Bicycle wheels are comprised of a tough outer tire, a soft inner tube, and a rim (the stiff outermost-edge which is often supported by spokes). The inner tube is inflated means of a valve stem that opens to let air in and then automatically closes and is kept sealed by the pressure in the chamber. The Presta valve (also called the ‘Sclaverand’ or ‘French’ valve) is a valve commonly found in high pressure (100 psi) road style inner tubes. It is comprised of an outer valve stem and an inner valve body. A lock nut to secure the stem at the wheel rim and a valve cap may also be present.
The outer valve stem is manufactured in various lengths for different applications, and has a narrower diameter at the base (6 mm) than the more common Schrader or American valve (8 mm) and is also used on most automobile tires. Japan, India, Russia, Germany, Britain, and several other countries use a third type of valve for their bicycles, the Dunlop (also called a ‘Woods’ or ‘English’ valve) which also has a wider base than a Presta valve. It is similar enough in size to a Schrader valve to use identically drilled valve holes in rims, but it can be inflated with a Presta valve adapter.
Unlike the Presta, the inner mechanism of the Dunlop and Schrader valves (the valve core) can be replaced easily, without the need for special tools, which is why the Dunlop is also common in developing nations. In recent years removable core Presta valves have become more common. The presta is popular with road bikers because its small base allows for very narrow tires, which are fast on smooth surfaces like pavement. The weakest point of a bicycle rim is usually the hole for the valve stem. The smaller hole for a Presta valve makes it possible to have extremely narrow wheels while maintaining sufficient strength in the wheel.
The air pressure in an inflated tire holds the internal parts of a presta valve shut. A small screw and captive nut on the top of the valve body permits the valve to be screwed closed securely. The nut must be unscrewed to permit airflow in either direction. The screw is attached to the valve body even when unscrewed fully; it is only untightened to pump in or release air. The valve cap protects the valve body, keeps dirt and mud out of the mechanism, and also prevents the valve from damaging the tube when it is rolled for storage, but is not necessary to prevent pressure loss.
Because the rims of bicycles drilled for Presta valves cannot accommodate the wider Schrader valves, it is often the case that rims need to be drilled for such replacements. Conversely, when a Presta valve is fitted into the larger Schrader rim hole, grommets or reducers are sometimes used to take up the extra space. The standard Presta valve has an external thread. An adaptor can be fitted onto this external thread to permit the Presta valve to be connected to a pump with a Schrader chuck. The same adaptor, because of a coincidence of thread sizes, can convert a direct-fitting Schrader pump into one that can connect to flexible adaptors of either kind.