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Discussion Starter · #1 ·
I contacted Mr. Dirk Starksen, President of ACT, to see if I could get some more precise information that would be helpful to our members when they go to buy a clutch. Specifically, when I was shopping around for clutches, several points to ask him became apparent to me:

1. What is the stock pressure plate psi clamping force in the various Bseries engines?

2. What pressure is considered excessive to the point that it may be too much for the thrust bearing and result in premature excessive wear.

3. What affects clutch disengagement effort or pedal effort when you disengage the clutch in order to change a gear?

4. How much chatter and vibration can you expect from an aftermarket clutch as compared to a stock clutch, since there is extra clamping force both from the agressive disc material and higher clamping pressures from the disc itself and pressure plate?

5. What materials are found in the various discs in both high performance street versus race only discs?

For those of you who aren't familiar with the basic parts of a clutch and how it actuates to link the flywheel to the tranny and driveshafts, please look at this:

Clutch Movie

Mr. Starksen was kind enough to take the time to reply to my inquiries. The letter first introduces you to some basic principles about the clutch and then the second part answers my specific questions. You may want to compare the info to what is stated in Joe Pettitt's Honda High Performance Hanbook Volume 1 chapter on clutches which has the same basics but approaches the problem of getting higher torque capacity slightly differently. Joe's book uses Rob Smith's (of RPS) approach at tackling the problem. But you have to remember that Rob is dealing with high hp turbos only in that chapter and not N/A or daily drivers...good comparative info nonetheless :

from Dirk Starksen said:
Clutches 101: Let me start off with THE BASICS :

1)What does a clutch do?

The clutch engages and disengages the torque from the engine to the transmission. This engagement/disengagement period is a crucial element of the clutch. Without this engagement phase where the clutch slips and carries only a portion of the torque, the need for a clutch wouldn't exist.

2)There are two main components to the clutch, the pressure plate and the clutch disc.

The pressure plate bolts to the flywheel and clamps the disc against the flywheel with a prescribed load (clamp load) generated by a diaphragm spring working through fulcrum points (pivot diameters) within the pressure plate.

The clutch disc connects to the input shaft of the transmission and once the clutch is engaged carries the engine's torque to the transmission. The linings on each side of the disc are made of various friction materials. Springs are incorporated in most street clutch discs to dampen torsional vibrations to reduce transmission noises. A marcel (wavy metal) is placed between the linings and the drive plate to help make engagement progressive and reduce chatter.

3)The formula for the torque capacity of a clutch is :

T = NxRxPxF

T is Torque in ft. lbs., N is the number of surfaces , R is the radius of gyration (mean radius of the friction material in feet), P is the pounds of clamping force, and F is the friction coefficient.

Let's look a little deeper:

From the formula for torque capacity you can see that if the number of surfaces double (such as going from one disc to two) the torque capacity is double. If the disc size is double then the torque capacity is double, and so on.

With each variable to increase clutch torque capacity there are benefits and drawbacks, whether it is changing N,R,P or F and there are different methods to achieve these changes. This is where the plot thickens and the expertise (or sometimes voodoo) takes place.

Surface area: Notice from the formula that there is no variable for surface area, only size. This is because surface area has little to do with torque capacity. As surface area goes up, the clamp load per square inch goes down. So, the two in essence cancel each other out. More friction surface, less pressure. Surface area has everything to do with heat capacity and inertia, and not much to do with torque capacity. However, if the clutch gets too hot from having too little surface area, then it does affect torque capacity.

Number of surfaces (N) : Not usually a practical option. A double disc clutch will carry twice as much torque as a single disc clutch that is the same size and clamp load and friction. Expense, transmission clearance, floater rattle and inadequate disengagement are drawbacks that influence the twin disc setup. Two production cars that I am aware of (Acura NSX and Porsche 928) went back to a single disc clutch when the double disc proved too troublesome. It is really best suited for a racing transmission that doesn't have syncros because there is nothing to separate the clutch disc from the mating surfaces. This clutch drag can hinder fast shifting.

Radius of gyration (R): Increasing the size of a clutch will help to carry more torque and also dissipates heat over a greater area. The drawback is that also usually increases mass or inertia so the assembly is most likely heavier and finding a bigger clutch that will fit is a challenge. Centrifugal force is higher on the disc as well which means slower shifting. Unless there is an optional size already available then considering adapting to a bigger clutch is not usually practical.

