Clutch Technology Part 3, Clutch Disc Construction


 Unorthodox Racing’s 4 puck solid hub clutch disc is about as extreme as you can get for a single disc clutch.  This sort of clutch is barely streetable but if you have a powerful engine and don’t have the budget for a multi disc clutch, a clutch like this may be your only choice.  The lightweight solid hub is easy on the syncros and speeds shifting.

Solid hub discs can also contribute to the failure of other drivetrain components like the tranny, halfshafts or C/V joints as they transmit a lot of engagement shock to the rest of the driveline. They are very noisy as well, transferring all of the drivetrain noise and harmonics throughout the car. Usually road racing or circle track applications are solid disc as crisp shifting and minimal weight are important for these applications. Solid disc clutches should be popped and wheels spun to get them going. Trying to slip them for a smooth street start will wear them out quickly. No matter how carefully you slip them they will still chatter as they engage. This is not acceptable on the street for the vast majority of drivers.

ACT 4 Puck sprung disc
 ACT’s sprung hub 4 puck is a compromise to allow some streetabilty and some cushion for the drivetrain.  Note how the disc is built to minimize material and to lighten the disc as much as possible.
ACT solid hub puck disc
 The most interesting part of the ACT solid hub disc is that it is designed to reduce weight to speed shifting as much as possible, look at the hollowed out areas near the pucks to reduce rotational inertia.

For really hard use, a full metal disc is needed, usually of the puck variety. These discs have anywhere from eight to three pucks of friction material per side on metal arms extending from the center hub. The fewer number of pucks the lighter the disc, and the less rotating inertia and better shifting it will have as when the input shaft speed falls rapidly, the syncos will work better. A full race disc may only have three pucks with a solid hub. The three-puck disc will be unstreetable because it must be popped to get the car moving as slipping it will burn it out quickly. Aggressive full race discs also have fewer pucks because extremely aggressive metal friction materials sometimes work better with a higher pressure loading per square inch. This helps them come to operating temperature faster.

dual friction disc
A dual friction clutch disc has a semi metallic friction material on one side for smoothness.
duak friction
And metal on the other side for grip and long life under hard usage conditions. This is the most aggressive clutch that many people can tolerate for everyday use. This is a JWT disc.

As a compromise for street/strip or weekend warrior cars there are dual compound type clutches available. These types of clutches originated from the heavy-duty industrial and semi-truck clutch industry where severe use is coupled with the need for smoothness. The Centerforce dual friction or the Clutchmasters Stage IV are examples of this type of clutch. These typically have eight or more metallic pucks on one side of the disc with a Marcel and Kevlar or semi-metallic material on the other with a sprung center hub. The less aggressive side of the disc provides a smooth initial engagement while the puck side provides bite and high temperature handling. The abrasive metal side is usually put on the disposable pressure plate side to spare the flywheel. These clutches are probably the most aggressive that most people can tolerate for daily driving.


ACT Sprung semi metalic
 ACT’s full face semi metallic heavy duty disc has no marcel spring as a marcel spring with its indirect attachment of the friction material to the disc itself can be a point of failure.  The ACT disc has a sprung hub and is pretty smooth.  These difference in disc design reflect upon the engineering opinions from the various clutch companies.
ACT solid hub semi metalic disc
 ACT’s full face semi metallic sold hub disc is a good choice for lower power applications that need a lightweight disc.  The semi metallic material is fairly smooth engaging and somewhat easy on the drivetrain and the solid hub is light and speeds shifting.  A low to medium road racing car would benefit from a disc like this.

Although this subject can be confusing, we hope that we have given you enough information so you can make a wise choice when selecting your clutch. In our next installment of clutch tech we will delve in to the mysteries of flywheels, dual disc clutches and specialized clutches.


Nukabe Automotive
Cusco Clutches

Jim Wolf Technology
Nissan Clutches




Clutch Net

Clutch Masters



Unorthodox Racing

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