The tire air myth
 

I have flown with my bicycle numerous times, over various distances, within my own country, and to other countries . . . and the one thing I am consistent about is .......

..... forgetting to let the air out of my tires!! :eek:


However, in all my travels I've yet to have something happen to the tires.


So a question for the physics-minded among you ... why do some of the airlines recommend (and sometimes strongly encourage) me to let the air out of my tires when it doesn't seem to matter one way or the other?

This is probably due to the decreased atmospheric pressure at high altitude. The concern is it may lead to a higher exerted pressure with in the tire, leading to a blow out/off from the rim.

However it would seem your adventures show it's not that risky :) Most cargo areas are pressurized.

Well, the atmosphere at sea level is <15psi. I inflate my tires to 110psi for the sake of argument. Assume the air pressure at altitude is 5psi. The difference of 10psi is not going to pop the tires.

Wouldn't your example be a difference of 20 psi?

However, I only inflate my tires to somewhere between 90 and 100 psi at the best of times anyway.

Just make sure you don't pump them to 110 while scuba diving. Then you might run into some problems when you surface :)

Airline policies are not necessarily based on physics! They classify compressed air containers as explosives, and bike tires have compressed air in them, so....

The general rule is atmospheric pressure decreases by half for each 18,000-foot increase in altitude. So at 36,000 feet, the tire that was once at 110 psi at sea level could be high enough to blow it off the rim or pop a tube. Thats the Airline's concern.

While air pressure at 30,000 feet may be one-third the pressure at sea level, it's also significantly colder up there (like 100 degrees Fahrenheit colder). Who knows what climate controls are protecting your bike down below, though -- they put cats and dogs in there, too.

You're also supposed to detune guitar strings before checking a guitar on a plane.

Well, that's the thing ... if the atmospheric pressure doubles in the hold, where they put the cats and dogs, wouldn't the cats and dogs explode or implode or something ... at least suffer some very serious side effects?

Living things probably require a pressurized cargo hold, either that or the time spent at high altitude isn't long enough to cause swelling ;)

It would also need to be heated...if the air temperature can form ice crystals on the windows, I would not want my animal to be in a hold the same temperature as outside the plane when I am flying trans continental...

I didn't state it very clearly. The difference was the atmospheric pressure. It went from 15psi to 5psi, hence the 10. The presure in the tire doesn't change at all since no air is going in or out.

Nope, you just have to ascend slowly, 10 seconds per 10 feet and then stop at 15 feet, waiting 3-5 minutes before you surface with the wheels. ;)

100% right

The cargo area is heated and presurized to the same leve the cabin you are sitting in. which is about the same presure as living in Denver, Col

I always let some air out of the tires – get them maybe at 80% of maximum rated pressure. That way they can probably take the increase in relative pressure if the cargo hold depressurizes – which, I believe, doesn’t happen that often.

'


Exactly right.


End of thread.

WHAT ?#$%!&@ No exploding pets and gerbils stories???

I know a guy who once thought he should fill his tire to the maximum instead of lowering them. He flies to Europe from America for bike touring. He claims he had both tires blown off the rims once. In the right circumstances with the right wheels and tires 5 or 10 psi extra could be enough to get the tires off the rims. What if the tire gauge is off and he really filled them over the maximum to start with, for example?

Bologna! This is just another one of those bicycling myths that get repeated so often that many people believe them.

Air pressure at sea level is only around 15psi. So a tire that contained 110psi at sea level would contain 125psi in a pure vacuum. At 36,000 feet the internal tire pressure would still only be around 122psi. assuming the cargo compartment was unpressurized. But, since airlines routinely ship pets in cargo compartments, I assume the cargo compartments are both pressurized and heated just like the cabin.

I agree with with the sentiment. The 10psi pressure increase with in the tire is probably an insignificant amount to blow it off the rim, due to the decreased atmospheric pressure at high altitude. I was only stating it for what I think the concerns are of the Airlines if they ask/recommend you to deflate the tires. Either that or they don't want the slight chance that if the tube/tire blows for what ever reason, that it's bursting could be mistaken for a bomb :rolleyes:


I guess it's also possible, it may happen if the tire was already inflated to some crazy pressure that is near blowing it off the rim at sea level. That additional 10 psi could be the amount to push it over the limit ;) Such as 2manybike's example.

Hey, that gives me an idea... You could try bringing your bike into the main/passenger cabin as a "companion pet." (I don't know if that would require purchasing an extra seat, though...) ;)

Actually the pressure does change. The pressure you are reading on a tire gauge is really reading the differential pressure between atmospheric and what is in the tire. If you lower the atmospheric pressure, the differential is greater since no air is going in or out of the tire.

There's an obvious reason why her tires aren't popping.


It's because there's no one standing next to them to scare the crap out of when it happens. ;)

http://www.htguide.com/forum/images/...on_laughat.gif

I was at my friends once where he was fixing his flat on the MTB. Business as usual until the tube blew. The bang scared the crap out of us, however the percussive shock from the explosion really made our heads hurt :eek: He had not checked for bead pinches :lol:

Damn that was loud in that small room :eek: I haven't been the same since and inflating bike tires indoors makes me apprehensive :( I still do it, but I'm always taken back to that event many years ago. The funny thing was it happen to him twice that day :lol:

That reminds me. I was on a flight with a friend. He had one of those packs of airline peanuts. He put it right up next to the headerest of the guy in front of us and squeezed it until it popped. The guy jumped up about a foot out of his seat. Of course, the hard part was sitting there like nothing had happened when he was looking all over the place. :D