If I had to guess I'd say they were lowering the pressure to account for the heat. Since tire pressure goes up at higher temperatures, if you start at 100-110, it will be much higher after a couple of hours on the hot blacktop.

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Fairly easy to verify, assuming air at that pressure behaves as a perfect gas, we have

PV=mRT

Furthermore, assuming constant volume (tire volume will not vary significantly with increasing pressure), we have P1/T1=P2/T2

and finally: P2=P1xT2/T1

Using P1 = 124 psi (110 + 14 so it's an absolute pressure and not a gauge pressure)

T1=283K (10 Celcius or about 50F) and T2=303 (40 Celcius or about 110F)

we find: 132 psi or, 118 as a gauge pressure.

So, for a 30C increase in temperature (roughly 60F), a tire at 110psi would see its pressure increase to 118psi. This is insignificant considering the security margin built into these tires.

Tires do not explode because of a temperature increase or because they are put in a plane for that matter.

The reason why tubes explode in the transition area or in a car backseat, is because tires are improperly mounted (pinching the tube). In such a case, the 8 psi increase might be sufficient to let the tube out.

Francois in Montreal

For a 30C increase, T2 should be 313K ... Of course, the conclusion is the same, as 123 psi is pretty close to 118 psi.

Dre'

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My mistake,

Thanks for pointing this out !

If a black-rimmed wheel is stationary in the sun for a period of time, I'd guess that the temperature of the air in the tire can get pretty hot. Hot enough to exceed the blow-off limit?

Ken Lehner

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"Go yell at an M&M"

And that's why I didn't go into any numbers. I knew that the second I did an actual physicist would break it down for us.

I can't do physics anymore; anything more than cubic interpolation or bond duration calculations and my caveman brain starts to wander.

I will say this though: I believe that I have seen greater increases than that on hot sunny days. I think that a surface temperature(the asphalt) of 130-150 would make the effective temperature increase for the calculation higher than simply the ambient air temperature.

I think that tire pressure adjustments are more for comfort and handling than to guard against blowouts, but I'm not a bike mechanic, I'm a bike rider, so I could be wrong.

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I'm not sure what this rule of thumb is regarding blow-off pressure ... twice max? Something? So anyway, to get to 200 psi in a tire, from 110 psi, you'd need to get the inside air temperature to 215C, or 419F ...

I'm not sure what a reasonable "black rim in the sun" temperature could be, but 419F seems high ...

Dre'

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''If a black-rimmed wheel is stationary in the sun for a period of time, I'd guess that the temperature of the air in the tire can get pretty hot. Hot enough to exceed the blow-off limit? ''

To exceed the blow-off limit (supposedly twice the rated pressure although I have never seen any test results) the temperature would have to be unreasonably high as pointed out by Dr.Dre.

On top of that, remember that if you exceed the blow-off limit, the tire casing will rupture. At high pressure, the tire bead will be firmly tight against the hook-edge structure of the rim. If you simply get a punctured tube when the bike is in the transition area (or in the car, or even at night in your house, as it happened to me), it is clear that it's because your tire bead was sitting on top of the inner tube. A very common mistake.

Francois in Montreal

Now let's do all that again for "sew- ups" :~))

Aloha,

Larry

the reason they may do this is not for fear of the tire exploding but minimizing rolling resistance. I was talking with the Michelin man at a couple of the expos and some of these tires have optimum charateristics at one particular pressure. More or less and the rolling resistance goes up.

Frank

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Frank,
An original Ironman and the Inventor of PowerCranks

Frank's got it. If the tire is too hard, it slows the bike down. Presumably, the pros get decent enough advice on what pressures to use for racing and are making final adjustments. Pump 'em up hard while setting up the bike, and back it off at the last minute to the final pressure desired.

Was it during bike check-in on Fri? Or was it pre-race morning on Sat? If it was check-in on Fri they probably are just being careful. Sat am - just getting their desired pressure before the race.

Now...how does one go about finding out what the optimal tire pressure should be for your tire/wheel/course set-up? I have heard lots of folks swear no higher than 120 psi, even if rated to 170 psi. Also have heard that no higher than 140 psi because you will have to hard of a tire and the bouncing will cause more rolling resistance.

Feel free to enlighten me!

This bollocks about tires exploding because of heat is an old wives tail. Francois does a good job of debunking this myth.

Years ago, Jobst Brandt mentioned on rec.bicycles.tech that the reason why folks got into the habit of deflating tires was from the days when tubulars/sew ups were made with sidewall casing that were extremely fragile. If you were to hit them the wrong way when they were under pressure, the casings would fray and the inner tube would come through and explode. As a result, racers, especially track riders, would deflate their tubular tires to prevent such mishaps.

