A dustcap is a device used on a vehicle to prevent dirt and debris from contaminating a bearing or vital component as well as help retain grease and other types of lubrication. The two most common uses of a dustcap on a vehicle are found on the front wheel spindles and the tire's valve stems. The spindle dustcap is commonly a pressed-tight fit while the typical valve-stem cap is a thread-on application. The primary duty of the spindle dustcap is to retain the wheel-bearing grease as it becomes liquefied from the friction heat of the spinning wheel hub. The valve-stem dustcap is primarily used to prevent the buildup of dirt in the inner workings of the valve stem, which would render it useless and could even deflate the tire.
Most dustcap designs are manufactured from light steel, plastic or rubber. While the typical function requires no physical restraint of moving parts or pressure, the component is able to complete its task while requiring no strength or special assistance. In the case of a spindle cap, only the melted grease comes into contact with the cap, and a wheel cover or center cap commonly protects the dustcap from coming into contact with any obstacle or foreign object. Valve stems use a spring-loaded diaphragm inside the valve stem that actually seals in the air pressure and provides the retention of air within the tire or tube. Occasionally, a valve-stem cap will be constructed of stainless steel and will have a valve-stem core removal tool designed into the cap.
The original design and intended purpose of the dustcap may be uneventful and minuscule in the grand design of the vehicle to most vehicle owners; however, it offers an award-winning opportunity for many vehicle customizers. When searching for the most minute detail to separate one vehicle from another in the show car arena, Some customizers choose to add class and style to their vehicles' dustcaps. Milling spindle caps from billet aluminum and anodizing or chroming the finished part to add a little more flash has become commonplace in the show circuit.
Valve-stem caps have been created from billet stock to resemble bullets, dice and a wide array of other objects intended to individualize the owners' particular taste and likes. Some after-market manufacturers even offer valve-stem caps that incorporate small light bulbs and batteries to provide an illuminated valve-stem cap as the wheels rotate. This new focus on the dustcap has brought more attention to an otherwise overlooked component.