Why do car tires not need a tube or liquid sealant but bicycle tires do?
In: Engineering
Hi, tire guy of some time here,
The tire is sealed to the rim by air pressure alone. Sometimes the bead on the rim is corroded and air can escape through the gaps, but the solution to that is to clean it up with a wire wheel, apply a liquid rubber “bead seal” (it doesn’t seal per-se, it just provides a reasonably flat surface for the bead of the tire to sit).
Whether or not a tire requires a tube depends on the type tire and wheel used. Automobiles don’t need tubes because both the tires and wheels are designed to hold air without them.
Most bicycles still require tubes, but you can buy bicycle wheels and tires that do not. They call them “tubeless”, and they’re very popular amongst riders who spend a lot of time off-road. Most cyclists run sealant for convenience (to automatically seal punctures), but you don’t necessarily *have* to run sealant if you don’t want to.
Bicycle tires will leak down faster though, because the rubber is much thinner and the bead is much longer (bicycle rims are often close to 29″ in diameter. A very large car wheel is 22″ in diameter. The tire leaks around the circumference of the bead, which grows at a rate of 3.14159 times the diameter. This means that for every inch of diameter you add, you get more than 3 inches of circumference along which the tire will leak. Hence, most bicycle tires strongly recommend sealant to make them more air tight.
The tubeless tire for bicycles probably doesn’t have an air impermeable inner liner or a very thin one. It was designed to be used with sealant (for self sealing) so probably relies on the sealant for air retention.
Air retention in a car tire comes from the inner liner, which is thin layer of halobutyl rubber cured onto the inside of the tire. Without it the tire wouldn’t be able to hold air for long periods of time. I suppose a car tire could also use sealant but it wouldn’t be practical as car tires deal with freezing, high heat and the requirement for balance.
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