How does the clutch on my car work?
The clutch and flywheel on your vehicle provide the connection between the engine and the transmission. In simple terms they transmit the torque provided by the engine into to input shaft of the gearbox which in turn transmits this to the drive shafts and finally the wheels. An important operation the clutch provides is that it allows a “neutral” position where the engine can run without having the transmission engaged, this is important when the vehicle is at a stop and also during gear changes where engine and transmission speeds need to be matched. It also allows for gradual take up of drive to the wheels.
Ordinarily the clutch on your vehicle will be made up of 2 main components. The clutch pressure plate and the clutch friction plate. Clutch friction plate linings (that’s the non-metallic part of the clutch) are usually made up of a similar material to brake pad friction linings. In years gone by, just like old brake pad linings, these were made of that nasty substance asbestos. These days they are usually made up of fibreglass and Kevlar, alongside interwoven metal strands, usually copper. Being a friction material the linings naturally wear out over time. Often modern clutches can last the lifetime of the vehicle, but like all wearing parts, this can depend on many factors like driving styles and conditions of use.
What is a flywheel?
There are 2 types of flywheel fitted to modern vehicles. Solid (or single mass), and dual mass.
Single mass flywheels have no separate or moving parts and rarely need replacing. Replacement is usually only necessary if the flywheel has been damaged, unless of course the vehicle has covered very high mileage. Damage would usually be as a result of excessive heat from clutch slip, either caused by worn out friction material or from arduous use. Hot spots and cracks can form and may cause clutch judder as a result, even after clutch replacement. In the event of heat damage, the flywheel would need to be replaced.
Dual mass flywheels (sometimes referred to as DMFs) are very different from solid single mass flywheels. As the name suggests they comprise of 2 main parts. The primary flywheel section belonging to the crank shaft of the engine, and the secondary flywheel section which links to the clutch/transmission. These 2 ‘masses’ are connected by a central bush and the damping effect is controlled by special springs packed with grease known as arc damper springs. There are 2 main reasons for a manufacturer to fit a dual mass flywheel to a vehicle. The first is to reduce noise and vibration from the drivetrain. Due to the increasingly stringent CO2 emission regulations, modern vehicles are required to be as fuel efficient as possible. This means high gear ratios and often the addition of a 6th gear. This allows for decreased engine speeds and better fuel efficiency, but as a consequence of the lower engine speed, there’s an increase in vibration from the engine. The dual mass flywheel protects the crank shaft and transmission from these vibrations and also helps to reduce noise levels from the drivetrain. The other reason is to deal with the increased power and torque that modern engines produce. Again the dual mass flywheel is there to protect engine and transmission components from premature failure due to the increased stresses. Unfortunately, dual mass flywheels are prone to premature wear themselves. Often this can be down to driving style or lack of vehicle maintenance. For example, driving in a high gear at low speeds can cause the engine to ‘labour’ which can increase the vibrations produced by the engine to beyond the dual mass flywheels designed damping limits. Driving a vehicle with a misfire or poor running characteristics will also push the dual mass flywheel to surpass its damping abilities. Even slow or poor starting can result in early wear so it’s vital to keep the engine in your vehicle in good order to keep your dual mass flywheel in order.