Project EVO VIII part 4, Getting More Trackside Grip, Killing Some Slip

With all of this negative multi disc clutch experience, we were skeptical when the guys at Cusco recommended the Super Twin and said no problem when we expressed our past experiences and concerns.  The Cusco clutch has many unique ingenious features that tame the beast giving us the advantages of a twin disk clutch with few of the disadvantages.

The exploded view shows the complexity of a heavy-duty twin disc clutch in all of its machined billet beauty.

Cusco’s clutch addresses every single one of our concerns with clever engineering. To address the issues of rapid clamp load drop off with clutch disc wear, Cusco uses a special diaphragm spring in the pressure plate.  Using a special coating, material and geometry, the Cusco spring can maintain a consistent clamp load even when the discs wear quite a bit.  When Cusco benchmarked their twin disc against others, they found that their springs could maintain the clamp load over nearly twice the wear distance of other clutches.

To make the clutch have a longer engagement travel allowing it to be smoothly driven off the line, the fulcrum of the pressure plate’s diaphragm spring, located on top of the pressure ring, which is normally a round wire or a high point in the pressure plate stamping, is itself a flat springs with give in it.  The additional spring action of the fulcrum gives more pedal engagement travel to the stroke of the pressure ring as the clutch is being engaged. The fulcrum initially also works at a reduced leverage ratio further increasing pedal engagement travel and reducing pedal effort needed to disengage the clutch. The result is a lot of room in the pedal travel to modulate the clutches engagement, right as the clutch starts to grab giving an initial feel close to that of a factory clutch combined with a light, non leg busting pedal effort.

To cushion the drivetrain, make for a smoother engagement and to reduce drive train noise both of the clutch discs feature the use of a sprung hub.  This is an unusual concession to streeabilty in a twin disc.  The discs friction material is full metal, usually the best for burst resistance, torque capacity and heat resistance but the worst for niceness on the street. Cusco has perfected the materials characteristics so that the material has the good attributes of metal but somehow still allows a fairly smooth engagement.  This is a major feat as anyone who has daily driven a metal disc clutch on the street can attest to.

The center plate has these leaf separator springs, which help reduce noise , heat and reduce wear.  You can also see the venting which pumps cooling air through the center of the clutch.

The floater plate between the two discs has some unique features as well.  First it is fairly thick and has a ventilated center to help pump cooling air into the heart of the clutch aiding in the dissipation of heat.  Flat leaf springs help keep the floater actively centered and under tension when the clutch is disengaged, reducing the annoying chinging noise that all twin discs are affected with.  Twin disc clutches tend to wear the pressure plate side disc faster as well as its gap tends to close first as the clutch is engaged.  The centering springs insure that there is a gap between the floater and the discs, not allowing them to touch, thus wear consistent between the discs.

The clutch also features a billet machined aluminum pressure plate cover giving both lightness and stiffness over the typical stamped steel that most clutches use. The flywheel of the clutch assembly is forged steel with a lightweight thin cross section.  The support pillars for the clutch are also steel which is an advantage for street driving.  Many twin discs have support columns made of aluminum for lightness.  The trouble is that on a streetcar, the frequent use causes the drive tangs of the clutch floater plates to wear grooves into the pillars. The floaters get stuck in the grooves causing sticking and inconsistent clutch action. In this case the harder steel is the superior material.

Here are the separator springs in place, you can see how they compress and keep the center plate from vibrating and rattling.  It also keeps the separator from rubbing on the discs when the clutch is disengaged making heat and causing wear.

All of these technical features make for an amazing clutch.  It takes the punishment of AWD drag launches and will hold all of the power that the engine mods we will make in the future will produce.  Although its is not perfectly silky smooth like a stock clutch, the Cusco twin disc is still the smoothest engaging, least chattery and quietest twin disc we have ever driven by an order of magnitude.  Short of an ultra expensive exotic carbon carbon clutch, we have never experienced a twin disc so smooth and it’s the smoothest easiest to drive all metal disc clutch we have ever used period.  Other makers of multidisc clutches should take note on how this one works.

The back of the Cusco forged steel flywheel is machined to further lighten it.