So what's what two variables are left? : Clamping Force (P) and Friction (F).

I. Pounds of clamping force (P) : This is a variable that is played with quite a bit. There are four different methods listed below:

A. The first is to simply alter the geometry of the clutch by changing fulcrum points. This method is probably the most common because only the casting has to be removed and altered to achieve the change in pivot diameter. Since this method increases clamp load and not spring load the pedal effort remains stock and obviously the clutch will not put additional force on the thrust bearings. The tradeoffs are that the pressure plate requires more travel to operate and although the pressure plate exerts more force it is over a shorter range. In other words, the clutch will feel "soggy" and light and the clutch will wear out faster than stock, proportional to the clamp load increase. The larger the change, the more severe the negative symptoms.

B. The second method is to weight the fingers. This will gain benefit from higher clamp loads only at higher RPM's. Since the torque curve is usually relatively flat you may not have the force when you need it. Because of the flexible nature of diaphragm spring fingers the affect of the weights is kept very limited. Because centrifugal force increases exponentially, If allowed to add only 20% more clamp load from centrifugal force at 4000 rpm then at 8000 rpm the increase would be over 100% and the clutch would have trouble operating.

C. The third method is to stack two diaphragms on top of each other. This is effective at increasing the clamp load 100% but is somewhat crude. The design benefits are that the stress on the diaphragm spring is not increased and the additional thickness of the second diaphragm will take the additional loads placed on the fingers as well. The drawbacks are extremely high pedal effort, vague feel, and an inability to make smaller increases in clamp load. Since the second diaphragm rubs against the whole surface of the first diaphragm in an environment of clutch dust, heat, etc. friction between the diaphragm surfaces becomes very high. This additional friction is felt in the pedal.

D. The last method is to increase diaphragm spring load thus increasing clamp load. This method is the most widely used by automakers when using the same clutch size to handle more power. The diaphragm spring is replaced with a thicker, wider or taller diaphragm to produce more spring load. The benefits are that the geometry does not need to be altered from the original clutch, the engagement is predictable, small to large increases can be engineered, and typically there is an increase in working range (clutch life). Naturally, there are drawbacks. These are: higher stresses placed on the diaphragm, higher loads on the thrust bearings, and higher loads placed on the clutch linkage. The higher loads on the diaphragm must be carefully calculated and considered in order to prevent breakage and finger flex. There is no change to engagement quality like one competitor claims. Otherwise, automakers wouldn't be using this technique.

At ACT we engineer our diaphragms to gain a generous increase in spring load and then typically make a small geometry change for additional benefit. Since we have increased the working range by use of a stronger spring, the small change in geometry doesn't significantly take away from the clutch life.If we have a 30% stronger diaphragm and have a 5% increase in geometry you get actually get a 36.5% increase, not a 35% increase because the increase is amplified.

Pressure plates are really simple in design. There are few tricks, but a lot of engineering considerations. Since we have the ability to design and manufacture diaphragm springs, we feel we have broader design options than most of our competitors.

II. Friction (F): Usually, here is where the voodoo happens. To put it in basic terms, the higher the friction, the higher the wear and harsher engagement (chatter). The lower the friction the smoother the engagement and lower the wear. This is a general statement, not intended to be true in all cases. For instance, when using solid mounts clutch chatter usually will not occur even with very high friction coefficients. Also, some inferior materials will wear quickly even though the friction is not really high and some superior materials will not wear quickly even though the friction is high. Keep in mind that friction is not abrasion Friction is the two mating materials rubbing each other, not biting into each other. We calculate most stock organic friction materials to have a coefficient of around .25, while our sintered copper ceramic material is around .32 or slightly higher. There are some sintered iron materials that are higher. We consider the sintered materials for racing only.

Notice that automakers don't have sintered materials in street cars even though it can take more abuse and handle more power with less clamp load. Even the makers of high dollar sports cars don't use these materials. The reason is because of engagement quality. Automakers know to keep the friction within a certain range in order to maintain smooth engagement. If you are willing to give up this smooth engagement, a racing clutch is an option, but usually not necessary.

