Free Wheel Unit In some vehicles a freewheel unit (also called freewheel clutch) is provided just after the gearbox. Fig. shows the construction of this unit. It is very much similar in action to the bicycle freewheel. The inner driving mem her is connected to the gearbox shaft and the outer one to the propeller shaft. The driving member has the three steps as shown in fig. In each step, there are 3 spring-mounted rollers of various sizes.
When the driving member is rotating in the direction shown in the fig. the driven member will also be rotating in the same direction. But when the driven member becomes the driving member e.g., when the car is going downhill with the engine stopped or clutch disengaged, the inner member will not rotate along with the outer one.
Thus, during this case, the transmission and the engine will be isolated from the wheels. This results in fuel economy. In some instance, the freewheel has to be locked, e.g. in reverse drive. In such cases, the provision of locking the freewheel drive is there. There is a dog clutch provided whose teeth will interact with the teeth on the within of the outer freewheel member. The dog clutch is automatically engaged by a connection from the reverse selector. There is additionally a provision for the manual locking of the freewheel.
The advantages of using a freewheel unit are
l. The engine can idle simply by losing the throttle without bringing the gear in neutral. Thus, gear changing at low speeds is far simplified because of the clutch need not be disengaged for this purpose.
2. Less wear on the transmission because whenever the car freewheels, the engine, and the gearbox are disconnected from the propeller shaft.
3. On long downward slopes, an appreciable amount of fuel is saved. The saving may be up to 20%. However, the main disadvantage is that as there is no resistance due to the engine and gearbox friction while freewheeling, the brakes must be used more effectively to stop the vehicle on a downward slope.
The freewheel is also used as an essential component of the torque converter in modern automatic transmissions.