Presta valve

[3] The outer valve stem is manufactured in various lengths to cater for the different depths of the wheel or rim.

It is recommended the valve stem is to be at least 10 mm longer than the rim is deep to allow adequate room for the bicycle pump to be attached.

[4][better source needed] The air pressure in an inflated tire holds the inner valve body shut.

The screw remains captive on the valve body even when unscrewed fully; it is tightened again after the tire is inflated and the pump removed.

Conversely, when a Presta valve is fitted into the larger Schrader rim hole, grommets or reducers are sometimes used to take up the extra space.

An adapter can be fitted onto this external thread to permit the Presta valve to be connected to a pump with a Schrader chuck.

A push-on diaphragm Presta chuck tends to push the valve stem in to the tire cavity, which slows down getting a good seal.

Leaving the nut loose still allows the valve to seal, but may make it easier to have accidental (or malicious) deflation.

This avoids the Presta's nut, but leads to use of slower screw-on or heavier clamp-on Schrader chucks.

At very high speed - approximately 150 kph - Schrader valves can open under centrifugal force, leading to a flat tire.

The important observation is a Presta valve's nut is not an advantage in actual use, because ordinary bicycles do not go fast enough for it to matter.)