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."
The ubiquitous Tremec T56 is utilized in a variety of vehicles from the factory, but with varying clutch disc diameters depending on the application. So how does disc diameter affect clutch performance? According to Baty, LS1-powered Corvettes and Camaros come from the factory with an 11.5-inch clutch discs while Vipers are equipped with a 12.25-inch disc. "The Viper's clutch is only 3/4 inch larger, but in the clutch world that's huge," says Baty. "To give you an example, the effect is similar to using a larger breaker bar to loosen the lug nuts on your wheels. It's what we refer to as the 'arm' of the clutch, and the farther outward you get the friction material, the greater the holding capacity. In other words, if you compare an 11-inch clutch with a 12-inch clutch that has the same clamp load and point of friction, the 12-inch clutch will hold more torque hands-down every time."
When a traditional single-disc clutch system can no longer hold an engine's torque, multi- or twin-disc clutches are required. By utilizing a floater assembly and two discs, they double the surface area available to transfer torque to the transmission. The result is significantly enhanced grip for a given clamp load. There are, however, some caveats to keep in mind. "Since you now have two discs and a floater that require the same release rate as a single disc, the release ratios need to be fast, and the drawback is that they tend to work like an on/off switch," Baty says. "A single-disc unit certainly has a limit to how much torque it can hold, but we have customers using our 11-inch DFX assembly in cars that produce over 900 lb-ft of torque at the wheels without any problems."
"Centerforce was conceived in the early '80s when Bill Hays, who also founded Hays Clutches in the '50s, began brainstorming new clutch technologies. With the desire to engineer a solution to the 'sticking over-center' problem of diaphragm-style clutches, Bill designed and patented the Centerforce weighted clutch system. This design increased pressure plate clamping force while maintaining easy pedal effort and eliminated the diaphragm's tendency to 'stick over-center'. Later in the decade, he invented, designed, and patented the Centerforce Dual Friction clutch system, which was awarded Best New Performance Street Product by SEMA. A member of SEMA since 1977, Centerforce has been nominated for the PWA Manufacturer of the Year award many times, winning it in 1998. Bill was also inducted into the SEMA Hall of Fame that year for his work in the performance aftermarket industry. Over the decades, Centerforce outgrew its two original California-based manufacturing facilities and in 1994 relocated to a new, 47,000-square-foot shop in Arizona. New patents have since followed as well as continuous improvements to product offerings. Today Centerforce has over 1,700 clutch applications available, and plans are being reviewed to nearly double the size of our facility to keep up with the demands of the performance automotive world. To this day, Centerforce is still a three-generation family-owned business."
While OE clutches typically use full-faced designs for smooth engagement, high-performance aftermarket discs often employ a multipuck arrangement to increase clutch bite. On a traditional disc, the force exerted by the pressure plate is evenly dispersed throughout the facing. A puck-style disc, on the other hand, concentrates the pressure over a much smaller surface, thus increasing the force exerted on the flywheel. "Imagine a bare-footed 100-pound woman standing on your back giving you a massage who suddenly puts on high heels," Baty quips. "It's going to hurt a whole lot more with the heels. This is essentially the same concept puck-style discs employ."
Weight And Traction
Horsepower isn't the only thing to consider when selecting a clutch. For a given power output, as vehicle weight and traction increases, so do the loads on the clutch. "Heavier vehicles tend to require more clutch slip when taking off from a standstill, and this has an adverse effect on the life of the clutch," says Baty. "When you slip the clutch, it generates lots of heat, which is the number-one killer of a clutch. Increasing mechanical advantage through gear reduction can take some stress off the clutch and driveline, which is why five- and six-speed transmissions with low First gears are becoming so popular. Traction can tremendously impact clutch loads, which is why drag cars are the most demanding of all applications."
Whether they're organic or metallic, the materials used on a clutch disc impact clutch bite, streetability, and longevity. "Organic compounds are more widely used from the factory, and are made of various fibers, resins, and other additives," explains Baty. "They offer very smooth engagement, but deteriorate very quickly when subjected to heat. Ceramic-metallic compounds tend to be more aggressive, typically have a greater holding capacity than organic linings, and are made by sintering different metals together-much like a brake pad. These compounds have a high coefficient of friction but tend to have a shorter lifespan."
Certain clutch designs are more prone to chattering when engaged than others, but it's often caused by installation errors or other faulty areas of a car. Baty suggests checking the clutch disc for contamination by oil, hydraulic fluid, or grease. "Other potential culprits are an improperly resurfaced flywheel, or a bent clutch disc or pressure plate," he explains. "Chatter isn't always caused by the clutch system, either. Loose or damaged engine and trans mounts, or mounts that are too stiff, can cause chatter as well."
