A Presta valve is a type of inflation valve commonly used on some types of bicycle inner tubes or tubeless tire setups. This type of valve stem is narrower than the Schrader valve stem that is common on automobiles, motorcycles, and some bicycles; this is done to strengthen the wheel's rim, as the weakest point on the rim is usually the valve hole. Air is retained in the inner tube or tire by closing the Presta valve, which is done manually by tightening a barrel adjuster down until it presses against the valve stem. When the barrel is loosened, air can pass through for inflation.
Sometimes known as a French valve, the Presta valve is commonly used on road bicycles and on high-end mountain bikes to strengthen the rim and prevent air pressure loss when higher pressures are pumped into the tire or tube. The valve stem may come in a variety of lengths to accommodate shallow rims or deep dish rims, as many road style rims feature a deep dish for aerodynamics and strength. Every valve stem features an opening in which the core is placed, and unlike Schrader valve cores, the Presta valve core usually cannot be removed without permanently damaging the tube. Some tubes feature a removable core, but these tubes are rare and tend to be more expensive.
The only reason a core would be removed is to add puncture-resistant fluid to the tube, which is an easy process on Schrader valves. Some people will cut the soldered tip off the valve core to remove the barrel and the core beneath, though this is not advisable and can damage the tube permanently. Some Presta valve tubes come pre-filled with puncture resistant fluid to counteract this problem. Otherwise, the only way to get fluid into the tube is to puncture it, which is also inadvisable because this compromises the integrity of the tube.
Presta valves often come with a plastic cap to prevent the core from damage during use and storage, as well as a metal lock nut that threats onto the outside of the stem and secures the stem to the rim, preventing jostling and potential damage to the joint between the stem and the tube rubber. The plastic cap does not necessarily need to be used, and many cyclists avoid the use of the cap; some cyclists even go without the lock nut, as it is only an added security against movement and is not a necessary part for proper function.
