothing to do with pressure (think that the tire wouldn't blow if it were at 30 PSI instead of 100?). This will take a temperature a good bit higher than 130 degrees.

As for heat increasing the chances of a tire blowing, sure it can - but I would bet that it's because we ride our bikes a lot more in the summer than in the winter, not because being warm makes tires and tubes inherently more likely to fail.

Dang, someone beat me to it. Just remember that your temperature needs to be in an absolute scale. Either kelvin or rankine.