Freewheel

The condition of a driven shaft spinning faster than its driveshaft exists in most bicycles when the rider stops pedaling.

An analogous condition exists in an automobile with a manual transmission going downhill, or any situation where the driver takes their foot off the gas pedal, closing the throttle: the wheels drive the engine, possibly at a higher RPM.

The simplest freewheel device consists of two saw-toothed, spring-loaded discs pressing against each other axially with the toothed sides together, like a ratchet but with the usual stationary part also rotating.

Rotating either the wheel or cassette in the direction that produces the clicking sound causes the pawl to easily slide up and over the gently sloped edges of the teeth.

When the cassette or wheel is rotated in the opposite direction, the pawl catches against the steeper-sloped edges of the teeth, creating a lock.

Most bicycle freewheels use an internally step-toothed drum with two or more spring-loaded, hardened steel pawls to transmit the load.

This may make freewheel transmissions dangerous for use on trucks and automobiles driven in mountainous regions, as prolonged and continuous application of brakes to limit vehicle speed soon leads to brake-system overheating followed shortly by total failure.

Without a live PTO, a high inertial load can cause the tractor to continue to move forward even when the foot clutch is depressed, creating an unsafe condition.

Because of the extreme gear ratio between starter gear and flywheel (about 15 or 20:1) it would spin the starter armature at dangerously high speeds, causing an explosion when the centripetal force acting on the copper coils wound in the armature can no longer resist the outward force acting on them.

[citation needed] In addition to the automotive uses listed above (i.e. in two-stroke-engine vehicles and early four-stroke Saabs), freewheels were used in some luxury or up-market conventional cars (such as Packard, Rover and Cord) from the 1930s into the 1960s.

During the Second World War, the military Volkswagen vehicles produced by KdF (Kübelwagen, Schwimmwagen) were fitted with a ZF limited-slip differential system composed of two freewheels, which sent the whole of the engine power to the slower-turning of the two wheels.

This feature appeared mainly on large, luxury cars with heavy clutches and gearboxes without synchromesh, as the freewheel permitted a smoother and quieter change.

This was an option on Citroën 2CVs and its derivatives and, as the name implied, was marketed as a benefit for driving in congested urban areas.

This lets the driver select a lower range to provide engine braking at various speeds, for instance when descending a steep hill.

[11] In 1924 French firm Le Cyclo introduced a gear-shifting bicycle with a two sprocket freewheel, which let riders to go uphill with more ease.

In the late 1920s, Le Cyclo began using both front and rear derailleurs in combination with a double chainring, giving the bicycle twice as many gears.