dougmc

Left my bike in my hot car again (this is Texas, summer, so it gets quite hot inside.)

This often causes flats. Presumably not because the pressure increases so much (doing the math, even if it's 200 degrees F inside the car and it was room temperature when you filled the tire, that increases the pressure by only 25%) but I'm guessing because it makes the rubber in your tube softer and perhaps makes your rim tape softer (if it's plastic anyways -- shouldn't affect cloth.) Ultimately I'm not really sure why it seems to cause flats.

Well, it happened again -- got a flat tire. And I put air into it, and it's holding the air just fine, which doesn't make sense. Unless the heat affects the rubber seal in the Schrader valve?

DannoXYZ 

Could be the valve failing. If you pump it back up again, does it hold pressure indefinitely in cool temps?

cbchess

Common problem here in VA.
We have to air down our tires if we leave them in the hot car, then pump them up before the ride.
Just bring your pump and let the air out to you get 60 pounds or so. Then pump up before the ride.

DannoXYZ 

Hmmm, on my race car, 35psi cold tyres get up to 40psi @100F and 50psi @200F on normal air. Using nitrogen maintains +/- 1-2 psi from cold pressure.

Remember when calculating pressure-changes using the ideal-gas law, that you must use temperature in Kelvins starting from absolute-zero. Pressure is also absolute from 0-vacuum rather than gauge-pressure.

sch

I have had the same experience on two bikes, a bent and DF. Valves seem to get leaky above a certain temp and hold air fine when cooler.. Easy for a car to get to 140-150F inside in our area, Austin likely worse. Every time the tires pump up fine when cooled down. I also reduce pressure when the bike has to stay in the car outside in summer. At 30-40# pressure the valve seems to hold ok.
FWIW, heavy braking will heat a rim too hot to touch, something readily
apparent on our tandem. So far this does not seem to have affected the
valves there. Maybe the valve doesn't spike up like the rim does.

KevinF

Say what? I lived in Northern Virginia for about 10 years, and I drove my bike all over the place to do various rides. I never "aired down" my tires or got a hot car induced flat tire.

Virginia. An awesome place to ride, except for the summer months.

exRunner

Run the numbers through the Ideal Gas Law and you get:

Cold = 120 psi @ 80 F
100 F = 125 psi
200 F = 150 psi
300 F = 175 psi

tatfiend 

True with dry air. That is the reason for auto racers using bottled Nitrogen, no water vapor in it which minimizes pressure changes and makes them more predictable compared to using ambient air with its variable amount of water vapor which is dependent on the humidity.

dougmc

It's true with moist air and pure nitrogen too, for that matter.

The only time that moist air behaves any differently is if it condenses, which will cause the pressure to drop when it happens. Well, that, or if you have liquid water in your tires, which will cause the pressure to rise when it evaporates (but it'll evaporate in pure nitrogen too.) But normally condensation happens because the temperature dropped, not because it got hotter.

As you said, the only real advantage to using nitrogen is that it's usually drier than plain air, so there's less change of any condensation happening.

DannoXYZ seems to have overstated the advantages of pure nitrogen. Going from 80 F to 200 F, the (total) pressure will increase by about 25%, be it dry air, moist air or pure nitrogen. (I'll assume that the tire didn't have any liquid water in it to begin with, but if it did, it would evaporate in pure nitrogen or dry air, and probably in moist air too, increasing the pressure further.)

DannoXYZ 

Find a chart showing the partial-vapour pressure of water-vapour. It's not a linear curve... It's not liquid-water at STP you're concerned with, it's the dissolved vapour and you can find roughly how much based upon the relative humidity at the time you pumped up the tyre.

DannoXYZ 

So you get a +5 psi increase when ambient temps go from 80F to 100F in a hot car. At 120 F, it's +10psi increase. I doubt the deflation-effect is due to pressure-increases in the tube, but most likely some other component that's failing.

dougmc

No, not exactly. It's the `ideal gas law', not the `real world gas law', after all. (Nitrogen isn't perfectly linear either, of course.)

However, it's extremely close for the pressures and temperatures we deal with in our tires. The only time that water vapor behaves significantly differently is when it condenses (which can be a real concern under certain conditions -- filling your tires when it's hot and humid, for example, which condenses when it cools, then you find the pressure too low the next morning, so you top them off, then it gets hot again, the condensation evaporates and the pressure goes up ...)

