For all practical purposes, the automotive clutch that has been in use for much of the last century was conceived by the Borg & Beck company in Kenosha, Wisc., around 1925. The traditional clutch disc, pressure plate and release bearing have remained largely unchanged other than a few modifications to friction materials and the use of multiple discs.
Most of the changes to clutches have come from the need — particularly in motorsports — to handle ever greater levels of horsepower. Of course, in single-plate applications, that extra capacity often came in the form of heavier clamping pressure, which increased pedal effort, sometimes to the point of inconvenience.
“The only way to make more torque capacity, with the same friction material, if you leave that the same, you have to add clamp load. We’re going to add more and more and more clamping pressure to keep that disc from slipping. At some point you end up with a very heavy pedal and the drivers complaining that he’s got to use two feet on the clutch pedal,” said Chris Bernal, vice president of engineering for Advanced Clutch Technology. “What you can do then is add another disc, so instead of two surfaces, now you have four surfaces. Of course, you need a floater plate in between. What that does is it takes, and it doubles, the amount of torque capacity for the same clinical element.”
To be able to hold more horsepower, you need more clamping pressure, or more surface area, among other things. The former is the first resort in single-plate applications. The latter is the solution presented by dual-disc clutch setups.
ACT had developed twin-disc applications for big-power V8s such as General Motors LS applications and Ford Coyote V8s and Chrysler Hemi engines. Those applications didn’t serve the import market, whose engines are smaller but are still plenty capable of making big power. That left a void in its product line.
They filled that void with a new line of twin-disc clutch packages. The Forged Twin clutch line uses a forged-aluminum cover with reinforcing ribs extra rigidity, stability and higher efficiency, which means they can add more clamping load with less pedal effort.
However, what makes the Forged Twin unique is not all in the pressure plate, but also the hub and friction surface system ACT developed. Rather than using two separate discs, either rigid for racing or sprung for street use, the Forged Twin uses a single hub driven by both discs.
This video highlights the modular Forged Twin dual-disc clutch setup, which works with sprung and unsprung hubs, and with organic and cerametallic friction materials.
They accomplish this by adding splines to the outside of the hub, which is thicker than normal, and incorporating matching splines on the inside of the friction surfaces, again either solid or sprung. So, essentially they have reinvented the twin-disc clutch by separating the friction surface from the hub and making the components completely modular. One pressure plate design can accommodate sprung and unsprung hubs, organic, full circumference materials and cerametallic “pucks.” The technology is as old as the spline itself. It’s just a new application for it.
“The way you calculate torque capacity in the clutch, it’s four factors. You have the clamp load, coefficient of friction of the friction materials, the number of surfaces, and then the size of the clutch, the working radius of the clutch, and then simple multiplication, and that’s what gives you the torque capacity of the clutch,” said Dirk Starksen, founder and president of Advanced Clutch Technology. “It’s four factors and that’s it. And now you notice it’s number of surfaces, not surface area. You can make this (friction surface) super wide, it doesn’t hold any more power. It’ll handle a lot more heat.
“So you can tell that in this case we haven’t changed the size much. You haven’t changed the coefficient of friction, because if you increase the friction a lot, then you’ll lose drivability,” Starksen continued. “So now you have clamp load and the number of surfaces. We’ve doubled the number of surfaces. So, everything else, if it’s the same, this will give you double the torque capacity. In this case, we want to lower the clamp load a little bit, so that it’s not as stiff on the pedal. That’s what we’ve done.”
In addition, the ribs on the aluminum clutch ribs are fitted with stainless steel sleeves on which the steel pressure and floater plates ride. This lets the user replace just the sleeves rather than the whole pressure plate when they wear out.
“In that formula, those four factors, what isn’t in there is the surface area, because the surface area of the friction material doesn’t really determine the torque capacity. It will help the heat capacity, certainly,” Starksen said. “You have more material to do the work, but with the organic material, you want to have a full surface, because it can’t handle the heat that a ceramic material can. So, you want to have the full surface there, because it has a resin that binds it together and in high heat the resin will melt. Or the ceramic material, it’s heavier, and it’ll handle a lot more heat. So you want to use less material, because if this was full surface it’d be too heavy, but it’ll handle a tremendous amount of heat, so you don’t need it.”
That’s why the organic materials are full-circumference discs, and the ceramics typically use “pucks.”
“These are meant to go with specific flywheels, but they need to be specific clutch-size, and right now we’re hitting the market, and the EVO’s, and the Subarus, STI’s, WRX’s, initially,” Bernal said. “And we’re going to have a whole bunch of more applications as time goes on.”
Take the full tour of the Advanced Clutch Technology plant and see how all its clutch assemblies are made.