Bicycle tire pressure on airplane
April 10, 2006 9:15 PM   Subscribe

Should bicycle tubes be deflated for an airplane journey?

Should I deflate them to a certain pressure or are they going to be fine on a cross-Atlantic journey in the airplane's hold?
Aersol cans are going to be fine too, right?
posted by wavejumper to Travel & Transportation (17 answers total)
deflate the tires
posted by unSane at 9:42 PM on April 10, 2006

It's not necessary at all.

Atmospheric pressure at sea level is about 15 psi. Mountain bike tires are usually inflated to between 20 and 40 psi above atomspheric pressure -- they can certainly take the extra 10 to 15 psi pressure differential in near vacuum. Road tires can withstand upwards of 130psi (usually inflated to less for the sake of comfort).

I would in fact suggest against deflating your tires, because then they don't hold their shape and stay on the wheel rims if the bike isn't boxed/bagged and it is rolled around. The ignorant airline person might still insist on it, so just make a PFFFT sound with a slight deflation while leaving most of the air in your tires.
posted by randomstriker at 9:45 PM on April 10, 2006

IMHO, I'd say there's no reason to, unless you infliate your tires to, or beyond, their maximum p.s.i. rating. And even then, If they're tough enough to handle the rider's body weight, plus the jumps in pressure that come from hitting potholes/curbs/roots/10' dropoffs, they can deal with the loss of pressure at altitude.

So do I deflate my tires when I fly with my bike? Yes. But not because I'm worried about blowouts, I do it because it saves room in the bike box.
posted by bicyclingfool at 9:51 PM on April 10, 2006

I've always partially deflated mine. Flying with a bicycle information. What's in the aerosol can and are they in the hold or the cabin?
posted by tellurian at 10:00 PM on April 10, 2006

the reason to deflate them partially is not to avoid a blowout of the tyre but an argument with the airline staff when they ask you to deflate them.
posted by unSane at 10:12 PM on April 10, 2006

unSane nailed it. Check-in staff aren't likely to be physicists and their procedures say you deflate, so deflate you will. A few times I've flown with a bike I've tried just half-deflating the tires (in the hope of protecting the wheel rims in transit). On retrieving my bike at baggage-claim, they've always been fully deflated. I suspect this is a deeply ingrained meme among airline staff.
posted by normy at 10:34 PM on April 10, 2006

Air pressure at 30-40,000 feet will drop to about 3-4 psi, by the way. So we're really only going to get an increase in gauge pressure of 11-12 psi in the tire, at most.
posted by normy at 11:09 PM on April 10, 2006

unSane, the original question was "should I deflate", not "will I be forced to deflate". The answer to the original question, if the bike rolls unboxed, is no.
posted by randomstriker at 1:07 AM on April 11, 2006

"should" is as much a societal construct as a physical one. he should do it to avoid social conflict. for well-adjusted people who are integrated into their community (ie not physicists) that is a valid argument.
posted by andrew cooke at 5:20 AM on April 11, 2006

Air pressure at 30-40,000 feet will drop to about 3-4 psi, by the way. So we're really only going to get an increase in gauge pressure of 11-12 psi in the tire, at most.

I would be rather surprised if air pressure worked like that.

It's not simply, so at 0.1 psi you only have a 14.9 psi increase in tire pressure? That can't be right. It's just feels wrong!

*Goes off to learn some physics*
posted by public at 6:13 AM on April 11, 2006

Since you are interested in the relative force on the tires at different atmospheric pressures it's clearly more like.

tire pressure / atmospheric pressure

That gives graphs more like I imagine things to behave in real life at least.

Which means that you have ~5 times more force being exerted on the tires at 3 psi than at 15?

I expect the maths is at least a little more complex than that but it seems like a much more reasonable explanation for not letting people fly with inflated tires.
posted by public at 6:30 AM on April 11, 2006

no, public. Force on the tyre is pressure on one side minus pressure on the other.

When the tyres says "inflate to 40 PSI", that means "atmospheric pressure plus 40PSI". Take away atmospheric pressure and you get about 55PSI, which the tyre is not going to care about much at all.

If you take the tyre out in a vacuum, there will indeed be zero force on the outside and therefore infinity times as much pressure on the inside. That doesn't matter though because the force is balanced by tension in the rubber.
posted by polyglot at 6:52 AM on April 11, 2006

I should say (difference in pressure) * area. But it's bedtime.
posted by polyglot at 6:53 AM on April 11, 2006

Polygot is right. That's why we can visit space but not the deepest parts of the oceans. A space ship only has to resist an internal pressure of ~15psi. A submarine capable of visiting the deepest parts of the ocean would have to resist a ~15,000 psi external pressure.
posted by malp at 7:28 AM on April 11, 2006

Cargo holds are pressurized the same as the cabin (for structural reasons, but if you still need convincing: they transport pets in that compartment too), to about 10-11 psi, and a 5 psi change in pressure is negligible on a 100psi tire.
As others have mentioned, this has no bearing on the cargo policies of the airlines.

Aerosol cans are considered dangerous goods and can not legally be transported by air without taking special shipping precautions.
posted by cardboard at 7:48 AM on April 11, 2006

public, resolving forces doesn't really help think about this situation. There's no net force on any part of the tire, or the air in it for that matter. If you want to think about the physics of it all, this is statics, not dynamics. Start with Newton's first and third laws: The tire is at rest (relative to our frame of reference - the aircraft hold). Forces from the difference in air pressure inside and outside the tire are equal and opposite to tension in the threads of the tire casing.

Calculating the actual tension in the tire casing is a more complicated problem involving a bit of calculus, but we don't need to know about that here. The tire manufacturer has done that for us by giving us a recommended maximum pressure. In practice, that pressure can often be greatly exceeded without catastrophe. The recommended pressure from the engineers who designed the tire has not only been filtered through the manufacturer's lawyers, but if you inflate much above the recommendation, you're bike ride will start to get uncomfortable for negligible gain in lower rolling resistance, anyhow. Besides all of that, there's an additional bit of failsafe engineering in bicycle hand-pumps - with most of them it's very difficult to reach the sort of pressure that will blow a properly seated, undamaged tire off the rim.

Another complication is that the everyday pressure gauges we're all familiar with that might be part of any bike pump or at a gas station are notoriously, horribly inaccurate. Like +/- 10% kind of error. To make precise pressure measurements in the modern world an electronic device employing a carefully calibrated pressure transducer is used. Unfortunately, that sort of instrument is much more expensive than the sloppy dial-gauge jobs we all generally use. So when we say "my bike tires are at 100 psi", we're really saying "my bike tires are probably somewhere around maybe 90-110 psi, but I don't really know for sure, but they seem to work ok, so let's not worry about it".
posted by normy at 11:49 AM on April 11, 2006

He should do it to avoid social conflict. for well-adjusted people who are integrated into their community (ie not physicists) that is a valid argument.

Alright, if we are going to be pedants, then I say that a bit of social conflict is necessary if the principle you are conforming to is fundamentally wrong. Besides, rolling an unboxed bike around on deflated tires will likely cause pinch flat, and possibly damage the rims. Screw the feelings of the ignorant check-in chick.
posted by randomstriker at 1:15 PM on April 11, 2006

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