In its most basic form, a bicycle pump functions via a hand-operated piston. During the up-stroke, this piston draws air through a one way valve into the pump from the outside. During the down-stroke, the piston then displaces the air from the pump into the bicycle tire.
Higher end models have a built in pressure gauge to indicate when the tire
is optimally filled.
Caution must be used when using a gas station air pump. Some are designed to cut off before the high pressures used in many bicycle tires
are reached. Others may pump enough air into the small volume of a bicycle tire
in a short time that will blow out the tire.
There is also a slight difference between the modern standard for Schrader valves on an automobile and that on a bicycle, which makes some more recent valves on gas station pumps a poor fit.
Be careful to make sure that the pump head you are using is compatible to the valve on your bike. The two most commonly used valves and Presta and Schrader valves. See below for more information about them both.
The Presta valve is a valve commonly found in high pressure road bicycle tires
and higher end mountain bicycle tires,
although it is now used on all UST or "tubeless" mountain bike tires.
It consists of three major components: the valve body, a threaded valve stem, and a lock nut. In order to accommodate different rim
wall depths, the valve body is available in various lengths. Like the Schrader valve, air pressure inside the tire
holds the valve shut; however, unlike the Schrader, the Presta has no return spring to help keep the valve shut. Instead, a threaded stem extends out of the valve core
to hold a locking nut that can be screwed down to ensure the valve remains closed. A Schrader core
can be unscrewed and replaced; some Presta valves also have removable cores.
In addition, the Presta valve has a much smaller diameter than the Schrader valve. This helps the strength of wheels
with narrow rims
because the weakest point on such rims
is the location of the hole for the valve. A Presta valve requires a different chuck on the bicycle tire
pump than the larger Schrader valve; simple threaded brass adapters are available to allow a Presta valve tube to be inflated by a pump or air chuck designed for Schrader valves.
The Presta valve must be manually unscrewed to permit airflow in either direction. The screw, located at the tip of the valve, remains within the valve housing even after it is unscrewed. Unscrewing the valve permits the plug to move up and down within the housing. It must be unscrewed before attaching a tire
pump. Once the tire
pump is removed, the valve should be screwed down to prevent accidental air leakage. A screw cap provides protection
to the valve top and prevents the valve from puncturing the tube when it is rolled for storage, but it is not necessary to keep air from leaking.
The standard Presta valve has an external thread.
A Schrader valve consists of a hollow cylindrical metal tube, typically brass, with the exterior end threaded. The interior end takes a variety of forms depending on its application. A new development is Schrader valve stems with integrated transmitters for tire
pressure monitoring systems (TPMS).
In the center of the exterior end is a metal pin pointing along the axis of the tube; the pin's end is approximately flush with the end of the valve body.
Generally, all Schrader valves used on tires
have threads and bodies of a single standard size at the exterior end, so caps and tools generally are universal for the valves on all pneumatic tires
on automobiles, bicycles, and even lawnmowers, hand trucks and wheelbarrows (at least in the United States). Most air hose fittings for inflating tires,
generally referred to as chucks, actually are not threaded. Instead they seal to the threads by compressing a rubber collar around the threads, or they simply seal to the end of the valve tube with a gasket (usually of rubber), which is sealed by manual pressure. For the former type, the threads are still important because they have essentially the same effect as concentric ridges around the valve tube, giving the rubber something to conform around for a firm grip against the force of pressure, which would tend to push the fitting off of the valve. Both tire
chucks and tire
gauges include a built in fixed pin to depress the valve core's stem as you apply the tool to the valve.
Depressing the pin is also the method for manually relieving the pressure retained by the valve (for example, when a tire
is over-inflated or needs to be deflated but the core
removal tool is unavailable). As mentioned above, a pressure gauge, when pressed firmly onto the end of a valve, depresses the pin to open the valve so that the pressure inside can be measured. Deliberately not pressing the gauge firmly is a common practice when a tire
is to be intentionally deflated.
Presta vs. Schrader
Presta and Schrader valves are both good at sealing high pressures. Their chief differences are that Schrader valves are larger and have springs that close the valve except when the pin is depressed. Schrader valves are used in car tires, tires
of inexpensive bicycles, mountain bike
air shocks, and in many types of compressed gas and compressed liquid systems. Presta valves are used only for bicycle tires.
Schrader valves used for bicycle tires
have a greater diameter than Presta valves and the larger diameter hole required for a Schrader valve will weaken a narrow wheel rim.
For this reason, Schrader valves are not used in the narrow wheel rims
of racing bicycles. Another disadvantage of the Schrader valve is that, when inflating a tire,
the air chuck must depress the pin before air can flow. The chuck and its use is therefore more complex. The Presta valve relies solely on internal air pressure—not a spring—to keep it shut (although it can be manually tightened after inflation). Schrader valves close and stay shut regardless of pressure until the pin is depressed.
drilled for Presta valves may be converted to accept Schrader valves, by drilling it out with a 21/64" drill bit, but care must be taken to de-burr the resulting hole to prevent damage to the tire
and inner tube.