The Schrader valve is a valve stem invented by August Schrader and is used on virtually all automobile tires and wider rimmed (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.
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The valve
A Schrader valve consists of a hollow cylindrical metal tube, typically brass, with the exterior end threaded. The interior end takes a variety of forms depending on its application. A new development is Schrader valve stems with integrated transmitters for tire pressure monitoring systems (TPMS). 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 tube. Pushing this pin into the valve opens it. There is a spring inside the Schrader valve which pushes the pin outward to close the valve when the pin is released.
The threaded exterior end of the valve is used to attach a hose fitting for pumping fluid (gas or liquid) in through the valve, to momentarily fit a pressure gauge to the valve for measuring the pressure behind it, and to secure a protective cap such as the combined cover seal and installation tool pictured. The illustrated tool is normally only used to removed the valve's core, which rapidly deflates the tire, pursuant to the repair or replacement of a damage or worn-out tire. (Beware: unscrewing valve cores used in high pressure applications, especially applications other than tires (e.g. refrigerants), can be very dangerous.) 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 pneumatic tires on automobiles, bicycles, and even lawnmowers, hand trucks and wheelbarrows (at least in the United States). Most air hose fittings for inflating tires, generally referred to as chucks, actually are not threaded; instead, they seal to the threads by compressing a rubber collar around the threads, or they simply seal to the end of the valve tube with a gasket (usually of rubber) which is sealed by manual pressure. For the former type, the threads are still important because they have essentially the same effect as concentric ridges around the valve tube, giving the rubber something to conform around for a firm grip against the force of pressure, which would tend to push the fitting off of the valve. Both tire chucks and tire gauges include a built in fixed pin to depress the valve core's stem as you apply the tool to the valve. Depressing the pin is also the method for manually relieving the pressure retained by the valve (for example, when a tire is over-inflated or needs to be deflated but the core removal tool is unavailable). As mentioned above, a pressure gauge, when pressed firmly onto the end of a valve, depresses the pin to open the valve so that the pressure inside can be measured. Deliberately not pressing the gauge firmly is a common practice when a tire is to be intentionally deflated.
The valve cap
A valve cap is essential 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 the handy 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. Eliminating oxygen and water is said to 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 vs. Schrader
Presta and Schrader valves are equally good at sealing in high pressures. Their chief differences are their outside dimensions, internal resistance to inflation and their reliability at sealing under low pressures. Schrader valves used for bicycle tires have a greater diameter than Presta valves and the larger diameter hole required for a Schrader valve will weaken a narrow wheel rim. For this reason, Schrader valves are more common in the wide wheel rims of mountain bikes, Presta valves in the narrow wheel rims of racing bicycles. The drawback of the Schrader valve is, when inflating a tire, the pump has to overcome both the tire pressure as well as the internal resistance of the spring loaded valve seal. With racing bicycles using high tire pressures, any added inflation resistance is especially undesirable. The Presta valve gives minimum resistance to inflation as it relies solely on internal air pressure - no spring - to keep it shut (although it can be manually tightened after inflation). The advantage of the Schrader valve's spring is however that, although air pressure helps it to stay shut it is not essential: the Schrader valve closes and stays shut even at very low pressure, making it ideal for low pressure tires. A rim drilled for Presta valves may be converted to accept Schrader valves, by drilling it out with a 21/64" drill bit but care must be taken to carefully de-burr the resulting hole to prevent damage to the tire and inner tube.
The Schrader valve's dimensions
The standard Schrader valve has the following thread: External thread
- 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 (the valve core)
- Metric: 5.30 mm OD x 0.706 mm pitch
- English: 0.209 in OD x 36 tpi.
