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HUMOR: Why Do Tires Lose Pressure?
Acute observation has noted a strong tendency for bike tires to deflate when the bicycle is not being used by the owner. Expert analysis has revealed profound truths.

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Why Do Tires Lose Pressure?

This challenge racked us in wreck.bikes.mischievous. We did our very best to solve the following problem, but we could not come to an agreement.  However, we feel that one of these solutions must be correct!

Keren Hamel asked: Has anyone ever noticed that if you ride every day, your tires deflate slower than if you leave the bike alone for weeks without riding? I can go for weeks riding every day, without pumping the tires, and the minute I don't ride for a week, the tires are totally flat. Why is this? 
Mike Miller said: As you ride along, the air caught between the tire and the ground is pushed into the tire, replacing the air that has leaked out. 
Mark Woodhead said: I think (egad! an opinion!) that the white powder in the tubes works to clog the pores of the rubber. When you ride, that powder is constantly being spread around to fill those pores. When your bike sits idly by, waiting for you to give it some attention, the powder settles down and the air begins to slowly leak out. 
Ken Kifer said: This is a great thread! However, I think everyone else is completely wrong! 

The real reason is this: The tires are really enthusiastic about riding! As long as you keep riding, they stay pumped up! 

But when you go without riding for a week, they become really deflated! 

Jack Dingler said: Come on guys! Tell him the real reason!

Years ago, a few tube manufacturers, under pressure to reduce the weight of their inner tubes began looking for ways to make a lighter tube. First they did a little market use analysis and determined that the worst time for a tube to lose air is when it's being used. As a tube is being spun while in use, they realized that due to the laws of centripetal acceleration, the air presents the greatest pressure on the outer wall of the tube. This lead to the obvious answer, they could shave the thickness of the inner wall!

By doing this, they've created tubes that are lighter, and yet, just as effective at holding air while being used. When the tube is not spinning, it of course, loses air at a faster rate, as the air is no longer constrained by centripetal forces, and presents more pressure to the inner wall (the thinner one).

For racers, this was no big deal. They fastidiously check their tire pressure before every ride anyway.

Under pressure from racer wannabes, the tube manufacturers eventually had to make all of their tubes to match the higher performance racing tubes and so through market forces now sell the multi-thickness tubes exclusively. 

Wendi Tobler said: Actually, it's because they lose muscle tone when not being used, thus they become flabby. 
Jim Chinnis said: You are probably failing to park your bike with the valves up. If the bike sits around idle, the air eventually stops moving and ends up in the bottom of the tire. If you park with the valve stems near the bottom, the valves may not be able to handle the additional air and will leak.

Performance has a vibrator you can attach to each wheel that will keep the air molecules moving, thus avoiding the problem. The Ti versions are $149 apiece. CroMo versions are a lot cheaper and give a sweeter shake. 

CDale SR900 said: Friction. 
David Casseres said: If you ride fast, it's relativistic time dilation at work. 
Ken Kifer said: I think this has to do with the uncertainty principle and Schroeder's cat (or was it Charlie Brown's dog?). If no one is watching the tires, they have a 50-50% chance of being flat, so one day they're flat, and the next day they're full of air. But as long as you're watching them, they have to behave normally. The solution is to keep the bike in the living room, so the tires will have to behave. Another solution is to put the bike in the closet when the tires are flat and not take it out until you find them full of air. 
Hobbel N. Kramp said: What's this? Humor? We here in wreck.bikes.technogeek do not engage in humor. We are serious. Please do not waste our time with such rubbish. You must all be buffoons.

All attempts at humor will be studiously avoided. A serious response to each of the above responses will be prepared, debunking the myths and urban legends in each. Now, go away.

Besides, as any true technical expert will tell you, or if you will just obtain Fred Fryburger's Fatuous Fysics Formulas, or the Physics for Pinheads encyclopedia published by Pedantry Press, the increased centripetal force of air molecules against the inside surface of the tube creates an osmotic hysteresis effect which closes the pores in latex and butyl compounds. Anyone who says it is the talcum powder is a black liar!

