Clutch 101

Categories: Tech Articles

Clutch 101- By Robert Martin



The clutch is one of the most important components in a high performance build up. No other single component will be used as much by the driver and affect the way the driver interacts with his/her car.

My Background

My name is Robert Martin and I have been building Honda engines, trannies, doing swaps, etc. I also have been selling and installing clutches for the past 4 years. I have had the luck to be very good friends with Alex of Action clutch and because of this have seen many many clutches. Here are some things I would like to share with you so you can make a better decision on which clutch to run.

The Players

In no particular order:

  • ACT
  • ClutchNet
  • Clutchmasters
  • RPS
  • HC Racing (Herson Racing)
  • Action Clutch
  • Clutch Specialties

Anatomy of a clutch setup

This diagram is far from complete but it will help you see what’s going on.

Basically the clutch locks the disc onto the flywheel using the diaphragm as the loading mechanism. When you push in the diaphragm it lifts the FRICTION RING and the CLUTCH DISC is free to spin independently from the engine so you can change gears without grinding. Any flexing of the CLUTCH COVER will hinder the FRICTION RING from releasing the CLUTCH DISC. Currently the only company to offer their own custom CLUTCH COVER (that is affordable) is ACT although OEM FCC or Exedy CLUTCH COVER’s work fine. As you can see if you move the LEVER POINT’s closer to the PIVOT POINT the DIAPHRAGM will have more leverage and exert more force on the FRICTION RING. ACT, Clutchmaster, Action, and Hersons do this by machining the LEVER POINT so that the peak of the LEVER POINT is closer to the PIVOT POINT. The side effect of doing this is that it requires you to push on the DIAPHRAGM further than you would if the LEVER POINT were not moved. Clutchmasters does a unique thing in that they place spot welds to build up the LEVER POINTs and then do machining to be be able to move the LEVER POINT the furthest. Another way to increase pressure is to install a stiffer DIAPHRAGM. ACT, Action, and maybe RPS does this (I have never seen RPS since they are not popular in LA). ACT has an extra thick 3mm vs. 1.8mm DIAPHRAGM that they use in there Extreme pressure plates. This gives good pressure but is on the thick side and has been known to crack due to it’s thickness and the hardness of the steel that it is made of. It’s very hard steel. It has the highest hardness of any diaphragm I’ve seen. You can also install another diaphragm and have two diaphragms. This is what is called a DUAL DIAPHRAGM clutch. Herson, Action, ClutchNet, Clutch Specialties, and some smaller shops make these. Note: to properly make a double diaphragm clutch is not as easy as slapping in another diaphragm. The rivets holding in the DIAPHRAGMs must be longer, the FRICTION RING or LEVER POINTS have to be checked for clearances to make room for the extra DIAPHRAGM. The only companies I know of that can do this properly is Action and Hersons.

What’s the best disc?

The answer is which ever one will work properly for your application at the right price.

Here is the best stock disc. It’s a Exedy “Unity” disc and incorporates some nice features like forged spring retaining plate (purple) and decently strong springs.

Here is an aftermarket sprung six puck. This particular disc is used by Action, Hersons, Clutch Specialties, and RPS (from what I can see from the pics on their website). If you compare the features of this disc with the Exedy you’ll notice it’s held together with 6 rivets rather than the 4 that hold the Exedy together. Also notice that the springs are much shorter.

Here’s an aftermarket full face sprung disc with kevlar facing. The kevlar lining will literally last forever. I am not exaggerating here. We have many customers coming back to fix their clutches but the kevlar lining just needs to be sand blasted after that there’s no way to even tell that it’s been used. Kevlar is so easy on the flywheel and pressure plates that you can still see the machining marks on the friction surfaces of the flywheel and pressure plates after years of use. Note the same heavy duty construction as the sprung six puck.

Here is an aftermarket solid six puck used by Action, Clutch Specialties, and Hersons. This is what I consider the standard of discs and works really well in performance applications since it is light and will be easy on your tranny when it comes to shifting. With the Miba friction pads it’s also possible to engage chatter free but engagement will be much faster than stock organic.

Here is the baddest of the bad dogs. It’s commonly called the “Iron Disc” and is made by Raybestos and used in truck applications and also used in twin disc apps on Ram Clutches in NHRA Pro-Stock cars. This is used in Clutchmasters, Action’s and Hersons most heavy duty clutches. This disc needs a lot of pressure to work properly since it’s a full face and has a lot of surface area.

