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.
Carbonite is a brand new carbon fiber friction material made by Friction
Composites Company for use on high performance clutch discs. The
benefits of this new material are twofold. Carbon fiber out
lasts both the ceramic and Kevlar facings, and the carbon fiber is
significantly lighter than other facings. Remember, the lighter
the disc, the faster the shifts!
At the time of this article, Carbonite is available only from
RPS Performance
Products with their Carbon Claw and Turbo Clutch Carbon High Performance
Clutch kits.