And the relativistic time dilation theory has been disproved by the detailed measurements of Raymond Diehard. Read the FAQ!!!! 

Bob von Moss said: Maybe it's because when you ride, it heats up the tube (due to pressure the rider exerts down and the resistance of the tire on the road), which may cause the rubber material to expand a little bit like a piece of dough rising. If the bike remains cold for many days, the tube material may shrink and permit some of the pressurized air in the tube to escape. 
Mike J1 said: But then how do we rationalize the mysterious "High pressure leak" that only occurs under high-inflation, and disappears when you inflate the tube at a lower pressure and check for holes in a tub of water?

These leaks would seem more likely to occur when riding than when not. Can't the tubes get their act together and be more consistent in their behavior? 

Jay Wenner said: The answers given thus far certainly lack any scientific basis, and are typical of people with limited scientific background. The air escapes from the tires at equal rates regardless of if the bike is ridden or not as shown in the partial derivatives below:

[15 pages of ASCII impossible to read equations omitted. They've been omitted because some nerd will actually go through the equations and find some simple fact that ruins the whole theory.]

After re-taking introductory psychology twice, I can safely say that this whole dilemma can be simply solved with Freudian theory. You see the bike ride represents sex, and pumping up the bike tires represents really outstanding sex. If you ride every day, there's no problem. If you take a couple weeks off, you need to pump up your tires. 

Verne A. Aebli said: Actually, all of you that responded so far have made an obvious but incorrect assumption: that tubes are filled with only air.

Anyone who's purchased a new tube fresh out of the box is familiar with that wonderful "new tube smell". New tubes emit this smell profusely. If you assume that both the inner and outer surfaces of the tube emit (outgas) equal amounts of new tube smell, there must be a lot of this aroma confined within the tube. (Since everybody knows that smells don't penetrate rubber.) The heat generated by the friction of riding excites the rubber molecules to further outgassing, filling the tube even more. The more you ride the more new tube smell is released into the tube. Those of us that spend 24hrs/day on our bikes know for a fact that air does not needed to be added to tires, though occasionally you do have to bleed off a little of the odor or risk rupture. This is especially important prior to transporting your bike by air.

As further evidence of this theory I submit that it is common knowledge that new tubes hold "air" better and longer than old tubes. This is due to their higher aromatic potential. 

Tim Chambers said: The reason this is especially important for air travel is that the prospect of a holiday/tour/meeting new bikes causes further excitement of the rubber, thus producing more of the aroma. This can lead to blowouts. 
Ed Chait said: Good guess but no aromatic cigar.

The reason that frequently ridden tubes hold air better is because the molecules of air inside the tube are pushed around in a high pressure environment. This produces an increased physical robustness that prevents them from escaping through the rubber pores as easily as their wimpy less exercised brethren. 

Jim Balter said: I explained to Keren that her cat was sucking the air out of her tires (that's what happens to mine), but she claims not to have a cat. Perhaps it's dust mites, then. 
David Martin said: Oxygen deficiency and pollution.

As the oxygen is depleted and heavy molecules such as carbon dioxide and ozone (and metal residues from combustion byproducts) become more abundant in the atmosphere, the air in the tyres becomes relatively light by comparison and so rises to the top of the wheel leaving a flat spot at the bottom where the 'heavy' gasses outside have pushed it in.

When you then stop and pump it up by the roadside you are pumping in 'heavy' gasses that will fill the flat spot at the bottom.

It should of course be noted that when tyres go flat they are more susceptible to damage from bits of glass etc. so these will rapidly find their way into the tyre before you stop.

And so the great search for truth goes on, with the most astute minds straining their abilities to establish beyond doubt the power of science and intellect in our lives!

Thanks for all the great contributions! This is an epic work that shall exist when the pyramids are dust!

Further contributions will be accepted only if they can meet the rigorous scientific standards above.

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