You may be thinking of the MAXIMUM partial vapor pressure of water vapor -- that does indeed vary with temperature. However, it increases as temperature increases, so if you air up your tires when it's cool and not near 100% humidity you aren't likely to have any problems as they warm up (at least not any problems that you wouldn't also have if they were filled with dry nitrogen.)

fuzz2050

problem with the ideal gas law, it only really applies to IDEAL gasses,

dougmc

Compare to the good thing about the ideal gas law -- it applies extremely well to gasses like oxygen, nitrogen and water vapor (and their mixtures) at the pressures and temperatures typically seen in tires.

The only place where it breaks down in any significant way for our purposes is when a gas turns into a liquid, or a liquid turns into a gas ...

Doug64

About 100 years ago (well 1978) when I was racing, I got the bright idea of pumping up tires (sew-ups@120lbs) the night before so I didn't have to mess with the bike on the ferry ride to the course in the morning. I leaned the bike up against the wall in the motel room pretty close to the heater. It was February so the heater ran all night. About midnight the front tire literally blew. I spent the ferry ride over sitting out in the cold fixing my tire so I could have a spare. Fixing sew-ups even in an ideal situation is challenging, especially when you have to do it in 30 minutes. Everyone else was inside having coffee!

DannoXYZ 

Yes, this law assumes that gases are infinitely small particles with no volume. There are numerous correction tables to apply to calculations made using the ideal gas law. Some of them skew the results by over 100%.

Ganesha

Have you taken a look at the tube to see how it failed? I've seen two types of 'high temp' failures. One where the tube is missing a circular patch of material. Another common failure point is just by the valve.

dougmc

The tire held air perfectly for a few days (in the somewhat temperature controlled garage) and then went flat all of a sudden. Pulled the tube out, and it's got a big hole at a spoke. And I didn't think this bike did, but it looks like it's got plastic rim tape.

So, it looks this particular incident was another case where the rim tape got soft and the tube grew into the space and popped. But since the rim tape is airtight, and the tube was a slime tube, the slime managed to seal between the rim tape and tube pretty well, at least for a few days, then it went loose all at once (well, within a few hours.)

I need to stop using plastic rim tape, even if it's nice and convenient. (It fits nicely anyways, but I'm doing cloth from now own.)

neil0502

Don't recall anybody ever recommending switching from Velox....

DArthurBrown

I think you guys are overthinking this a bit. The gas heats and expands, but the rubber in the tube and tire doesn't expand uniformly. It can burst at the seams, develop a hot spot, etc. If the rim is in the sun, it can get so hot that it can melt the tube and ... "BANG!"

Remember what used to happen to VHS tapes left in a hot car?

DannoXYZ 

Dude... have you ever actually measured the highest-temps a rim can reach in an enclosed car in direct sunlight? I assure you it's not hot enough to melt the tube. In fact, it's cooler than what the rim faces under braking...

And vulcanized rubber CANNOT melt! Take a blowtorche to a tube and see what happens. It'll burn before melting significantly enough to develop a hole.

And he's found the problem, bad rim-tape. One of the most common causes of tube punctures. Going with Velox tape is always the best upgrade, even on a brand-new bike.

dougmc

Well, this time the hole was right at the rim. And it was big -- a few milimeters long.

My theory is that the rim tape got soft enough to let the rubber push it into the hole. The rim tape never broke. The rubber stretched a lot right there, and then it popped. But the slime eventually sealed it to the rim tape, allowing it to hold air when refilled, at least for a while. Once the temperature went back down, the rim tape went back to normal.

All a theory, but it seems likely.

I've had hot car flats with fabric rim tape too, but never really investigated it that carefully. Ultimately, my plan for the future is to 1) not leave bikes in hot cars, 2) if I must break #1, let the air out first (and throw a good pump in the car so I don't have to use a crappy frame pump), and 3) use fabric tape (velox sounds good) when I replace rim tape.

EDIT: `right at the rim' probably wasn't the right choice of words. It was on the inside of the tire, pointing towards the center of the tire.

DannoXYZ 

Also be careful when mounting the tyre, be especially aware of pinched tubes. Before pumping up the tyre, pull it sideways and inspect the gap between the tyre and rim-edge; no tube should be showing. Move over about 6" and pull tyre sideways again and inspect. Work your all the way around the tyre. Then inspect the other side. This will prevent the majority of self-popping tubes. People are always reporting tyres popping in basements, cars, etc.

This is usually caused by a pinched tube that slowly works its way out and blows up. By far the most-common cause of pinched tubes is starting & stopping your tyre-mounting at the stem. The thicker tube there will pretty much guarantee you'll pinch the tube. You'll need to push up on the stem into the tyre to un-pinch the tube. If you start & stop somewhere else other than the valve-stem, the step of pushing the stem into the tyre won't be necessary. I know, it's not that big of a task, but A LOT of people forget about it and end up with self-popping tubes.