The life of a clutch is rought: lots of stress, unreasonable expectations, and a tremendous amount of heat that accompanies a slip in performance. Actually, it's worse. Muscle cars are heavy, and many late-models are even heavier. Horsepower is exploding, and tires are getting stickier. All this is compounded by the knucklehead in the driver seat whose left foot does its best impersonation of a high-stall converter at the Tree. On top of all that, people demand soft pedal pressure, smooth engagement, and component longevity. And you thought your job was tough! Although the conditions inside a bellhousing are bleak, there's hope for the poor clutch. Companies like Centerforce have learned to transcend these challenges and developed methods to get clutches to grab like they should. Maximizing clutch performance is a delicate balance of clamping force, disc design, and high-tech friction materials. To help you solve your clutch woes, we hit up Will Baty and had him explain it all to us.
"There seems to be an ongoing debate over the virtues of lightweight aluminum flywheels, but definitive answers are hard to come by. When it comes to the weight of a flywheel, altering it significantly impacts the inertia exerted on the crankshaft. This inertia is similar to the effects of gearing. A heavy flywheel is like having short gears, and a light flywheel is like having tall gears. Keep in mind that flywheel weight can't be measured on a bathroom scale. Since it is bolted to a spinning crank, a flywheel's dynamic weight is far more important than its static weight. Where the weight is located on the radius of the flywheel determines how much inertia a flywheel has. For a given amount of weight, the farther it is from the crankshaft the more it will affect dynamic weight. To illustrate this point, get a piece of string, tie a small weight to the end, hold the string about a foot from the weight, and spin it over your head like a lasso. You should feel a small amount of force, or inertia, pulling on your hand. Next, hold the string three feet from the weight and once again spin it over your head. You should feel a drastic difference in force because you just changed where the weight is on the radius of the circle. Consequently, an aluminum flywheel with less inertia will make the vehicle feel heavier from a standstill and will require slipping the clutch a little more to get the vehicle moving. Vehicles that have a deep First gear or develop a lot of bottom-end torque can benefit from a lower inertia flywheel since it allows the torque to be transferred to the ground instead of breaking the tires loose. The engine will rev quicker with a low-inertia flywheel and will typically accelerate through the gears faster. Heavier flywheels, on the other hand, are better suited to cars that don't produce much low-end torque or have taller gears."
Engineering a clutch assembly isn't as simple as lining a disc with gripper friction materials and calling it a day. Centerforce starts by putting the OE pressure plate through a series of tests to find the limits of a vehicle's clutch and hydraulic assembly. "Some clutch systems require little modifications, but we often end up engineering an assembly that's completely different from the OE unit," Baty explains. "Many of our clutch designs utilize our patented ball-bearing fulcrum that gives us the ability to increase the clamp load and pressure, increase the rate of release, and reduce the throwout bearing load. With modern vehicles, we look at the hydraulics system very carefully, since a high throwout bearing load may cause the hydraulic system to fail prematurely. Once we have engineered a clutch assembly, we find a vehicle and install the clutch on it in-house to address any issues with the installation that our customers may run into."
Pressure Plate Design
The three basic types of pressure plate designs are: Long, Borg & Beck, and diaphragm. Long and Borg & Beck pressure plates both use three large fingers and a series of coil springs to generate clamping force. "These pressure plates are adjustable, but have a large amount of pedal pressure so they're typically used in drag cars these days," says Baty. "The diaphragm-style plate is used in just about every manual transmission vehicle today due to its having an ideal balance between clamping force and pedal pressure. As they wear out, the diaphragm-style plate actually gains clamp load."