Every car engine on the road is connected to a transmission either automatic or manual. For those connected to a manual transmission this connection usually consists of some sort of clutch assembly. This clutch assembly in your car has two basic functions. First, it must be able to momentarily disengage the engine from the transmission for gear changes and to slow the car. Next, it must be able to transfer maximum engine torque without slipping when it is engaged. Sounds relatively simple. The problem is with all the details.
The Flywheel Stores the Car Engine Energy
Let us start with the flywheel. This is the disc that is bolted to the end of your crankshaft. A flywheel can be made from iron; steel or aluminum and it can come in a variety of diameters and weights. The flywheel has three main purposes.
The primary purpose of the flywheel is to store kinetic energy and maintain the revolving inertia of the engine as it passes through its various cycles. Engine’s power comes from the pistons going in a vertical or horizontal direction. These pistons are connected to the crankshaft in such a way that the vertical, or horizontal, motion is converted to rotational motion.
Keep in mind that the pistons only provide force 25 per cent of the time. Most of us have 4-stroke engines and only one of the four strokes produces power. The mass of the flywheel creates inertia to keep the crankshaft running between the power strokes. This is especially critical at idle.
Next, the flywheel serves as part of the engine starting mechanism. There are teeth all around the outer circumference of your flywheel. The teeth on the starter gear engage with the teeth on the flywheel to turn the engine over for starting. Finally, the machined face of the flywheel acts as part of the clutch mechanism. The face of your clutch disc interacts with the flywheel to transmit the engine forces to the rest of the drivetrain.
When you apply throttle, you are not only turning the crankshaft faster but you are accelerating the flywheel as well. It takes more power and time to accelerate a heavy object than it does to accelerate a light object. A lightened flywheel will not increase horsepower overall, but it will allow the engine to accelerate faster and will free up a small amount of horsepower your engine is already making. Slightly less power is needed to turn a lighter flywheel and this power can go to your wheels instead.
While you might want a big heavy flywheel to smooth out the power strokes on your streetcar you want a light and small flywheel for your racecar. Aluminum flywheels are better for maintenance since the friction surface is generally a steel insert that’s bolted in place. That makes replacement inexpensive. You only need to replace the friction surface, not the entire flywheel. It also makes it almost impossible to wear the flywheel out. When the surface that contacts the clutch face wears you only replace the insert. The rest of the flywheel is just fine.
The Clutch Pressure Plate Controls Engine Torque
Pedal Feel | Linkage Type | Best Use | |
Borg and Beck | Heavy Effort | Mechanical | Race/Limited Street |
Long | Heavy Effort | Mechanical | Race/Limited Street |
Diaphragm | Light Effort | Cable, Hydraulic or Mechanical | Daily Driver / Race |
A car’s clutch pressure plate is the mechanism that transfers the engine torque to the transmission input shaft through the clutch disc. Thereby, it is a vital part in the functioning of a manual transmission. A clutch pressure plate lives up to its name in the fact that it is round-shaped. Plus, it is a metal component that has springs and levers inside of it that are controlled by a release fork attached to a shifter.
As the car’s driver steps onto the clutch pedal, the springs inside the pressure plate are compressed, pull the plate away from the clutch flywheel, and stop the clutch disc from moving. Then, the driver shifts into the desired gear and releases the pedal and the levers release, and the springs expand and push the clutch plate back into the disc, which engages the flywheel and gets your vehicle wheels powered again.
Three Kinds of Pressure Plates
The job of the pressure plate is to apply pressure and squeeze the clutch disc firmly between the pressure plate and the flywheel. Some people call this the clutch cover, and it consists of the pressure plate and the springs. There are basically three types of pressure plates: the Long style, the Borg & Beck, and the Diaphragm:
Long Style Pressure Plate
The Long style pressure plate is easily identified by the three thin fingers that engage the release bearing. Under the pressure plate cover you will find a series of nine coil springs. To release the clutch, you need to compress these springs. A lever arrangement allows you to add small weights to increase centrifugal loading on the pressure plate as engine speed increases. Drag racers are constantly playing with the Long style pressure plates in between runs. The idea is to adjust the static load separately from centrifugal load.
