A tire valve uses a one-way flow system, operated by a metal pin, to allow air into an inner tube or tire without letting trapped air escape.
Most performance bicycle tires contain two main valve designs: Schrader and Presta, or the french valve. A third type, the Dunlop valve, is rarely seen in the United States, but may be found on bikes in Europe and Asia.
The Schrader valve incorporates a heavy stem with an internal pin system. It is a robust design that handles large amounts of air, making it ideal for mountain bike tires.
The Presta, or french valve, is thin and fragile with an external, lockable pin. It is mainly used for performance road tires, where light weight and high air pressures are important.
Dunlop valves have a similar design to the Presta valve and can be inflated using a Presta pump. However, the valve seal is maintained with a rubber gasket, which can become cracked with age, rendering the valve useless.
Presta valves can be found on performance mountain bike tubes, but Schrader valves are not often seen on road bikes.