The Schrader valve (also called American valve) invented by August Schrader in 1891 consists of a valve stem into which a valve core is threaded, and is used on virtually all automobile tires and most wider rimmed bicycle tires. The valve core is a poppet valve assisted by a spring.
In addition to tube and tubeless tires, Schrader valves of varying diameters are used on many refrigeration and air conditioning systems to allow servicing, including recharging with refrigerant; by plumbers conducting leak-down pressure tests on pipe installations; on the fuel rail of some direct fuel injection engines; and on the buoyancy compensator hose of SCUBA regulator systems, allowing the user to remove and attach the hose while in use.
The valve Edit
A Schrader valve consists of an externally threaded hollow cylindrical metal tube, typically of brass. In the center of the exterior end is a metal pin pointing along the axis of the tube; the pin's end is approximately flush with the end of the valve body.
Generally, all Schrader valves used on tires have threads and bodies of a single standard size at the exterior end, so caps and tools generally are universal for the valves on all common applications.
A new development is Schrader valve stems with integrated transmitters for tire pressure monitoring systems (TPMS).
The valve cap Edit
A valve cap is important on a Schrader valve because if one is not fitted, dirt and water can enter the outside of the valve, potentially jamming it or contaminating the sealing surfaces and causing a leak. Rock salt and other chemical deicers used in the winter are especially damaging for the brass components in the Schrader valve.
Metal valve caps usually have, in addition to a handy deflating tool, a small rubber insert to permit a good seal against the valve body; a cap of this kind also helps to prevent air escaping from a slightly leaking valve. However, the vast majority of Schrader valves used for tires are fitted with plain black plastic caps which effectively serve only to keep contaminants out of the valve stem.
There are also special pressure monitoring valve caps available that use a spring loaded piston to raise a green flag when the pressure is at or above the correct setting. Upon losing pressure the green flag is retracted to reveal a red pin, hopefully catching the attention of the owner before fuel is wasted by running the tire under-inflated.
Recently, colored plastic valve stem caps have appeared. Certain automobile tire dealerships are promoting the use of dry nitrogen to inflate tires. Dealers claim that eliminating oxygen and water will prolong the life of both tires and wheels. These dealers install green caps to signify that the tires are filled with nearly pure (typically about 95%) nitrogen.
Other vendors are selling caps in a variety of other colors for purely decorative purposes. The decorative category even includes caps that light up when the wheels move.
Presta valves vs. Schrader Edit
Presta and Schrader valves are both good at sealing high pressures. Their chief differences are that Schrader valves are larger and have springs that close the valve except when the pin is depressed. Schrader valves are used in a wide variety of compressed gas and compressed liquid applications. Presta valves are used only for bicycle tires.
Compared to Presta valves, Schrader valves used for bicycle tires have a greater diameter, requiring a larger diameter hole in a bicycle rim. While not a concern on wider bike rims, it will weaken a narrow wheel rim, precluding their use on racing bicycles. Another disadvantage of the Schrader is that the air chuck must depress the spring-loaded pin before air can flow during inflation, whereas the Presta valve relies solely on internal air pressure to keep it shut. This means that some air is lost while attaching and detaching pumps to Schrader valves.
A rim drilled for Presta valves may be converted to accept Schraders by drilling with a 21/64" bit, but care must be taken to de-burr the resulting hole to prevent damage to the tire and innertube.
Screw-on adapters are available at bike shops to give a Presta valve the diameter required for using standard air fittings.
Schrader valves are classified by their material, diameter of intended rim hole, length, and shape.
- TR-4 - straight metal stem (8 mm dia.)
- TR-6 - straight metal stem (8 mm dia.)
- TR-13 - straight rubber stem (11.5 mm dia.)
- TR-15 - straight rubber stem (16 mm dia.)
- TR-87 - short 90° metal stem (10 mm dia.)
- TR-87C - tall 90° metal stem (10 mm dia.)
The standard Schrader valve has the following threads:
- Metric: 7.7 mm OD, thread root diameter is 6.9 mm x 0.794 mm pitch.
- English: 0.305 in OD, thread root diameter 0.302 in x 32 tpi (threads per inch)
Internal thread (to accept the threaded valve core)
- Metric: 5.30 mm OD x 0.706 mm pitch
- English: 0.209 in OD x 36 tpi.
For refrigeration, a 1/4" male flare fitting is used, with the same internal thread as above.
See also Edit
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