ANSWERS: 1
  • A valve stem is a part of the small valve most often used in bicycle and automobile tires. It maintains the pressure in the tire and prevents the pressurized air inside from escaping.

    Function

    Valve stems use the pressure of the gas inside the tire pushing out on the seal to maintain a seal. Often a small plastic or rubber ball rests in the valve, being pushed against the hole at the end by the air pressure inside. This ball is what creates the seal.

    Filling

    In order to add more air to a tire, the pump must first push the valve stem's seal in. This reason is why bicycle tire pumps often have a needle-like shaft inside the clamp that attaches the pump to the air valve.

    Presta Valves

    Presta valves are one type of valve stem. Presta valves are narrow, which prevents them from weakening the tire. They often contain a small nut that must be screwed into position before they can be filled.

    Schrader Valves

    Schrader valves are larger and thicker than Presta valves. They often have a cap that must be unscrewed before they can be filled. In Schrader valves, a spring helps keep the seal in place.

    Woods Valve

    Woods valves are an older type of valve that is not very common in the US. Woods valves are narrow like Presta valves and use rubber tubing.

    Source:

    Sheldon Brown's Bicycle Glossary: Presta Valve

    techno-fandom.org: Valve Stem Problem

    Sheldon Brown.com: Presta vs. Schrader valves

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