Schrader Valve - Cap

Cap

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 de-icers used in the winter are especially damaging for the brass components in the Schrader valve.

Some valve caps have a rubber-washer seal (or a synthetic-rubber-washer seal) inside to help make a hermetic seal (airtight seal). Such a rubber-washer-sealing-type cap also helps prevent air from escaping from a slightly leaking valve. In addition, the rubber washer prevents the cap from loosening and falling off due to vibrations and hence being lost, by acting as a cushion between the cap and the valve stem and in this way damping the vibrations. The caps without these rubber-washer seals inside them effectively serve mostly to keep contaminants out of the valve stem, without providing a good hermetic seal. The caps without rubber washers inside them can also loosen and fall off due to vibrations when undertightened, especially if they are made of hard plastic or hard-plastic-lined metal. Valve caps are mostly made of black plastic and also of metal or plastic-lined metal, with rubber washers found inside some caps made of plastic and most metal caps. However, to prevent corrosion due to electrolysis and malfunction of the electronics in a direct TPMS (tyre-pressure-monitoring system) and avoid costly repairs, metal valve caps without insulating plastic linings should never be used on direct-TMPS-equipped tyre valve stems. Therefore, only plastic caps or metal caps with plastic linings should be used in direct-TMPS-equipped tyre valve stems. Moreover, metal valve caps without plastic linings can also cause corrosion and may become stuck to the valve and require breaking of the valve in order to remove it when the cap metal (such as brass) is different than the valve metal (such as aluminium). When the valve caps are installed, care must be made not to leave them loose, as they may not seal properly and may even come loose and fall off. Overtightening the valve caps might damage the rubber washer inside the cap, damage or break the cap itself, or damage the valve. Also, installing the cap at a slight angle can cause stripping of the threads and, therefore, it should be gently finger-tightened while ensuring that it is straight and the threads aren't stripped.

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 tyre under-inflated.

Recently, colored plastic valve caps have appeared. Certain automobile tyre dealerships are promoting the use of dry nitrogen to inflate tyres. Dealers claim that eliminating oxygen and water will prolong the life of both tyres and wheels. These dealers install green caps to signify that the tyres are filled with nearly pure (typically about 95%) nitrogen. In contrast, regular air is approximately 78% nitrogen. The moisture in the air-filled tyres condenses in below-freezing temperatures and causes a sudden drop in tyre pressure. Dry-nitrogen filling helps prevent this problem. As an alternative to green nitrogen-ID (nitrogen-identification) caps, green nitrogen-ID O-rings to be installed on valve stems under the caps, while still using black- or metallic-colored caps, are also available.

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. When used on a bicycle these can also enhance safety by improving the visibility of the rider.