"Stock pressure plate and flywheel bolts can be reused as long as they are in good condition. We suggest using some type of thread locker on the flywheel and pressure plate bolts and to tighten everything back up to the factory torque specs. Aftermarket bolts can also be used, but we strongly suggest that you use bolts designed specifically for pressure plates and flywheels. You shouldn't go down to your local hardware store and buy some Grade 8 bolts and expect them to work. For example, the 3/8-inch pressure plate bolt commonly used on GM vehicles is a small-shouldered bolt. The shoulder fits snuggly into the pressure plate boltholes and goes into the flywheel, centering the pressure plate to the flywheel. This is critical for proper balance of the engine. The shoulder also takes some of the shearing force when the clutch is engaged. Some flywheels use dowel pins to center the pressure plate and absorb this force, but it's still advisable to use a small-shouldered bolt."
With new car manufacturers transitioning from cable-actuated to hydraulic clutch systems, the challenge for the aftermarket is increasing clutch bite without the luxury of significantly increasing pressure plate clamping force. It's a problem that requires innovative solutions. "Many modern hydraulic systems seem to be marginal when simply trying to operate the stock clutch," says Baty. "That is why we pay very close attention to the throwout bearing load, which is the amount of pressure it takes to operate the clutch. Some clutch manufacturers change the diaphragm or machine the fulcrum point to increase the clamping force of the pressure plate. This, unfortunately, increases the throwout bearing load and leads to hydraulic system failure in some vehicles. Consequently, we utilize our patented ball-bearing procedure that allows us to decrease the amount of pressure required to operate the clutch."
A botched install is the quickest way to destroy a clutch. According to Baty, the most common mistake is not checking to see why the original clutch failed. "If you have a leaky oil seal and put a new clutch in, you're just going to ruin the new clutch with oil in a short time, so we advise our customers to always check and inspect every component in regard to the clutch and driveline when replacing a clutch," he explains. "Also, people often use too much grease on the input shaft. Remember the input shaft spins with the engine so the grease will eventually reach its way to the disc. Our suggestion is using an aerosol spray can of dry graphite lubricant on the input shaft instead of grease."
"Over the years, Centerforce has continually updated and improved its product line to meet the needs of a variety of different applications. Our Centerforce I clutch system is a good stock replacement over OE clutches and features better wear characteristics. The Centerforce II series is designed for the vehicle with basic performance bolt-ons, and for the 4x4 market where additional holding capacity is needed. The Dual Friction family of clutches is designed for vehicles with substantial power increases over stock while retaining excellent street manners. It uses a patented ball-bearing design coupled with a clutch disc that incorporates an aggressive lining on the flywheel side for excellent grip, and a less aggressive lining on the pressure plate side for smooth engagement. The DFX is designed for all-out performance vehicles and features a six-puck ceramic-metallic friction disc that can handle some serious torque. Its pressure plate is also a ball-bearing design with reinforced drive straps and a nodular friction ring. Lastly, the Light Metal Clutch, or LMC, was designed for circle track race cars but is now used widely across the board. It has an aluminum ball-bearing pressure plate that reduces weight by 5-6 pounds, and it uses an organic dual-puck disc."
Although modern piston ring technology has turned breaking in a fresh motor into somewhat of a formality, the same isn't the case with clutches. "We know what it's like to want to go out and test the clutch immediately, but the break-in period is the most important part of a clutch's lifecycle," Baty advises. "We suggest normal around-town driving or just cruising the town for the first 500 miles. An improper break-in can drastically reduce the clutch's holding capacity and life. The old method of burning in a clutch is not recommended at all since this will glaze the friction material and greatly diminishing the holding capacity of the clutch."
Mixing & Matching
Wear on a clutch disc is easy to spot, but wear on a pressure plate isn't. This tempts many hot rodders to reuse their existing pressure plate from one manufacturer with a performance clutch disc from another, but there are some potential pitfalls to keep in mind. "All clutch manufacturers have different ways of achieving clamp load, so some companies use thinner or thicker discs to work in conjunction with their pressure plates," Baty explains. "They may also suggest using shims, and the problem is that one brand of pressure plate requires a specific disc thickness. So even though it may be for the same vehicle application, if a disc from one manufacturer is used with a pressure plate from another, the required specs for disc thickness will be off and the potential for failure is high."