Perhaps I am propogating more myth, but when Jobst Brandt speaks, I generally listen.

Craig

'the reason they may do this is not for fear of the tire exploding but minimizing rolling resistance. I was talking with the Michelin man at a couple of the expos and some of these tires have optimum charateristics at one particular pressure. More or less and the rolling resistance goes up'

Quite a bit of data on rolling resistance is available. None supports this claim. Rolling resistance decreases with higher pressure, although this is a power function and the curve flattens out rapidly at higher pressure (that is, there is minimal gain in rolling resistance once a certain pressure is achieved in the tire).

Some of the worst technical misinformation comes from the bike industry. I can buy that cornering traction may be optimized at a certain pressure, but definitely not rolling resistance (and that includes rolling on rough pavements, in case someone wants to bring that one up).

Francois in Montreal

I read some where years ago that the inflation rating was one third of the pressure it took to blow the tire from the rim,anyone else hear this? And how can a tire builder know what the BEST inflation is when the weight put on them is so different?

Um, not too sure about that Craig,

I don't have tons of data, but when I was bike racing, I went to some races where air temps went from 70-95F and we were racing on black asphault. I do recall several tire blow outs. Why? My guess was temperature. And what about Beloki - not sure the chain of events, but he was doing a lot of braking on the decent and that heated up his rim enough to blow the rear tire. That day was a scorcher.

Just some thoughts about it...

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Animal!!!

The amount of temperature change required to blow a tire because of pressure is extreme. I think Francois did some math in an earlier post in this thread. You'd have to have about 400F degrees in order to achieve the necessary pressure. The tire casing is going to be compromised at a much lower temperature than that I suspect. If the tires are blowing, I am guessing that it has to do with a compromise in the tire casing because of heat as opposed to tire pressure.

Craig

Beloki was riding tubular. Heat generated from braking during long, steep and agressive descents can heat up the rim significantly, and soften tubular glue sufficiently to enable the tubular to roll off the rim. That's what happened to Beloki (and not a blow-out). Tubulars will not blow off simply because of increased temperature for the same reasons already discussed.

Francois in Montreal

This is why people still let the air out of their tires before races. Because even after being presented with the physics and the near impossibility of blowing a tire from a change in temperature, people still hold onto the myth and refer to previous incidents where they are sure a temp-blowout happened. Oh well! Don't confuse 'em with the facts I suppose.

The one thing I did learn from this post...

A clever man would overinflate his tires before pre-race day checkin. Then on race morning all he has to do is let out some air to get to his sweet spot (120psi right???!!!). All he needs is a gauge. No need for a pump. Of course the clever man would hope he doesn't accidentally let too much air out and he would hope HE didn't get a pinch flat overnight, in which case clever man would have to borrow someone's pump. Oh, and this clever man's pump has his best gauge on it, so he'd have to buy a pressure gauge just for this purpose. Or not.

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Justin in Austin, get it? :)

Cool races:
- Redman
- Desoto American Triple T

At least in relation to the Michelin's it is specific tire dependant and optimum pressure is on the packaging.

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Frank,
An original Ironman and the Inventor of PowerCranks

I didn't say that this was definite. All I'm saying is it was a hot day, and tires were popping like balloons. Seemed to me to be a correlation, despite what the physics says. Could also be other factors. The tire isn't the only expansion point. There is the area where the valve sticks out of the rim...

As for Beloki, wasn't sure about that one... I'll back off that hypothesis.

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Animal!!!

Uh...not quite true there, my friend. On a perfectly smooth surface, rolling resistance does generally go down with pressure going up. Real roads are nothing like that.

It's not the tire pressure per se. It is shock absorption. That's why race cars have suspension. The smoother the track, the harder the suspensions settings. Rolling vehicles go faster when minimizing the transfer of road shock to the unsprung mass. Suspensions accomplish this. A suspension allows the "negative" force that a rough surface places on the wheel to be turned into heat, rather than transfered up to the body of the vehicle and turned into negative acceleration.

In the case of a bike, the tire is virtually the only suspension on the vehicle. It has to be calibrated to the surface to be ridden, and road courses are not like a velodrome or lab testing machine.

BTW -- the people at Michelin know a thing or two about this. They build Formula 1 and lots of other tires, after all. Their people have long advised road racers to ride at 120-140 and no higher.

'... but when Jobst Brandt speaks, I generally listen.'

Can't agree more.

  I used to run those contis up to 160psi thinking harder was better. I now like the 120-130 range for racing as they are way more comfortable and hop less in corners. Last summer my buddy and I were going down a mountain pass with v-brakes and blew a tire on the rear. By the grace of God we landed safely, I licked my finger and touched the rim and it was like a fryingpan with the sizzle. I believe the blowout was heat related. G