Full surface or pucks?: Because most materials except sintered materials are bonded with a resin, they have a much lower melting point than sintered materials. This means that they are better suited having more surface area to distribute the heat to keep them from overheating. Since the material is fairly light weight the additional material is not a problem. With sintered materials, the binder is metal so the melting point is higher and so is the weight. Therefore it makes sense to have less material to do the job. Excessive disc weight can cause problems for syncros. once again, the surface area has little to do with torque capacity.

That's some of the basics. There is much more to it than that but it's a start. For really in depth clutch tech. I would suggest a book called Manual Transmission Clutch Systems (AE-17) published by the Society of Automotive Engineers, Inc., ISBN 1-56091-923-X. For more information on ACT clutches, see our website at .


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Discussion Starter · #2 ·
cont'd email from Dirk Starksen of ACT said:
To answer the specific questions:

1. What is the clamping force provided by your different stages as compared to a stock B18C1, B18C5,B18B, B17A, B16A pressure plate?

We have seen basically two stock pressure ratings on OE applications. I am not sure of all the B18 designations and which clutch is which but we have tested them all.

We see around 1000 lbs. on the weaker units (about three different Integra part numbers) and around 1200 lbs. on the GSR and type R.

The 2000 Civic SI Daikin unit we tested is around 1000 pounds. Most aftermarket dealers will interchange the pressure plates back and forth.

If you are looking for a good stock pressure plate, the 1200 lb. FCC part with the red dot on the diaphragm (diaphragm #P72) is preferred.

ACT offers three clamping force ratings:

Our heavy duty tests 1600 lbs new, the Xtreme tests 2100 lbs. and the MaXX Xtreme is around 2600 lbs.

We are re-engineering the Xtreme and MaXX diaphragms for improvements to be released later this year. Our pressure plates are now made from scratch. We no longer use modified stock units because of the design limitations.
This has given us the ability to use larger rivets, wider torque straps, strengthen the cover, and provide ductile iron castings on every pressure platet. We are concluding testing for SFI Certification as I am writing this.

2. What do you consider as excessive clamping pressure enough to effect the thrust bearing and synchros?

This varies from model to model,and sometimes from car to car. We haven't had complaints about Honda thrust bearings going out at all. Second Gen. Talons are another story, but then again Talon thrust bearings seen to go out for no reason at all, even with a stock clutch. Since the thrust load from the clutch only occurs when the clutch is pushed in you have to consider how much of the time the clutch is pushed in. If the car is driven in stop and go traffic every day then the additional load from a heavy clutch will add wear. If you add 50% more thrust pressure from a heavy clutch, then it stands to reason that the bearing may wear 50% more when the clutch is in than usual.

Clamp load has no affect on syncros. Syncros are adversely affected by inadequate clutch disengagement, heavier disc weights, and more commonly, from trying to shift faster than the tranny wants to.

3. What affects clutch disengagement or pedal effort and what can a consumer expect in terms of difficulty in disengagement at the different stages you have? Do you change the fulcrum of your springs, weight them,
rib them, or thicken them?

Pedal effort - affected by springs loads only. ACT clutches will feel stiffer, HD - a little stiffer, Xtreme - much stiffer, MaXX (considered racing only) - a lot stiffer

Disengagement - Affected by pressure plate geometry, spring load, cover flex, disc marcel, anything that affects travel.

Pedal adjustment may be necessary for adequate disengagement and proper pedal feel is using a ACT pressure plate and a stock street disc. Minor adjustment may be desired for ACT Performance disc, and no adjustment should be required for our race disc unless using our MaXX pressure plate.

4. How much chatter can people expect after proper break-in seating, flywheel resurfacing, and installation for the various stages you have? Is the marcel thickness similar across the various stages?


ACT Stock disc - no chatter, full marcel

ACT Performance Street - minor, if at all, due to reduced marcel for faster action. We are looking for a good compromise here.

ACT Race disc (copper ceramic) - Moderate chatter, takes getting used to, no marcel. We don't recommend them for street but many people use them and like the chirp they may get in fourth gear! For street use, I would recommend a stiffer pressure plate and a performance street disc over a lighter pressure plate and a race disc.

Break in period -

Street disc, about 4-500 miles give or take. Break in helps to lap the surfaces so that the material will form a glaze that is slower wearing. Race disc, several hard slips before racing will provide better contact for heat dissipation.

