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?
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.
Early bicycle tires were made out of solid rubber. (Before that, iron covered the edges of wooden bicycle wheels.) Solid rubber tires made bicycling a bumpy experience because they were unable to provide any cushioning on rough roads. When the air-filled rubber bicycle tire was invented, it made riding a lot more comfortable.
But along with the comfort of air-filled tires came the frequent task of filling them up. The rubber that is used to make bicycle tires is thin and porous, which means that it has tiny microscopic pores, or holes, through which air can escape over time. Air that is pumped into bicycle tires is pressurized, meaning it is compressed into a much smaller space than it would ordinarily occupy. Without pressurized air inside, a bicycle tire would not have its firm shape. Air under high pressure, like all gases, moves or migrates to surrounding areas that have lower pressure, traveling even through fairly solid materials. So air in a bicycle tire naturally tries to escape through the valve stem that is used to fill it and the inner tube that holds it. So even bicycles that don’t undergo the wear-and-tear of frequent use eventually end up with flat tires.