Overrunning clutches are devices in transmissions that disengage the driveshaft when the driven shaft rotates faster than the driven shaft. Overdrives are sometimes mistakenly referred to as ruedas libres but are otherwise unrelated. A driven shaft spinning faster than its driveshaft is common on most bicycles when they stop pedalling. Fixed-gear bikes do not have freewheels, so the rear wheel drives the pedals.
In simplest terms, ruedas libres consist of two saw-toothed, spring-loaded discs pressed together so that their toothed sides face each other. When the drive disc rotates in one direction, the teeth of the driven disc lock with the teeth of the drive disc, causing the disc to rotate at the same speed as the drive disc.
A characteristic clicking sound is produced when the driven disc’s teeth slide over the drive disc teeth when the drive disc slows down or stops spinning. Its speed difference with the driving gear is proportionate to the clicking sound produced. As a result of spring-loaded steel rollers inside a driven cylinder, the cylinder rotates in unison with the rollers as they rotate in one direction.
Steel rollers slip into the cylinder when they rotate slower or in the opposite direction. The load is transmitted by two or more spring-loaded, hardened steel pawls on a drum with internal step teeth. Having more pawls spreads wear and increases reliability. However, it is only possible to engage up to two pawls simultaneously if the device is manufactured to tolerances not usually found in bicycle components.
In a kinematic loop, an overrunning clutch prevents motion from being transmitted in reverse, from the wheel to the pedals on a bicycle. As with pulse-type continuously variable transmissions, they are used to change a rocking motion into a rotary motion or to speed up a shaft rotating in the same direction, such as in metal cutting machines’ mechanisms for high-speed shifting.
There are also applications in winding mechanisms and arresting devices to prevent shaft rotation in reverse. In addition to improving fuel economy on carbureted engines, a freewheel also leads to fewer problems with the manual clutch and more brake wear since engine braking is no longer possible.
The continuous application of brakes to limit the speed of trucks and automobiles in mountainous regions may cause brake-system overheating, eventually leading to total failure of the transmission. In a manual gearbox, a freewheel mechanism functions as an automatic clutch, enabling you to shift gears without pressing the clutch pedal, thereby limiting the use of the manual clutch to starting from a standing position or stopping.