Clutch Tech Part One: The Pressure Plate, Get a Grip!

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What does a clutch do?

When a clutch is engaged, the clutch cover, which is bolted to the flywheel, squishes the diaphragm spring down onto the pressure ring, which presses the clutch disc into the flywheel. The clutch disc is attached to the transmission via the input shaft and the flywheel is attached to the engine’s crank. Drive torque from the engine is now transmitted to the tranny.

Parts of the clutch

A clutch consists of two parts, the pressure plate and the disc. The disc contains the friction material and is coupled to the input shaft of the transmission. The pressure plate contains the diaphragm spring, the pressure ring and the cover. The pressure plate bolts to the flywheel, which bolts to the engine.

The pressure plate

The pressure plate design that just about all production cars use is called a diaphragm type. The main advantage of the diaphragm pressure plate is that it has a light pedal for the amount of clamping load delivered with a smooth linear engagement. Older V-8 domestic cars, trucks and pure drag racers can sometime use other pressure plate designs such as the Borg and Beck or the Long type designs. We won’t get into these because the diaphragm clutch is what most performance clutches are nowadays.

The pressure ring is made of heat resistant cast iron. It applies pressure to the disc from the diaphragm spring, squishing it into the flywheel.

Diaphragm clutches use a circular, cone-shaped spring aptly called a diaphragm spring to apply clamping force to the pressure ring, a heavy circular plate of cast iron that pushes the clutch disk against the flywheel. The flywheel is a solid piece of metal that bolts to the engine’s crankshaft. The clutch disc has splines that engage the input shaft of the transmission.

The clutch cover is the clutch’s housing, containing the clutch assembly. It is usually made of stamped steel that bolts to the flywheel. Some racing clutches have covers made of machined billet aluminum for stiffness and lightness in weight. The diaphragm spring is squished under the cover when it is bolted tightly down to the flywheel. This presses the pressure ring and disc hard to the flywheel forming a direct coupling from the engine to the transmission. Now power can flow from the engine to the transmission. In this state the clutch is said to be engaged.

The drive straps transfer torque form the pressure ring to the clutch cover. They also help the pressure ring pull away from the clutch disc when the clutch is disengaged.

Squishing the diaphragm spring to the pressure ring and disc is what provides the clamp load that keeps the clutch disc from slipping when power is applied to it by the engine. The pressure ring is held to the cover by thin flexible metal strips called drive straps. The drive straps transfer the engine’s torque from the pressure ring to the cover and helps the pressure ring retract, holding it against the diaphragm spring to keep it from rattling around when the clutch is disengaged.

When the clutch is disengaged the clutch pedal is pushed in which moves the release fork pushing down on the throwout bearing. This pushes the diaphragm spring inward, where it levers against the fulcrum on the clutch cover and pulls the pressure ring away from the clutch disc. The clutch is now disengauged and the engine can spin freely from the transmission.

To disengage the clutch to allow the engine to spin freely as when coming to a stop or shifting, you push the clutch pedal in. The clutch pedal moves a pivoting arm on the transmission case, called a release fork, via a cable or hydraulics. The release fork pushes on the throwout bearing which is a simple thrust-type ball bearing. The throwout bearing pushes on the center of the diaphragm spring. The diaphragm spring is visible in the center hole of the clutch cover and has multiple “fingers” that the throwout bearing rests against.

The throwout bearing pushes on the center of the diaphragm spring bending it inwards. The diaphragm spring pivots about on a fulcrum which is either a rim stamped into the clutch cover, riveted or bolted in pedestals, or a round wire held in place with rivets around the inside of the clutch cover. The outer end of the diaphragm spring is attached to the pressure ring. When the spring pivots on the fulcrum as the diaphragm spring is pushed in by the throwout bearing, the clamp load is released on the pressure ring and the clutch disc is allowed to spin free, disengaging the engine from the transmission.

This clutch uses a wire ring fulcrum. Moving the fulcrum locations can keep a high clamp load pressure plate’s clutch pedal effort manageable by increasing the leverage that the throwout bearing can have on the diaphragm spring.  Moving the fulcrum outward will reduce pedal effort without loosing clamp force by increasing the leverage.   It will also increase engagement travel which can make a heavy duty clutch easier to drive smoothly as well.

The worst thing that a clutch can do is slip under power when the clutch is engaged. A clutch that slips means that the engine’s power is not getting to the drive wheels but is being converted into heat energy, not driving the car forward. To reduce the chances of slipping, a heavy-duty (HD) clutch will generally have a pressure plate with a higher clamping load and a clutch disc with a higher coefficient of friction and greater heat resistance.