Clutches 101

May 15, 1999

Separating the facts from friction!

Clutches are basically made up of two parts: the pressure plate (clutch cover) and the clutch disc. A clutch’s main purpose in life is to smoothly transfer the engine’s power to the wheels. The following is the actual formula used to calculate the amount of torque the clutch system can transfer.

Clutch Torque Capacity:

Pressure plate strength * Clutch Disc Size * Number of Clutch Discs * Friction Coefficient / Constant

Or in other words…

Pressure plate strength, multiplied by clutch disc size, multiplied by the number of clutch discs, multiplied by the friction coefficient, divided by a constant.

By looking at this equation, we can see that torque capacity increases by doing any of the following: increase the strength of the pressure plate, increase the size of the disc, install more discs, increase the coefficient of friction of the disc.

In most cases, it is not practical to install a larger clutch or go to double or triple disc setup so the only two areas a clutch engineer has to play with is making the pressure plate stronger and increasing the coefficient of the disc.

Mechanically it is important to understand how a clutch is attached to the engine. Since the pressure plate is bolted to the flywheel and the flywheel is bolted to the crankshaft, then the flywheel, pressure plate and crankshaft all turn as one. The clutch disc has a hole in the middle with splines which slides over the transmission’s main input shaft. All of the engine’s torque is transmitted though the clutch disc to the transmission and then to the wheels.

When the clutch is engaged, the pressure plate squeezes the disc against the flywheel making the disc rotate at the same speed as the engine. When the driver presses down on the clutch pedal to disengage the clutch, the casting surface of the pressure plate (the surface that the disc rides against) pulls away from the disc and releases the disc from the flywheel. The flywheel still spins, but the disc and the transmission input shaft do not. This is why a car can be stopped with the engine running while in gear.

Going back to the above equation, if we increase the clamping force of the pressure plate, then we increase the torque capacity of the clutch system. Almost all of the pressure plates used today use a diaphragm spring to exert its clamping force. Over the years clutch companies have tried various ways to increase clamping force. One of the most recognizable methods is using the centrifugal force of weights attached to the fingers of the diaphragm spring. There have been many arguments over the years if this technique really works or is it just great marketing at work. There is no reason to get into the debate here, except to explain how to test the theory yourself.

The faster the weights spin, the higher the centrifugal force. If the force is directed in the direction of pulling back on the diaphragm fingers, then the clamping force will go up. To test if the force is in the right direction, pump the clutch pedal at idle and feel how stiff the pedal is. Then rev the engine to near redline and pump the pedal again. If the centrifugal force is pulling back on the fingers, then the pedal will be stiffer at the higher RPM.

Another old-time method of increasing pressure plate pressure is to move the fulcrum point or pivot point that the diaphragm spring rides against. The spring acts as a lever with the pivot point of the lever being the fulcrum. With the help of leverage, a lever allows you to lift a heavier object than without leverage. Moving the pivot point closer to the object requires more movement at the other end (longer stroke), but gives you more leverage.

So the only down side to more leverage is a longer stroke. When you move the fulcrum point in a pressure plate you get more clamping force, but you also have to stroke the clutch pedal farther to get it to disengage. Poor release characteristics are the most common complaint you will get when using a pressure plate that has had the fulcrum point moved.

The newest method of increasing pressure plate pressure is to reshape the diaphragm spring to produce a higher spring force. This method has been so successful that it has recently been patented. When reshaping a diaphragm spring it is critical that the original shape has been “erased” from the metal’s “memory”. If you just reshape the spring without employing the patented step to eliminate the memory, over time the spring will bend back to its original shape.

Another area that clutch engineers have available to them is changing the coefficient of friction of the clutch disc. Raise the coefficient and raise the torque capacity. However, the problem most engineers have is that there are only a few clutch disc manufacturers in the world. Most of the aftermarket performance clutch manufacturers can only buy the same friction materials from the same manufacturers.

The “dual friction” or “puck” style clutches use either cut pieces of factory material or ceramic or Kevlar segments. The up side of the puck design is better holding power. The reason for the better holding power is the pucks have about half of the surface area of a full circle disc so the pressure plate pressure is distributed over a smaller area.

You can increase pounds per square inch (PSI) by leaving the pressure the same and decreasing the square inches. Of course, the less material you have the faster the clutch wears out. The down side of ceramic pucks is they not only wear themselves out, but they can be hard on the flywheel surface.

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