While inflating your car tires it is the valve stem that allows the air to flow into it and then automatically seals itself so that the air is retained inside at a constant pressure. The air is prevented from escaping by a spring or simply by its own pressure. Valve stems are typically found on any inflatable tire, whether it's for bicycle tires or automobile tires and valve stem caps provide the sealing for the valves. These caps are essential to also keep dust, dirt and other pollutants from entering the inner parts of the tire and causing damage.

Additionally, one of the functions of the valve stem cap is to provide protection to the valve stem threads and keep them from wearing out. This ensures that the cap remains tightly fitted on the valve to prevent escape of air. There are different types of valve stems which have been developed in different countries and for specialized purposes. Some like the Presta are used only on bicycles, while Schrader valves may be used on automobiles. However, the valve stem caps are manufactured to fit the diameter of the valve heads only.

Generally, valve stem caps are made of low-density polyethylene, as this material is highly resistant to all kinds of weather conditions, affordable, can withstand high impact and is also resistant to petroleum products. The domed head and ribbed sides design provides a good grip while turning and locking into place. However, they're available in a variety of materials like aluminum, brass alloys, chrome-plated, antique finish, etc. Better quality caps come with a rubber seal or gasket that fits snugly into the inner portion of the valve stem cap and provides a good, tight fit to seal the cap. This is essential to retain the tire pressure at a constant rate and prevent leaks.   

Auto enthusiasts may also prefer custom-designed valve stem caps to provide a unique look to their tires. Manufacturers have on offer specialized designs like chromium-plated or LED caps to give that quirky flash as you drive by! Skull-and-crossbones, smileys, eagle-wings, dice, chrome-plated cobras and bullet shapes are some of the unusually shaped tire valve stems caps available, in a variety of colors, textures and finishes. These caps, apart from serving a very essential function, also help in making a style statement!

Another innovation in tire valve stem caps is the caps embossed with the required air-pressure. This makes it convenient for the driver to remember what the ideal air-pressure should be for his tires. Some caps also have inbuilt pressure indicators, which display when the pressure falls below the optimum.  

Your tire valve stem cap can be a perfectly functional component but you can also make a personality statement of it if you like!