Will an inflated thermarest break or explode if I put it in my airline's checked luggage?
April 14, 2006 12:50 PM   Subscribe

Above the pressure at sea level, how many additional PSI can I expect checked luggage to experience ?

I'm trying to come up with a bike bag that would protect a bicycle while in transit on an airplane. The idea is that most hard cases for bikes are strong, but heavy, and with recent changes in maximum checked luggage weight, often the traveler gets charged extra fees for excess weight. I already have a full sized mountain bike that collapes into a suitcase size that is within the maximum luggage dimensions allowed. (See www.sandsmachine.com) Now I'd like to make a nylon case with padding made of aircells of some sort, much like Thermarest camping pads. Not only are these things light and potentially could protect the bike, but I may collapse the big bag and take it with me while bike touring, a huge advantage when one wants to fly into one airport and fly out of another (or train or bus or whatever).

However, I can't imagine using breath inflated thermarests if they will break when the plane flies at those altitudes. I think the question comes down to: How many additional PSI of pressure will be added to an already inflated thermarest when at cruising altitude?
posted by jldindc to Travel & Transportation (15 answers total)
recent thread

Note though, that most cargo holds are pressurized.
posted by randomstriker at 12:57 PM on April 14, 2006

Didn't we just have a question about a bicycle tire?

The cargo hold is pressurized. Just like the passenger cabin. The pressure in the cabin and hold is not going to drop much below 10 psi (as compared to 14.7 at sea level). The bicycle tire thing is sort of, but not entirely, ludicrous - any bicycle tire normally inflated will easily withstand 5 extra psi pressure differential. But it would be possible to massively overinflate a bicycle tire, such that it was just about to pop, and then ship it, and have it pop.

5 extra psi pressure isn't going to explode anything that is normally inflated. But the airlines may object to anything inflatable being carried.
posted by jellicle at 1:05 PM on April 14, 2006

Even if you were to fly that plane into outer space with an unpressurized cargo hold, only 14.7 psi. I think this point was made in the last thread but despite its common sense it seems easy to overlook.
posted by caddis at 1:05 PM on April 14, 2006

Response by poster: Randomstriker, you're right, they are pressurized, but that does not mean they don't experience changes in pressure. I believe that's why our ears pop on takeoff and landing.
posted by jldindc at 1:10 PM on April 14, 2006

Response by poster: Ok thanks everyone, I'll assume 5 psi in changes and take that into consideration in my case contruction project. I figure that if I make the nylon case effectively, the airlines won't even know there is an inflatable bladder/thermarest in there.
posted by jldindc at 1:15 PM on April 14, 2006

Thermarests are notoriously easy to pop. If you use one, be sure to protect it from the chainrings, etc.
posted by exogenous at 1:16 PM on April 14, 2006

You won't experience additional PSI unless you fly below sea level.
posted by knave at 1:33 PM on April 14, 2006

Doesn't S&S sell a backpack case?
posted by fixedgear at 1:34 PM on April 14, 2006

Randomstriker, you're right, they are pressurized, but that does not mean they don't experience changes in pressure. I believe that's why our ears pop on takeoff and landing.

Well, that's because pressurized doesn't mean pressurized to identical pressure as sea level. Aside from that, it can take very little pressure change to make your ears pop; mine do simply driving around here in the VA/DC area.

How things work talked about cabin pressurization, albeit briefly. A more concrete number is over on this thread which matches what I have seen elsewhere, that pressure is equiv to 8000 ft, not the often bandied about (incorrect) claim that sea level is simulated.

Using this calculator here you get a result of 0.7192735652003696 atmospheres which multiplied by 14.7 gets you 10.57 psi. Which is an even smaller diff that jellicle's 5psi.
posted by phearlez at 1:41 PM on April 14, 2006

I've found thermarests to be surprisingly tough. I used one for over 20 years on everything from sharp rock to boggy swamps to fractured ice. I've carried them to over 20,000 feet. Even deflated they swell up considerably at high altitudes and compress to as thin as cardboard when you go back to sea level. An airplane cargo hold is pressurized to 8000 feet so the changes are less extreme. Just allow for a little extra swelling at altitude.
posted by JackFlash at 1:41 PM on April 14, 2006

Here's a link to some cases for this sort of bike, as fixedgear mentions above. The backpack case retails for $235, hard cases significantly more. They also describe a cardboard box and cover as a cheap option.
posted by exogenous at 1:48 PM on April 14, 2006

I'll assume 5 psi in changes and take that into consideration in my case construction project.

That reasoning sounds fine, but be careful!

I've limited knowledge when it comes to pressure vessels, but.. The much larger surface area of the thermarest concerns me greatly. What is the normal inflation pressure?

I found an air mattress with a maximum inflation pressure of 40mmHg, which is apparently about 0.77psi. Based on those numbers I suspect the thermarest will burst!

Earlier tonight I was looking at the pressure rating of PVC pipe. Notice how the pressure rating goes down with increased diameter? This is because pressure acts on surface area so, the larger the diameter of the pipe, the larger the tension in the pipe wall per psi of internal pressure.
posted by Chuckles at 7:17 PM on April 14, 2006

I should elaborate - the pressure rating goes down despite increasing wall thickness!

The effects of pressure grow quickly.
posted by Chuckles at 7:20 PM on April 14, 2006

I don't see how this can work. The bike is most likely to experience jolts and potential damage at landing and takeoff, right? And in order for the inflatable padding not to burst during the flight it will have to be partially deflated upon take off and landing making it the least effective at the point of highest risk.
posted by fshgrl at 8:28 PM on April 14, 2006

Response by poster: Thanks everyone for great insight on this topic.
I have used the S&S backpack case, and even made my own internal folding alluminum frame for it. However, given the number of times I flew with the case, the frame ended up bending more than I wanted it to, and the nylon backpack case started to rip at the seams from all the pressure points of the internal frame. I know I'll never have a perfect case to protect the bike, and I'll always have some compromises between weight, strength, durability, and protection of the bike. It's interesting to note your comments about surface area and pressure. I may end up flying at some point with a test case using thermarests without the bike inside, just to see if they leak. If not, I will have to evaluate how much of my breath is required to protect the bike at sea-level without causing it to break at altitude. I will certainly report back here on this thread if anyone is interested in my results. I also will post a note on my favorite site on this topic, Travel with Bicycles

I have opted not to use a cardboard case, since they are just not very durable, especially when I'm traveling with the bike so much. Most of my travels are trips around the world, and with multiple stops (last trip, 11 flights). If there was some sort of lightweight durable plastic for this purpose, that could be rolled up and taken with me on the bike, and wasn't too bulky, that might work. The idea of using Thermarests in some way would provide a dual purpose - protection of the bike while in transit, and as camping pads when camping.

Again, thanks everyone. I really like your feedback.
posted by jldindc at 9:07 AM on April 15, 2006

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