tire valve caps

A valve stem is a self-contained valve that opens to admit gas to a chamber (such as air to inflate a tire), and is then automatically closed and kept sealed by the pressure in the chamber, or a spring, or both, to prevent the gas from escaping.
There are many types of valves, and they are most commonly used on automobile, motorcycle, and bicycle tires, but also for many other applications. The most common type is the Schrader valve, but some road bicycles with skinny tires instead use the Presta valve.

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