TYPES OF PRESSURE PLATES
What’s a Pressure Plate?
The pressure plate applies pressure to the clutch disc to clamp it against the flywheel and engage the clutch. Pressure may be generated by spring (static) pressure or centrifugal pressure. Static pressure is constant, meaning that whether the engine is turned off or spinning 7000 RPM, the pressure never changes. Centrifugal pressure is not constant. It is a function of engine RPM. Centrifugal pressure is generated by the clutch levers and increases to the square of the engine RPM. It is less pronounced at lower engine speeds but very effective in the higher RPM range.
Diaphragm Pressure Plates
The diaphragm pressure plate utilizes a Bellville or conical spring to apply pressure to the pressure ring. This type of pressure plate has multiple fingers that the release bearing presses against to disengage the clutch. Diaphragm clutches rely completely on static pressure which is unaffected by engine RPM.
The Belleville spring allows the pressure plate to be released and engaged with a relatively light pedal effort compared with coil spring clutches. This design is used almost exclusively in late model vehicles that have hydraulic or cable release mechanisms, due to the lighter effort required to engage and disengage the pressure plate.
The diaphragm clutch is excellent for use in street and heavy duty street applications where drivability and pedal effort are a major concern for the user.
CENTRIFUGAL WEIGHTS IN DIAPHRAGM STREET/STRIP APPLICATIONS
Some aftermarket diaphragm pressure plates feature a centrifugal weighting system. Centrifugal assist is useful in drag racing and other high RPM applications to apply additional clamp load to the clutch disc. The drawback to centrifugal assist in highway performance applications is that the centrifugal assist is low until the engine reaches high RPM. The load on the clutch (the effort required to turn the driveshaft) is very high at low RPM and decreases at higher RPM. In most highway applications centrifugal assist is not effective in increasing holding power or performance because it fails to solve the high torque load at low RPM problem.
Borg and Beck Pressure Plates
The Borg & Beck pressure plate uses three levers to engage and disengage the clutch disc. It is a coil spring design, where the pressure of the clutch is applied to the disc using coil springs similar to a valve spring. By combining these springs at a specific installed height, different pressures can be attained for the pressure plate. Borg & Beck clutches rely completely on static pressure which is unaffected by engine RPM.
Borg & Beck pressure plates are found in GM, Chrysler, and AMC early model applications with mechanical linkage. Borg & Beck clutches and can be identified by looking at the width of the clutch fingers, which is about one inch. The coil spring design by nature will require more pedal effort to engage and disengage the clutch.
Borg and Beck clutches are best suited to street and heavy duty street applications for older muscle cars and trucks.
CENTRIFUGAL OR COUNTERWEIGHT LOADING
Centrifugal pressure is additional clamping pressure on the clutch disc that occurs as a function of engine RPM. As engine RPM increases, centrifugal pressure forces the clutch fingers of the Long Style pressure plate outward. As this occurs, the clutch fingers pivot against the cover and apply additional clamp loading to the clutch disc.
Long Style Clutches
Counterweighted lever (top) and standard lever (bottom)
Typical Long Style pressure plate
Weights are added to the Clutch levers to increase the plate pressure as RPM increases.
Long style pressure plates are the Ford version of a three lever, coil spring pressure plate. The Long Style is the most popular type of pressure plate for drag racing applications. It’s design is the basis for today’s professional drag racing clutches.
The inherent advantage of Long Style pressure plates is their ability to apply centrifugal clamping pressure. As engine RPM increases, the levers in the plate pivot against the cover and apply additional clamp load to the clutch disc. This is true in both counterweighted lever Long Style pressure plates and also non-counterweighted designs. (Counterweighted levers have provision for installation of weights to the backside of the levers to further increase the centrifugal clamp effect.)
The The Long Style pressure plate is best utilized with mechanical release linkages. Long Style clutches were prevalent in early Ford muscle cars and trucks.
Some Long Style clutches feature adjustable static pressure. By turning an allen screw located on top of the spring, the pressure can be increased.
ADJUSTING STATIC PRESSURE ON RAM LONG STYLE CLUTCHES
Increasing the static pressure of a RAM Long Style pressure plate is achieved by turning the allen adjuster screw COUNTERCLOCKWISE to increase the static load. When the screw is turned, it pushed against the cover and the adjuster base compresses the spring, increasing the spring rate. Most pressure plates have a maximum of 7 turns adjustment. Turning the adjuster screw further will cause it to come out of the adjuster base. When this occurs, the entire plate must be disassembled for repair. Some racers mark the turns they have in the clutch on the side of the bellhousing in order to keep track of where they are set.
As the adjuster screw is turned counterclockwise, the spring is compressed to increase clamp pressure.