Friction Materials

This is the most common stock lining and is fine for normal driving but should never be used in performance applications. Organic lining is basically wood (cellulose) with a polymer binder and some metallic strands. I am not saying it won’t work but it won’t last too long if you dump the clutch during high rpm launches. My favorite term for organic lining is cookie dough. And it doesn’t matter if you have a heavy pressure plate working on it, organic is still weak.

Here’s a pic of a new organic replacement lining.

Here’s a pic of a used glazed organic lining. I broke this piece by hand to show that it is fairly weak and that it’s composed of fiber.

Street disc usually means “beefed up” stock organic disc. The difference between 4 and 6 puck is that with less pucks the harder it is to engage smoothly (harder to control slippage).

Sprung vs. Unsprung

Which is better, depends on usage. For any performance application I always recommend un-sprung. Would you want a one piece hammer or a three piece hammer connected by springs? If you want something fail safe then unsprung is the way to go. The reason why there are springs in the hub is not to prevent chatter. Chatter is a engagement property and is mostly dictated by the friction material used. The springs in the clutch disc prevent crank vibrations (piston engines have jerky rotational motion due to combustion) from entering the transmission. This is the principal behind sprung hubs. In practice I have yet to see any ill effects of running un-sprung hubs. I’ve taken apart a few transmissions and have not noticed anything unusual. It’s not like I’ve opened up a tranny and say look Bob, this is what happens when you run a unsprung disc. I just haven’t found any signs of bad effects. Now if you want your tranny to last 200-300k miles then I would choose to run a sprung disc. If not the lighter un-sprung disc is my choice.

Comments on certain brands

Located in Chatsworth. CA, close to where I live, its run by Robert Smith and sounds like a small operation when I called them once. RPS is really expensive for a clutch. I haven’t seen one though so I can’t say what trick stuff they are doing. The rib diaphragm design is gimmicky in my opinion since it should not affect diaphragm springiness. The fingers just won’t flex. That’s all.

Located in El Monte, CA, it’s owned by a Russian family and has been in business for 40 years. They are probably the biggest and best aftermarket disc manufacturer. They make discs for everything from Audi to VW. Their pressure plates are Valeo brand units which I don’t like. Also their friction pads are too soft and will eat your flywheel and pressure plate.

Located in Palmdale, CA. ACT is also known as Kennedy Engineering Products. They make awesome pressure plates (they manufacture their own). Nice and stiff but the friction pads they use on their pucks are too aggressive and chatter and disintegrate and eat up your flywheel and pressure plate. They are also actively involved in import drag racing and have a booth at every Battle of the Imports in Palmdale.

How to recommend a clutch

This is a question I am asked very often, “Which clutch should I get?” I usually have 2 to 3 clutches in mind but since the clutch affects the way you interact with your car, a clutch seller should try as best as possible to understand BOTH buyer and the buyer’s car. The general rule is the more hardcore you are the more likely I recommend a puck disc. The more conservative you are I would recommend kelvar. Action has good segmented Kevlar discs like the one pictured earlier. The reason for segmented vs. full face Kevlar is that Kevlar needs more pressure to work properly. Yes, torque holding capability can dramatically increase with different friction materials. It’s measured by what engineers call coefficient of friction. The lower the coefficient of friction, the more force is needed to produce the same holding power. Copper Ceramic has one of the highest (but tends to break up under heat). Kevlar actually also has a high coefficient of friction, almost approaching that of the sintered metallic pucks.

Pressure plate assembly

Here’s the underside of a OEM P72(GSR) pressure plate


Here are the difference between the stock STRAPS and the STRAPS used by Action

Note that the Action strap is pre-bent. This prevents the strap from bending at the rivet. It is 100 times more likely to break if it bends at the rivet instead of bending at a pre-determined point away from the rivet. Also note the thickness of the strap. The material of steel is also superior. Also not that the Action strap is shorter. This is important since the straps keep the friction ring centered in the cover. Sometimes stock straps can’t keep the friction ring centered and on high HP turbo engines the friction ring tends to shift and hit the cover. Once this happens the clutch has a hard time releasing properly. ACT also use their own straps but I am not too impressed since they just use cut steel ribbon and do not have the pre-bent sections like Action’s. Here’s a pic of what the stock strap looks like up close in the stock P72 pressure plate. Notice how the stock strap can bend at the rivet. This bending occurs when the strap is in compression (when you down shift). When you launch, the straps are in tension (being pulled upon) so theres no bending.