Schrader valve
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The Schrader Valve is a valve stem invented by August Schrader and is used on all automobile tires and lower pressure bicycle tires. It is a poppet valve assisted by a spring. Schrader valves are also used on the fuel rail of some direct fuel injection engines and on some automotive air conditioning systems, in a diameter larger than those used on tires, to allow recharging with refrigerant. In addition, Schrader valves are sometimes used on irrigation systems to provide a place to temporarily attach a portable pressure gauge.
It uses air pressure inside the tire to help shut the valve, but has a spring to assist in keeping the valve shut (especially important at lower pressures). The size used in bicycle tires has a greater diameter than the Presta valve. The spring pressure makes it better than the Presta valve for low pressure tires but the larger diameter hole it requires can weaken the rim. Due to this, they are more common on mountain bikes, which have wider tires and lower pressure.
The valve cap is essential on a Schrader valve because dirt and water can enter the outside of the valve if one is not fitted, potentially jamming it and causing a leak later. The cap also helps to prevent air escaping from a slightly leaking valve. Metal valve caps usually have a small rubber insert to permit a good seal against the valve body.
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