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.
To disengage the clutch to allow the engine to spin freely, as when coming to a stop or shifting, the clutch pedal is pushed in. The clutch pedal moves a pivoting arm in 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.
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 and 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.
To prevent slipping, high-performance pressure plates have stiffer diaphragm springs to give a higher clamp load. The more clamping force, the harder the pressure ring smashes the disc to the flywheel and the less likely the clutch disc will slip. Most aftermarket clutches have a thicker diaphragm spring or a stock spring that is re-stamped more conically for more preload and/or heat-treated for more spring tension. Some extreme clutches even have a double diaphragm spring, which is usually two stock springs stacked on top of each other.
Beware of going too crazy with clamping force; some clutches have so much clamping force that they can cause excessive wear on your crankshaft thrust bearings. A general rule of thumb is: Do not increase the clamp load more than 50 percent over stock.
Some HD clutches have beefier, thicker pressure rings to resist warping with heavier clamp loads and high heat. Several HD clutches have a cover stamped of thicker gauge steel to hold up to the increased clamping pressures of thicker diaphragm springs without flexing. This gives more consistent clutch action and improved clamping.
Some pressure plates have a ring of weights on a cable attached to the fingers of the diaphragm spring. These weights are supposed to offer a centrifugal assist, increasing clamping pressure at high rpm. Although this is an innovative idea, care must be taken to avoid overcentering with this sort of device.
Overcentering is when the diaphragm spring's fingers get pushed past the normal centerline of the diaphragm spring. When this occurs, centrifugal force acting on the weights and diaphragm spring keeps the fingers of the spring bent down causing the clutch to stay disengaged during high-rpm shifts. This may cause your motor to overrev or damage the transmission. Overcentering can occur with any diaphragm clutch, but centrifugal-assisted clutches are especially sensitive to this. If you can feel the pedal get lighter with increasing revs, that is the clutch wanting to overcenter. All diaphragm-type clutches are prone to overcenter to some degree so you want to be sure that your clutch is adjusted for proper throw.
Although the penalty for having a slip-resistant high clamp load clutch is a stiff, hard-to-drive clutch pedal, many HD high-performance clutches use some tricks to keep the clamping force high but the pedal pressure low. The most common trick is to increase the throwout bearing's leverage ratio by moving the diaphragm spring's fulcrum inboard so the throwout bearing has more leverage to bend the spring inward. This also makes the engagement slower, which can help make slipping a grabby clutch off the line easier. Another trick is to use a smaller throwout bearing. Some clutches come with a clutch slave cylinder with a larger bore to reduce pedal effort. The only disadvantage to this is that it can slow clutch release by a few milliseconds and therefore slow shifting somewhat.
A less common type of pressure plate is called a pull-type. Pull-type pressure plates are found on cars like the EVO, STi, some Porsches, and the Skyline GT-R. Pull-type pressure plates are also common on full race applications. A pull-type pressure plate releases by pulling on the diaphragm spring instead of pushing on it.
The throwout bearing is attached to the inside of the diaphragm spring. Pull-type pressure plates have a lighter pedal effort for a given clamp load and are more efficient in developing clamp force because the diaphragm spring's fulcrum is at the outer edge of the cover and the outer diameter of the spring. Because the clamp load of the diaphragm spring is on the outer edge of the cover, there is less stress and thus less flex in the cover. With less flex, a higher clamp load can be applied to the pressure ring. This reduction of flex allows a pull-type pressure plate have up to 30 percent more actual clamping force per pound of load that the spring makes over a conventional push-type pressure plate.
The pull-type pressure plate also has a much lighter pedal effort because the outboard fulcrum gives the release arm more leverage to flatten the spring and take load off of the pressure ring. Because the pull-type pressure plate's cover is near the bolted down periphery it reduces bending load on the cover. As such, the cover can be both smaller and lighter in construction, thus reducing rotating weight. These features are the reason why many true racing clutches are pull-type.
Now that you are familiar with the operation of the pressure plate, in our next installment we will discuss the ins and outs of an even more complex subject, the clutch disc.