Why Do Tires Lose Pressure?
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
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
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.
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.]
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
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
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,
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
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.