How a one-way clutch lets a bicycle coast, a Saab shift without a pedal and a starter motor step away from a spinning engine.
Freewheels are one-way clutches that let a shaft spin freely once it outruns its driver. Bicycles coast on them, manual cars can change gear without the clutch, and starters step away from a running engine. An overdrive, by contrast, is a separate gear arrangement.
A freewheel, also called an overrunning clutch, is a device in a transmission that disconnects the driveshaft from the driven shaft when the driven shaft turns faster than the driveshaft. The idea appears in bicycles, car gearboxes, starter motors and helicopters, and it solves the same problem in each case: a part that should be free to spin must not drag the rest of the system around with it.
What is a freewheel?
A freewheel is a one-way clutch that lets a driven shaft run faster than its driver without transmitting that extra speed back. The condition it deals with is common. It appears on most chain-driven bicycles whenever the rider stops pedalling, because the rear wheel keeps turning while the pedals stay still. A fixed-gear bicycle, which lacks a freewheel, makes the rear wheel drive the pedals round instead.
People sometimes confuse a freewheel with an overdrive, but the two are otherwise unrelated. An overdrive is a higher gear ratio, while a freewheel is a one-way coupling. Some cars include both, and the freewheel may be fitted to smooth the change between them.
A part that should be free to spin must never drag the rest of the system round with it.
How does a freewheel mechanism work?
The simplest freewheel uses two saw-toothed discs held together by springs, with their toothed faces pressed against each other. When the drive disc turns in the working direction, its teeth lock with those of the driven disc, so the two rotate together. If the drive slows or stops, the driven teeth slip over the drive teeth and keep spinning, producing a clicking sound that grows with the speed difference.
A more rugged design places spring-loaded steel rollers inside a driven cylinder. In the working direction the rollers lock against the cylinder and make it rotate in unison. Turning slower, or in the opposite direction, lets the rollers slip freely inside the cylinder. Both designs achieve the same one-way action through very different means.
Bicycle freewheels use a similar principle. Most have an internally step-toothed drum with two or more hardened steel pawls. Turning the cassette in the coasting direction makes the pawls slide up and over the gentle slopes of the teeth. Pedalling forwards makes the steeper edges catch, so the drive is transmitted. More pawls spread the wear, although only two or so usually engage at once unless the tolerances are unusually tight.
| Design | How it engages | Typical use |
|---|---|---|
| Saw-toothed discs | Spring-loaded teeth lock in the drive direction and slip the other way | The simplest one-way coupling |
| Steel rollers in a cylinder | Rollers wedge against the cylinder in one direction and slip in the other | Rugged industrial and automotive uses |
| Step-toothed drum with pawls | Hardened steel pawls catch on the steeper edges when pedalling forwards | Bicycle freewheels and cassettes |
Why do cars use freewheels?
A freewheel mechanism acts as an automatic clutch. In a manual gearbox it lets a driver change gear, up or down, without pressing the clutch pedal, so the clutch is needed only for starting from rest and for stopping. The Saab freewheel could be engaged or disengaged by the driver with a lever, which locked or unlocked the main shaft with the freewheel hub.
There are benefits and costs. A freewheel gives slightly better fuel economy on carburetted engines, because the engine can idle while the car coasts, and it reduces wear on the manual clutch. However, it removes engine braking, so the brakes take more of the load. On mountain roads that can be dangerous, since prolonged braking can overheat the brake system until it fails.
How are freewheels used in starters and farm machines?
Starter motors need to spin at about 3,000 rpm to turn an engine over, and their gear ratio to the flywheel is about 15 or 20:1. Once the engine is running, the flywheel may turn at about 1,000 rpm at idle. Without a freewheel, the starter would need to turn at 15,000 to 20,000 rpm whenever the key was left in the start position, risking damage to the armature.
The overrun clutch releases the starter from the flywheel as soon as the engine runs and prevents the gears from meshing again while the engine is running. The freewheel has largely replaced the older Bendix drive on car starters, because it cuts the electrical demands and complexity of the starting system. Many motorcycles and combustion mowers use the same arrangement.
Agricultural equipment uses overrunning clutches too. Hay balers and other high-inertia equipment are fitted with one, because without a live power take-off a tractor can keep moving forward with the foot clutch pressed. Disconnecting the load under those conditions makes the machine safer. Push lawnmowers use freewheels to protect their high-speed blades from transmitting momentum backwards through the drive.
What happens when the wheels drive the engine?
