A valve cap is the plastic cap that screws over a valve; usually, valve caps are used on a car or bicycle tire. Because a valve cap has nothing to do with actually sealing the air inside a tire, many wonder why companies use them. Despite what most assume, valve caps do serve a purpose depending upon the valve.
There are two types of common valves, which means their are two different types of valve caps. With each type the valve cap serves an important purpose. Schrader valves and caps are by far the most common type. They are used on car tires and bicycle tires. Presta valves and caps are used only on bicycle tires.
Between Presta and Schrader, Schrader caps are the most useful type of valve cap. The reason being is because a Schrader valve has cylinder-shaped chamber where it houses its valve. Without a valve cap dirt and other contaminants have the opportunity to penetrate the valve seal. The cap also serves as a covering to protect the internal and exterior parts of the valve from rocks and other road debris.
Unlike Schrader valves caps, a Presta valve cap is relatively useless once installed. This is because Presta valves are narrower and have no metal housing that will collect dirt around its valve core. Yet, this isn't to say the cap serves no purpose. Manufacturers reason that caps protect thinner tubes during shipping--usually the tube is tightly packed against the sharp medal threads of the valve. However, after a tube is installed on your bike there is little if any reason to use it.
Another reason companies use valve caps is to market their products and distinguish them from other companies. Many times these plastic caps will range in color depending upon the brand you choose. Some bike stores actually sell specialty caps in shape eight balls, dice or even in the shape of bullets. Manufacturers make a variety of different styles.
Your bike or car tire will most likely be all right if you lose the cap, unless of course you regularly submerge your wheels in 2-foot-deep mud puddles. However, if you can't bear the possibility of something happening to your valve, make a trip to your local bike shop. Usually bike shops have jars of extras they are more than willing to part with at no extra cost.
