In mechanical or automotive engineering, a freewheel or overrunning clutch is a device in a transmission that disengages the driveshaft from the driven shaft when the driven shaft rotates faster than the driveshaft. An overdrive is sometimes mistakenly called a freewheel, but is otherwise unrelated.
The condition of a driven shaft spinning faster than its driveshaft exists in a bicycle when the rider holds his or her feet still, no longer pushing the pedals. Without a freewheel the rear wheel would drive the pedals around.
An analogous condition exists in an automobile with a manual transmission going down hill or any situation where the driver takes his foot off the gas pedal, closing the throttle; the wheels want to drive the engine, possibly at a higher RPM. In a two-stroke engine this is a lethal situation: as the engine depends on fuel for lubrication, a shortage of fuel to the engine would result in a shortage of oil in the cylinders, and the pistons would seize after a very short time causing extensive engine damage. Saabs used a freewheel system in the two-stroke models for this reason and maintained it in the Saab 96 V4 and early Saab 99 for better fuel efficiency.
A more sophisticated and rugged design has spring-loaded steel rollers inside a driven cylinder. Rotating in one direction, the rollers lock with the cylinder making it rotate in unison. Rotating slower, or in the other direction, the steel rollers just slip inside the cylinder.
Most bicycle freewheels use an internally step-toothed drum with two or more spring-loaded, hardened steel pawls to transmit the load. More pawls help spread the wear and give greater reliability although, unless the device is made to tolerances not normally found in bicycle components, simultaneous engagement of more than two pawls is rarely achieved.
By its nature, a freewheel acts as an automatic clutch, making it possible to change gears in a manual gearbox, either up- or downshifting, without depressing the clutch pedal, limiting the use of the manual clutch to starting from standstill or stopping.
In agricultural equipment an overrunning clutch is typically used on hay balers and other equipment with a high inertial load, particularly when used in conjunction with a tractor without a live power take-off (PTO). 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. By disconnecting the load from the PTO under these conditions, the overrunning clutch improves safety. Similarly, many unpowered 'push' cylinder lawnmowers use a freewheel to drive the blades: these are geared or chain-driven to rotate at high speed and the freewheel prevents their momentum being transferred in the reverse direction through the drive when the machine is halted.
A freewheel assembly is also widely used on engine starters as a kind of protective device. Starter motors usually need to spin at 3,000 RPM to get the engine to turn over. When the key is turned to the start position for any amount of time after the engine has already turned over, the starter can not spin fast enough to keep up with the flywheel. 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 centrifugal force acting on the copper coils wound in the armature can no longer resist the outward g-forces acting on them. In starters without the freewheel or overrun clutch this would be a major problem because, with the flywheel spinning at about 1,000 RPM at idle, the starter, if engaged with the flywheel, would be forced to spin between 15,000 and 20,000 RPM. Once the engine has turned over and is running, the overrun clutch will release the starter from the flywheel and prevent the gears from re-meshing (as in an accidental turning of the ignition key) while the engine is running. A freewheel clutch is now used in many motorcycles with an electric starter motor. It is used as a replacement for the Bendix drive used on most auto starters because it reduces the electrical needs of the starting system.
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 Rovers and Cords) from the 1930s into the 1960s. The freewheel meant that the engine returned to its idle speed on the overrun, thus greatly reducing noise from both the engine and gearbox. The mechanism could usually be locked to provide engine braking if needed. A freewheel was also used in the original Land Rover vehicle from 1948 to 1951. The freewheel controlled drive from the gearbox to the front axle, which disengaged on the overrun. This allowed the vehicle to have a permanent 4 wheel drive system by avoiding 'wind-up' forces in the transmission. This system worked, but produced unpredictable handling, especially in slippery conditions or when towing, and was replaced by a conventional selectable 4WD system.
In the older style of bicycle, where the freewheel mechanism is included in the gear assembly, the system is called a freewheel, whereas the newer style, in which the freewheel mechanism is in the hub, is called a freehub.
The freewheel is sometimes known as the "Stieber Clutch" named after its German developer Ortwin Stieber.