A Schrader valve is a type of gateway that allows air to be pumped into a car tire or other pressurized object. Present on just about all cars and trucks on the road, the Schrader valve protrudes from the rim of the wheel, and a pump can be attached to it. The pump presses against a pin inside the valve, which opens the chamber to allow air to pass through. When the pump is removed, the spring-loaded pin closes, sealing off the air inside the tire. A Schrader valve is often fitted with a plastic cap when not in use to prevent dirt and moisture from clogging the valve.
While the Schrader valve is present on just about all motor vehicle tires and rims, it is only present on some bicycle tires and rims. Bicycle tires and rims are often quite narrow, meaning a drilled opening for a Schrader valve would be too wide and could weaken the rim. A Presta valve is often used in place of the Schrader valve. The Presta valve is much narrower than the Schrader valve, and it seals off the chamber using just the pressure inside the tire or inner tube, unlike Schrader valves, which are sealed off by a spring loaded pin.
Schrader valves can be used with a tubeless system or they may be present on an inner tube. An inner tube is a rubber insert that sits in between the tire and the rim; it has a valve attached to it, and when the tube is inflated, it presses against the inside walls of the tire, pressurizing the entire set-up. Many cars used to use an inner tube system, but the inner tube has become antiquated in lieu of tubeless tires. A tubeless tire seals to the rim when it is inflated, essentially acting as its own inner tube. Schrader valves are inserted into the rim and are an independent component rather than a part of any inner tube system.
Tires and tubes are not the only uses for Schrader valves. Many other pressurized components use Schrader valves as a means of initially pressurizing the component. Refrigerant systems commonly use these types of valves, and some types of pneumatic vehicle shock absorbers use Schrader valves as well. SCUBA systems employ the use of these valves, as do some aircraft that use hydraulic lift systems for various flight components. Other areas on cars can use the valves as well, such as on certain types of engines.