What is the Freewheel Mechanism in Automotive?

A freewheel mechanism in automotive is a mechanical device that allows the wheels to rotate independently of the engine and transmission when certain conditions are met. It is primarily used in older vehicles and is not commonly found in modern cars due to advancements in transmission technology. In this article, I will discuss the freewheel mechanism in automotive and how the system works in a transmission system.


Freewheel Mechanism in Automotive

Freewheel unit is also known as an overrunning clutch, sprang clutch, or one-way clutch. It is an essential part of every overdrive. It transmits power in one direction only the transmission mam shaft to the output shaft when the sun gear is unlocked and releases the main shaft from driving the output shaft when the planetary gears are in overdrive.

When you lift your foot off the accelerator pedal, the engine in a car equipped with a freewheel mechanism can continue to run without directly powering the wheels. The freewheel mechanism decouples the engine from the transmission. This means that the engine is not directly connected to the wheels, and there is no power transfer from the engine to the drivetrain.

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Now let’s talk about the construction of the freewheel mechanism in automotive.

Freewheel unit in Automotive Diagram


Construction of Freewheel Mechanism in Automotive

A freewheel mechanism consists of a hub and an outer race. The hub has internal splines to connect it to the transmission main shaft. The outer surface of the hub contains twelve cams designed to hold twelve rollers in a cage between them and the outer race. The outer race is splined to the overdrive output shaft.

The details construction details are –

Outer Housing: The freewheel mechanism is enclosed in an outer housing, often referred to as the freewheel body. This housing typically has threads on the inner side that allow it to be screwed onto the hub of the wheel.

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Inner Gear: Inside the freewheel housing, you’ll find an inner gear with a set of teeth that are designed to engage with the drivetrain or axle. This gear is typically fixed to the hub of the wheel and rotates with it.

Springs: A freewheel mechanism contains one or more springs that provide tension or resistance. These springs are usually arranged around the inner gear and serve to engage the mechanism when necessary.

Pawl Mechanism: The key component of a freewheel mechanism is the pawl system. Pawls are small, spring-loaded levers that engage with the inner gear’s teeth in one direction and allow it to spin freely in the other direction. When the vehicle is moving forward, the pawls engage with the teeth, transmitting power to the wheel. When the wheel tries to spin backward (coasting or moving downhill), the pawls lift off the teeth, allowing the wheel to spin freely without driving the drivetrain.

Types of Freewheel Mechanism

In automobiles, freewheel units, also known as overrunning clutches or one-way clutches, come in different types, each designed for specific applications. These units allow the wheels to spin freely in one direction while preventing backward motion. Here are some common types of freewheel units in automobiles:

  1. Sprag Clutch: The sprag clutch is one of the most common types of freewheel units. It uses small, asymmetrically shaped wedges or sprags that engage with a ratcheting mechanism. When the input (engine) rotates in one direction, the sprags lock, providing a direct connection to the output (wheel). However, in the opposite direction, the sprags lift off, allowing the output to rotate freely.
  2. Roller Clutch: Roller clutches use cylindrical rollers that lock when rotated in one direction and disengage when rotated in the opposite direction. This type of freewheel unit is commonly found in automatic transmissions and some drivetrain components.
  3. Cam and Roller Clutch: This type of freewheel unit combines a cam and roller mechanism. The cam’s shape causes the rollers to lock when the input direction is correct, providing a solid connection. When the input direction is reversed, the cam allows the rollers to disengage, enabling free rotation.
  4. Bearing Freewheel: In a bearing freewheel, a bearing arrangement is used to allow the input to rotate in one direction while providing minimal resistance to rotation in the other direction. This type is often used in specific automotive applications like alternator pulleys.
  5. Overrunning Alternator Pulley (OAP): An OAP is a type of freewheel unit commonly used in alternators. It allows the alternator’s pulley to rotate freely when the engine decelerates or idles, reducing the load on the engine and improving fuel efficiency. When the engine accelerates, it locks to transmit power from the engine to the alternator.
  6. Backstop Clutch: Backstop clutches are used in applications where you want to prevent reverse motion, such as in certain parts of the drivetrain. They lock in one direction and allow free rotation in the other. These are common in various automotive systems like winches and some transfer cases.
  7. Crankshaft Damper Pulley: Some vehicles use a freewheel unit in the crankshaft damper pulley to reduce engine vibrations. This type of freewheel unit allows the pulley to spin freely when the engine is running smoothly, but it can lock to absorb vibrations and prevent damage to the engine’s components.
  8. Overrunning Clutch for Starter Motors: In some starter motor designs, overrunning clutches are used to prevent the starter from engaging with the engine after the engine has started. This protects the starter motor and prevents potential damage.

Working of Freewheel Mechanism

When the hub is driven in the clockwise direction, as shown in the diagram above, the rollers ride up the cams; and by their wedging action, they force the outer race to follow the hub. Thus, the outer race moves in the same direction and at the same speed as the hub. When the hub speed slows down and the outer race is still moving faster than the hub, the rollers move down the cams. releasing the outer race from the hub. Thus, the outer race moves independently of the hub and the unit acts like a roller bearing.

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The transmission main shaft is connected to the hub and the output shaft is connected to the outer race. Thus, the freewheel unit can transmit power only from the main shaft to the output shaft.

Freewheel Mechanism Advantages and Disadvantages

In an Automotive transmission, the freewheel mechanism transmits the torque from one end to the other. So the advantages and disadvantages of this system are-


  • One of the primary advantages of a freewheel mechanism is improved fuel efficiency.
  • Freewheel mechanisms can extend the life of the engine and transmission by reducing wear and tear.
  • Freewheel mechanisms enable smoother and quieter coasting, as there’s no engine braking effect.
  • In some manual transmissions, freewheel mechanisms can make shifting gears easier because you don’t need to perfectly match engine and wheel speeds.


  • This system needs more maintenance than other mechanical Devices.
  • Depending on the design and quality of the freewheel mechanism, it can add to the overall cost of a vehicle or system.
  • In modern vehicles, there is no need for freewheel units as the degradation of the system.


So the freewheel unit is the system the system where the torque or the speed transmits in just one direction. Some limitations of this system are why modern vehicles do not need this system. Thank you for reading the article.

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