At the end of my race on last Sunday, while loading up my bike, I noticed the rear tire was flat. Not soft, not squishy, but dead flat. I had filled the tire before the race, and knew it was good, as I had had no leaks the day before. Being a space cadet, I guess I didn’t notice it flatting on the bike portion…which is concerning, as there had been a downhill portion of the race where I hit over 50 MPH.
One friend I talked to after the race said that the heat had caused a lot of people’s tires to pop, as you could hear them popping coming out of the transition. It had been unseasonably hot, around 87F that day.
In warmer conditions, how much should one underinflate one’s tires, to account for expansion due to heat?
I live and train and race in central florida where the heat on the road is 100+ for a large part of the year. i have never flatted because of heat. i run my clinchers at 135+. the only time i have ever experienced what was likely a heat failure was when i left my bike with tires inflated high in the car all afternoon during 100+ weather. my guess is that you simply had a puncture. i doubt you were dead flat during the race. seems impossible not to notice.
Depending on conditions, I will run my MTB tires a little soft, but on the I’m too afraid to get snakebitten from the crappy roads we have here in SJ to do the same with my road bike. My chopper has cruiser tires to it’s tough as nails and I run at the recommended pressure.
Overpressurization from ambient temperature is almost never a cause of flats. The cause almost always involves human error. It amazes me when I hear people say they always inflate their tires to the max. rating of the tire. If you weigh over, say, 175 pounds, then maybe that works but otherwise…
There was a study (or two) awhile back that showed “properly” inflated tires were “faster” than simply pumping them to the max. pressure stamped on the sidewall. I put those terms in quotes because proper tire inflation is a relationship of rider weight, road conditions, and preference. Faster is generally meant to refer speeds/rolling resistance measured with the tire as mounted on a wheel, on a bike with rider weight and momentum applied.
Anyway, the whole deal has to do with vertical rebound of the tire on the road surface and loss of energy upward due to excessive high inflation pressures. Balance that against the opposite effect of insufficient pressure that allows things like excessive drag and the dreaded pinch flats and you see there is an optimal range of possible pressures.
It’s unlikely temperature caused your flat because the recommended inflation pressures always include an unpublished safety factor built in. Plus, flats due to overpressure usually go bang rather than hiss or no sound at all. Having worked in bike shops for years I’d rank flat frequency something like:
Under-inflation, pothole or curb cut induced pinch flats.
Tiny glass or radial car tire wire nicks and subsequent slow leak flats.
Old replacement tube, butyl fatigue, cracking seam, flats.
I may be forgetting a few.
Anyway, talc your tubes, use care when installing, center the valve in the rim valve hole, test inflate then deflate the wheel off the bike before installing to make sure the tire bead hasn’t trapped the tube and that the tube isn’t twisted inside, then inflate to your “proper” pressure.
Always inspect your tires, both outside and inside the tire, for embedded glass or wire bits that would just re-puncture a new tube when re-installing a tube. Use the right tire width for your conditions and make sure the tire selected is compatible with the rim on your wheel.
My friend blew out his front clincher tire after a ride. He inflates his 120 PSI tires to 130. It blew as he left it on his blacktop driveway and we were stretching. It was about 30C that day.
I always figured a heat induced blow out was much more likely during a long steep descent? Isn’t what happened to Beloki at the TDF? There are lots of mountains where I ride and sometimes I will stop halfway down and feel my rims and be amazed by the temp. They are way hotter than any road surface is likely to be.
I get kind of freaked out thinking about a front tire blowing during some steep technical section…is that a real risk?
No, this isn’t what happened to Beloki. His tires were not glued well, and when things heated up on that descent, the glue got soft. When he fishtailed, the tubular pulled right off the rim.
I have lost clinchers in the heat before - fully inflated tires in a hot car are a bad mix. I think Hinds57 blew out a tire when he left his bike in a hot garage. Could it happen in a hot transition area? Probably.
Where’d you read this? I’ve heard a ton of armchair-expert explanations (including TdF commentators) for what happened to Beloki, but never anything from Beloki or his team.
This past Sunday it was 90F and sunny. I set my bike outside for about a half hour before I rode. I’m certain that the cold pressure of my tires was about 100 because thats what it drops to within 24 hours of a previous inflation to 120, where I normally ride them.
I put the pump on to inflate them and immediately it registered 150! I was very surprised, and a bit skeptical, so I deflated them a bit. Yup, it was an accurate reading. So thats about 50 pounds of pressure due to tire heating. Amazing.
I can barely pump my tires up to 150 as I only weigh 140.
My suspicion is that unless you leave a tire in a very hot car, ambient air temp is not going to increase the tire pressure enough to blow it out unless it it already inflated beyond its max rating. I place blind faith in the beleive that there is a safety factor built into the max stated on the sidewall.
But heat may affect the tire in other ways which could increase the chance of a flat. The hotter the tire is the softer the rubber in the tire and tube are going to get. This will increase the exposure to any weaknesses in either the tire or tube. Also high temps may cause the tread to soften and become more sticky, which would make it pick up and hold onto more debris and the hot tread will be softer and more easily penetrated. I suppose it is also possible for a tire to get hot enough that the tube may stick to the inside of the tire which also could lead to a flat as soon as the bike starts rolling.
Underinflating will not solve any of these problems so I just don’t worry about it
I know this will sound equivocal but when folks say their tires blew out being left in a hot car or on the hot pavement, the expanding air is usually just a contributing cause to a rim/tire problem.
Some tire/rim combos are not optimal. Like my old Araya CT19 rims with anything Michelin. They’ll hold fine up to 100 psi but, get some water in there, or leave them in a hot car, and BANG!
Not every tire can go on every rim despite both being 700c. Mavic rims have very good hooks. Other rim mfg.s not so good. As a rule, the harder it is to seat your tire on the rim the better the resistance to bead seating issues.
Tim, I’m sure you are technically accurate about the cause of these failures, with the added pressure being only a contributory factor. Most folks probably don’t care really about the actual reason for the failure, they just know it blows. I doubt that anyone doubts your input here, but most don’t care to know to that degree.
Simple questions can often be the most frustrating. Should I underinflate my tires on a hot day? Hmmm…
It’s physics. Ideal Gas Law : PV = nRT. If the volume stays constant, then the pressure rises proportionately with temperature.
Say you inflated your tires in a 60d F garage to 100psi. You then stuck the tire in the back window of a car where the temps soared to 130d F. Convert to kelvin = 327.4/288.5 = 1.13. So in this example your tire pressure would rise to 113 psi - not that big of an increase.
Also, when you pump up the tire, the air going in increases in temperature as it is compressed by the pump. I guess it would be possble if you factor this into your calculation that the tire may actually not “gain” any pressure at all when it is heated by the sun/warm environment but just gets itself back to the pressure you read when you were done inflating it.
That’s all fine and good for a static bike, but you should also remember to take into account the conduction heating from the rim while braking. Then there’s the convection cooling of the air going past the tire and rim… I wouldn’t even want to guess what nusselt number is involved in calculating that.
