ACSCHRADERVALVES by POLARBEARINC

The Lowly Schrader Valve

The Schrader valve was invented by August Schrader, an enterprising German-American immigrant who set up a shop dealing in rubber products in lower Manhattan at 115 John Street. In 1845, he began supplying fittings and valves for rubber products made by the Goodyear Brothers, including air pillows and life preservers. Schrader was also a maker of daguerreotype photographic apparati. Shortly thereafter, he went into partnership with Christian Baecher, a brass turner and finisher.

After watching divers working at a nearby jetty, Schrader sought to improve the diving helmets in use at the time. In 1849, he created a new copper diving helmet which ultimately led to him to design an air pump for the same purpose.

Around 1890, after reports of English cyclists' success using pneumatic tires, August Schrader saw the need for a bicycle tire valve. By 1891, he produced the Schrader valve, his most popular invention which is still in use today. August’s son, George, is generally credited with the experimental work that resulted in the valve's creation.

Later in 1896, Schrader patented the tire 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.

The complete valve consists of a valve stem into which a valve core is threaded. The valve core is a poppet valve assisted by a spring. Schrader valves are used in a wide range of applications, some of which include the fuel rail of fuel injected engines and for access ports in refrigeration and air conditioning systems.

The fitting typically varies with the application and many are larger than those used on tires. Other uses include recharging ports for refrigeration systems and temporary gauge ports for pressure or pump driven systems. They are often found inside the Quick Disconnect fitting on a buoyancy compensator hose of SCUBA regulator systems to allow the user to remove hose and attach the hose while under pressure. Schrader valves are also used in bladder pressure tanks to introduce air when the air cushion needs topping up. An example of this would be air filled adjustable shock absorbers.

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 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 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

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. 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 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 car tires, bicycle tires, mountain bike air shocks, and in many types of compressed gas and compressed liquid systems. Presta valves are used only for bicycle tires.

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 not used in the narrow wheel rims of racing bicycles. Another disadvantage of the Schrader valve is that, when inflating a tire, the air chuck must depress the pin before air can flow. The chuck and its use is therefore more complex. The Presta valve relies solely on internal air pressure—not a spring—to keep it shut (although it can be manually tightened after inflation). Schrader valves close and stay shut regardless of pressure until the pin is depressed.

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 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.

All of the above information, including the animated gif was taken from a Wikipedia search on Schrader Valves.