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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 most when the rider stops Bicycle pedal. 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 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 cylinders, and the can soon seize, causing extensive damage. Saab automobile used a freewheel system in their Saab two-stroke 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.
Bicycles use freewheels to allow the cyclist to coast without pedaling. 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. This process is sometimes informally referred to as "slipping." In this scenario, the cassette rotates independently of the rear wheel. When the cyclist stops pedaling, the ratchet slips as the wheel continues to rotate while the cassette stops, producing the clicking noise. Consequently, a bicycle will not move in reverse if the cyclist pedals backwards. 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. As the cyclist pedals forward, the cassette spins forward causing the pawl to catch against the steep slope of the teeth and drive the rear wheel in the forward direction. This also explains why pushing the bicycle forward causes the pedals to rotate in the forward direction.
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.
A freewheel also produces slightly better fuel economy on carbureted engines (without fuel turn-off on engine brake) and less wear on the manual clutch, but leads to more wear on the as there is no longer any ability to perform engine braking. This may make freewheel transmissions dangerous for use on and driven in regions, as prolonged and continuous application of to limit vehicle speed soon leads to brake-system overheating followed shortly by total failure.
The freewheel meant that the engine returned to its idle speed on the overrun, thus greatly reducing noise from both the engine and gearbox and reducing oil consumption. 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.
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..
Other car makers fitted a freewheel between engine and gearbox as a form of automatic clutch. Once the driver released the throttle and the vehicle was on the overrun, the freewheel disengaged and the driver could change gears without using the clutch pedal. This feature appeared mainly on large, luxury cars with heavy clutches and gearboxes without synchromesh, as the freewheel permitted a smoother and quieter change. Citroën combined a freewheel and a centrifugal clutch to make the so-called 'TraffiClutch', which let the driver start, stop, and change the lower gears without using the clutch. 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. Similarly, the Saab 93 was available with an optional Saxomat clutch.
A common use of freewheeling mechanisms is in automatic transmissions. For instance traditional, hydraulic General Motors transmissions such as the Turbo-Hydramatic 400 provide freewheeling in all gears lower than the selected gear. E.g., if the gear selector on a three-speed transmission is labelled 'drive'(3)-'super'(2)-'low'(1) and the driver has selected 'super', the transmission freewheels if first gear is engaged, but not in second or third gears; if in 'drive' it freewheels in first or second; finally, if in low, it does not freewheel in any gear. This lets the driver select a lower range to provide engine braking at various speeds, for instance when descending a steep hill.
Overdrive units manufactured by Laycock de Normanville used a freewheel to facilitate a smooth gear change between locked mode (1:1) and overdrive mode without use of the conventional clutch pedal. The freewheel would lock the outgoing axle to the outgoing axle in the brief transition period between the conical clutch for locked mode disengaging and the clutch for overdrive mode engaging.
/ref> and fitted to the Chelseymore tricycle, but the pioneers of fitting the freewheel to the safety bicycle were Linley and Biggs Ltd (trading as the Whippet Cycle Syndicate) who fitted a freewheel from the summer of 1894, in part to assist the operation of their 2-speed 'Protean' gear.
By 1899 there was widespread adoption in UK bicycle manufacture of the freewheel, usually combined with the back-pedal brake, and conversions were offered to existing bicycles.The Free Wheel, Cycling, 26 Aug 1899, pp28-32"Fortune and Bantham are fitting the cheapest and most effective back-pedalling brake to any machine on the market", Bradford Daily Telegraph, 12 Aug 1899, p1
In 1899 the same system in the USA was known as the “coaster brake”, which let riders brake by pedaling backwards and included the freewheel mechanism. At the turn of the century, bicycle manufacturers within Europe and America included the freewheel mechanism in a majority of their bicycles but now the freewheel was incorporated in the rear sprocket of a bicycle unlike Van Anden’s initial design.
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 in combination with a double chainring, giving the bicycle twice as many gears. In the early 1930s, Le Cyclo invented a four sprocket freewheel, and several years later the company combined the four sprocket freewheel with a triple chainring giving the bicycle twelve gears.
In the 1960s and 1970s Japanese manufacturers introduced their own versions of the derailleur. SunTour notably introduced the slant parallelogram rear derailleur design in 1964, which is tilted to keep the pulley closer to each cog of the freewheel as it shifts, providing smoother and better shifting than its European equivalents. This version of the derailleur became the standard when SunTour's patent expired in the 1980s, and is still the model for today's designs.
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