Borg and Beck Type Pressure Plate
The Borg and Beck type is similar to the Long style but it is basically a street or road race version of the Long style pressure plate. Borg and Beck pressure plate can be identified by the slightly wider fingers found under the pressure plate hat. These then release the pressure by compressing the coil springs. Certain applications of the Borg and Beck also offer centrifugal assist for high-rpm, high-horsepower applications. The Borg and Beck uses rollers inside the cover that are forced to the outside under centrifugal force. Their purpose is to increase the plate load as the rpms increase that is all about reducing any possible clutch slippage.
The late Carroll Smith felt the Borg and Beck pressure plate was the only one to use in road racing. He wrote that it was not only the lightest possible unit, but it had the lowest moment of inertia. If everything was installed properly and maintained correctly a Borg and Beck unit should last forever.
Diaphragm Type Pressure Plate
The diaphragm pressure plate uses a single, large Bellville-style spring to load the pressure plate. There are several advantages to this style of spring. First, it loads the pressure plate evenly since the pressure is applied uniformly over the entire plate assembly. Secondly, as the Bellville spring is compressed (clutch released), there is a point where the pedal effort decreases because the spring over-centers. This makes holding the clutch pedal in on the grid, or a stoplight, much easier than a coil spring type pressure plate. The diaphragm may very well be the ideal pressure plate for driving on the street.
Clutch Disc Transmits the Torque
A clutch disc transmits engine torque directly to the input shaft of the transmission. The clutch disc, when coupled with the pressure plate and flywheel, makes, and breaks the flow of power from the engine to the transmission.
A clutch disc consists of a friction surface and a hub. The hub is what slides onto the input shaft of your transmission. Hubs are offered in sprung and non-sprung versions. These springs, which surround the center of the hub, act like a harmonic balancer. A coil-spring hub allows the disc to rotate a few degrees upon engagement. This softens the initial contact and reduces driveline torsional vibrations and minimizes noise and vibration. Clutch discs with springs make clutch engagement easier on the drivetrain.
Each type of clutch has its own merits and brings different benefits. While solid hubs are great for positive engagement, they do transmit more shock to the driveline. This can lead to wheel spin and increased wear to the driveline components. A solid hub could also mean harsher shifting and increased driveline vibration.
Moving out from the hub is the friction surface. This friction material is like the material on your brake pads. Just as you can purchase brake pads with different coefficients of friction you can do the same with your clutch disc.
We really have three broad categories of clutch disc friction materials to select from which we will discuss in a later post.
Throw-out Bearing
Release bearings are designed to pivot forward and compress the pressure plate levers, which disengages the clutch system. Although release bearings are all designed for the same basic function, they come in many shapes and sizes because they must work in conjunction with a variety of actuation systems as part of the clutch assembly.
Clutch Assembly Hardware
At some point you are going to have to bolt all your parts together. The local hardware store may not be your best source for pressure plate and flywheel bolts. ARP pressure plate bolts have a tensile strength of 200,000 psi and only cost roughly $15.00 for a set of 6 complete with washers. A set of 200,000-psi flywheel bolts for a Chevrolet is usually less than $15.00 too. Even Porsche flywheel bolts cost less than $40.00. Good hardware is critical for safety; do not skimp here!
Finally
The clutch assembly is a set of components that work together with one simple goal – disconnect the engine from the transmission (and consequently from the wheels) when you push the clutch pedal all the way down, and gradually reconnect the engine back to the transmission, when you release it. Here is a simple diagram of the clutch assembly. To get a sense of how this diagram is oriented, let us tell you that the flywheel is on the engine side and is fixed to the crankshaft, while the clutch disc is on the gearbox side, and is connected to the transmission.
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