Editing Freewheel (section)

{{Short description|Mechanism which disconnects a driveshaft from a faster-rotating driven shaft}}
{{distinguish|Flywheel||}}
{{Redirect|Freewheeling|the Italian comedy|Freewheeling (film)}}
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[[image:Freewheel en.svg|thumb|Freewheel mechanism]]
[[File:Roue libre cliquet.svg|thumb|right|[[Ratchet (device)|Ratcheting]] freewheel mechanism (van Anden, 1869)]]
In [[mechanical engineering|mechanical]] or [[automotive engineering]], a '''freewheel''' or '''overrunning clutch''' is a device in a [[transmission (mechanics)|transmission]] that disengages the [[driveshaft]] from the driven shaft when the driven shaft rotates faster than the driveshaft. An [[Overdrive (mechanics)|overdrive]] is sometimes mistakenly called a freewheel, but is otherwise unrelated.

The condition of a driven shaft spinning faster than its driveshaft exists in most [[bicycle]]s when the rider stops [[Bicycle pedal|pedaling]]. In a [[fixed-gear bicycle]], without a freewheel, the rear wheel drives the pedals around.

An analogous condition exists in an [[automobile]] with a [[manual transmission]] going downhill, or any situation where the driver takes their foot off the [[Car controls#Throttle control|gas pedal]], closing the [[throttle]]: the wheels drive the engine, possibly at a higher RPM. In a [[two-stroke engine]], this can be catastrophic—as many two stroke engines depend on a [[fuel]]/[[oil]]  mixture for [[lubrication]], a shortage of fuel to the engine starves oil from the [[cylinder (engine)|cylinder]]s, and the [[piston]]s can soon seize, causing extensive damage. [[Saab automobile|Saab]] used a freewheel system in their [[Saab two-stroke|two-stroke models]] for this reason and maintained it in the [[Saab 96]] [[V4 engine|V4]] and early [[Saab 99]] for better [[fuel efficiency]].