stiggywigget

I did an hour plus ride in 20ºF weather today. I have a new bike that I outfitted with 28c Continental Contact tires for commuting. They are rated to 102 psi. I fill them to that limit, roughly. I topped them off when I left home today. I was outside in the cold air when I filled the tires. I brought the bike into my warm office and 15 minutes later the tire blew off the rim with a bang. The tube is totally blown out.

Presumably this occurred because the air warmed and expanded, taking the psi way up beyond the 100 psi I'd put in before leaving. It seems feasible, but is it a known phenomenon?

zonatandem

Not likely. Never thad that problem. Look for another cause/reason for blowout.

bac

I've seen this happen when a high pressure tire is put inside a hot car .... BOOM! I don't know if your situation is the same, but it does seem to make sense.

... Brad

goofycc

100 psi is way too much for winter commuting. Go with a nice cyclocross tire (depending on your frame, a 30c tire should fit). This will allow you to run lower pressures, so you don't have to worry about tubes warming up and exploding. Plus, it will give you more traction when the snow flies.

stiggywigget

Yeah, I'm going to aim for about 85 psi. That's closer to the recommended psi for these tires anyway. For snow and slush I could drop it even lower than that, I think. I'm used to 120 psi 23c road tires and I like the feel and lower rolling resistance of a hard tire. Also, I'm 6' 3", 200 lbs, and carry another 15 lbs in my messenger bag so a higher psi seems justified. But not if it blows the tire off the rim, obviously.

Machka 

I had that happen once. I was out riding in temps around -10C or so, then brought my bicycle into my apartment, turned the heat up to about 27C because I was chilled, and maybe about half an hour later ....... BOOM!!! I had to peal myself and my cats off the ceiling!!

Rincewind8

The ideal gas law is:

pressure*Volume = mu(number of moles of gas)*R(Universal gas constant) *Temperature(in Kelvin)

Assuming the volume stays constant, p/T has to stay constant. So filling the tires at ~266ºK (20ºF) to a pressure of 102psi and then heating the air in the tires to room temperature of maybe 297ºK (75ºF) results in a pressure of 102psi*297/266=114psi.

Since there is always a safety margin, the tires should hold that pressure.

JasonC

For what its worth, thinking back to high school physics, if your pressure was at 100psi at 20ºF, it probably got to about 110-120 psi once you got indoors.

pV = nRT...

Edit: Hah... yeah, what the last guy said

Roody

How big is that safety margin?

The way I'm reading it, the tires are only rated for less than 85 psi, so he's already over-inflating them. It seems like the extra 12 psi could blow them off.

stiggywigget

I'm glad the office was empty or there probably would have been several calls made to 911.

stiggywigget

Awesome. Thanks. I would agree that the safety margin should be such that the tires wouldn't blow off. However, it's entirely possible that they were holding something closer to 110 psi due to an imprecise floor pump gauge reading. Then we'd be talking about 123 psi at 75ºF. Also, these tires go on and off these rims pretty easily. In fact, I can get them on with my bare hands. I've never been able to do that with any other road tire. Not even close.

goofycc

That's funny, I can get any tire on with only my bare hands. It's all in the technique. Think of kneading bread.

If you could get them OFF with your bare hands, then I would say you have a problem.

Rincewind8

If I recall right Continental gives a recommended pressure, which I guess is the 85psi mentioned, and a maximum pressure, which I guess is the 102psi (coincidently 20% higher than 85psi ).

I don't know what the safety margin is. For consumer products the safety margin is often high, maybe it is 50%, but that's just a guess.

grolby

No. I would be willing to bet a fair amount of money that the amount of pressure required to blow the tire off would be in excess of 150 PSI. It takes a LOT of pressure to just blow a tire off. Temperature changes resulting in blowouts due to temp changes is a persistent myth.

I vote for user error.

CastIron

^^ Ditto.

squegeeboo

I've done it also. It was prob. a combination of the heat uping the PSI a bit and a slight pinch

thebulls

I read somewhere that the "standard" method for determining max psi was to find the psi where the tire blows off and then take half of that. So that would say that the blow-off psi is 204.

Seems like the most likely cause is a bead that wasn't quite in place. Maybe the tires were low at the start, allowing the bead to come out of place, then when pumped up it was just marginally attached, and after bringing it in, the increase in pressure was enough to blow it off.