Pure freewheeling in a car means pushing in the clutch and releasing the throttle, which disconnects the engine from the transmission. The engine then idles while the wheels turn at whatever pace gravity and momentum allow. The opposite condition happens when a manual car runs downhill with the throttle closed but the clutch still engaged. The wheels then turn the engine, possibly faster than it would normally run.
That reversal can be dangerous in two-stroke engines, which depend on a fuel and oil mixture for lubrication. When the fuel supply is cut off, oil can be starved from the cylinders, and the pistons may seize and cause serious damage. Saab used a freewheel in its two-stroke models for this reason, and the Saab 96 V4 and early Saab 99 kept the system, which helped their fuel efficiency.
Why were freewheels fitted to historic luxury cars?
Freewheels were used in some luxury and up-market conventional cars, including Packard, Rover and Cord models, from the 1930s into the 1960s. On the overrun the engine returned to its idle speed, which greatly reduced noise from the engine and gearbox and cut oil consumption. Drivers could usually lock it when they wanted engine braking.
Other makers used freewheels to ease gear changes. When the throttle came off and the car entered overrun, the freewheel let go, so gears could be swapped without the clutch. The feature was mostly found on big luxury cars, which had heavy clutches and gearboxes lacking synchromesh. Citroen combined a freewheel with a centrifugal clutch to create the TraffiClutch, offered on the 2CV and its derivatives. The Saab 93 could be ordered with an optional Saxomat clutch.
The original Land Rover used a freewheel from 1948 to 1951 to control drive from the gearbox to the front axle, disengaging it on the overrun. That gave a permanent four-wheel drive system without transmission wind-up, but the handling was unpredictable in slippery conditions or when towing, so it was replaced by a conventional selectable system. During the Second World War, military Volkswagen vehicles built by KdF used a ZF limited-slip differential made from two freewheels.
Automatic transmissions also use freewheeling. Hydraulic General Motors units such as the Turbo-Hydramatic 400 freewheel in every gear below the selected one. Overdrive units by Laycock de Normanville used a freewheel to make smooth changes between locked and overdrive modes without the conventional clutch pedal.
Why were the Saab two-strokes given freewheels?
Saab's two-stroke cars were the best-known users of the freewheel for protection rather than comfort. Their fuel-oil mixture meant that a closed throttle could leave the engine short of lubrication if it was driven by the wheels. The freewheel prevented that condition, which is why the approach stayed in the design through the Saab 96 V4 and into the early Saab 99.
How do freewheels work on bicycles today?
On a bicycle, a freewheel can mean a single sprocket at the wheel, which is the simplest arrangement. The older style of derailleur gear includes the freewheel mechanism in the gear and sprocket assembly, and that whole unit is called a freewheel. The newer style places the freewheel mechanism in the hub instead, and that design is called a freehub.
Bicycles use the freewheel so that the rider can coast without pedalling, and it is the reason the pedals stay still while the wheel keeps spinning. Every modern rider experiences the clicking sound it makes, which marks the moment the pawls slip over the teeth.
Do helicopters use freewheels?
Rotorcraft use freewheels as well. Just as a bicycle's wheel must be able to turn faster than its pedals, a helicopter's rotor blades must be able to spin faster than the drive engines. A freewheel in the main transmission allows the main and tail rotor systems to keep turning independently of the drive system. That is especially important when an engine fails, because it preserves flight control and makes an autorotation landing possible.
How did the bicycle freewheel develop?
William Van Anden of Poughkeepsie, New York, invented the freewheel for the bicycle in 1869. His design placed a ratchet in the hub of the front wheel, which was the driven wheel on the velocipedes of the day, so the rider could move forward without pedalling constantly. Many enthusiasts initially rejected the idea, believing it would complicate a machine that should stay as simple as possible.
In Britain, J. White and G. Davies of the Coventry Machinist Company patented a roller freewheel in 1881, which was fitted to the Chelseymore tricycle. Linley and Biggs Ltd, trading as the Whippet Cycle Syndicate, fitted freewheels to the safety bicycle from the summer of 1894, partly to support their two-speed Protean gear. By 1899 widespread adoption had taken place in British bicycle manufacture, and in the United States the same arrangement was known as the coaster brake.
Gearing followed. Le Cyclo, a French company, offered a 1924 bicycle whose gears were selected through a two-sprocket freewheel, making climbs easier. Its later changes included front and rear derailleurs in the late 1920s and a four-sprocket freewheel in the early 1930s, which combined with a triple chainring to give twelve gears. SunTour introduced its slant parallelogram rear derailleur in 1964, a design that kept the pulley closer to each cog and became the model for later derailleurs.