5. What materials do you use in your organic and metallic content discs?

Performance Street - Steel backed, high quality premium OE organic material. The high copper content helps the disc to recover from overheating by conducting the heat away from the surface. Many high dollar sports cars (Corvette, Viper, 911 Turbo) come with this same material from the factory. No magic, just plain good stuff!

Metallic - Copper Ceramic Sintered material. The high copper content helps to conduct the heat away from the surface. Rigid center is meant for quicker shifts because of reduced weight. Less chatter than spring centered metallics because there is less wind-up affect which is what chatter comes from (rubber mounts, disc springs, etc)

I hope that's not too much information to swallow. Let me know if you need help to condense it down. Our best customer is the informed customer.

Best Regards,
Dirk Starksen
President, Advanced Clutch Technology, Inc.
my thanks goes out to Mr. Starksen for his detailed and informative reply. Now let's get into some discussions among the members, shall we? Please run a search using the terms clutch in the Performace Tech Forum and you will find 2 of my threads on clutches as well and I suggest you check those out as well.....

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Discussion Starter · #5 ·
Clutch Chatter:


The linings of the older clutch discs used to be made of asbestos. They're now made of fiberglass or other materials that do not contain asbestos due to the effects of asbestos to our health.

However, asbestos provided superior cushioning and better wear than the newer materials at this time. The (modern) linings contain copper and/or bronze metal strands to aid heat dissipation and to help prolong disc life.

The linings have grooves cut in their faces for air ventilation and to help prevent lining dust build up. Dust build up causes chatter and slipping due to the slip/grab action of the dust. The 6 to 9 springs around the disc hub may range from loose to tight. Not to worry, the looseness/tightness does not affect the operation of the disc. They're there to asborb shock and help eliminate engine pulsations (eg. from lumpy long duration cams).

By far the most critical element of the disc in eliminating chatter is the "marcel". This is the term given to the crimped plate, or wafer, between the 2 clutch linings. The purpose of the marcel is to prevent clutch chatter by giving the clutch disc some "give" during clutch engagement. The marcel also helps prevent the lining from sticking to the flywheel and/or pressure plate (due to the spring effect of the marcel) when the clutch is being disengaged. The marcel thickness (the distance the linings are held apart by the marcel when the disc is not under compression) will vary depending on the type of use the disc is designed and built for.

Basically the thicker the marcel, the smoother the clutch will engage and the spongier it feels to your foot. The absence of marcel makes the clutch grab, but makes for a much more postive lockup (less slippage and ability to handle more horsepower). As the marcel thickness increases, it will require more clutch lever travel to engage/disengage the clutch. A pure drag/race car clutch marcel will be from 0.000" to 0.010", since engagement is quick and abrupt and chatter is not a problem. Truck clutches use marcels in this area also. A street/strip clutch will generally have marcels from 0.015" to 0.025". Pure street clutches will use marcels in excess of 0.025". A super soft clutch uses marcel in the neighborhood of 0.030" to 0.040". Mine is 0.025" and works well with no slippage and no chatter. When the marcel gets up to 0.030" and more, it may require as much as 60% of the clutch travel to engage.

Other Reasons for Chatter:

1-Flywheel not machined. Grinder it with a Flywheel Grinder Machine (not with a Lathe)

2-Oil on facings.

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When your picking between HD, Extreme, and MaXX XXtreme and there clamping force ratings of 1600, 2100, and 2600 lbs how can you pick the best one based on these numbers in correlation to how much power your going to make or your intended power goal?

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Discussion Starter · #10 ·
The torque capacity equation is your friend when you shop for clutches. :

T = N x R x F x P

* T = torque capacity in ft. lbs.
* N = number of friction surfaces
* F = coefficient of friction
* P = lbs. of pressure plate clamp force
* R = radius of gyration in feet

R = (OD + ID) / 4 = Average radius of the disc in mm.
Then convert to ft where 25.4 mm = 1 inch,
12 inches =1 ft.

eg. a typical 225mm outer diameter disc will have an
R=0.308 ft

Most people get a clutch after their stock one dies.

By then, you know how hard and often you launch at high rpms at the strip or race at the track or whether you are just street driving. You also know how much the engine makes. Selecting the clutch based on peak tq then is easy.

The difficulty is, as you have pointed out Tony , when you are starting from scratch on a motor build up and trying to predict the peak tq that you will end up with and buying the clutch ahead of time.

I suggest building the motor, breaking it in, and tuning it to your liking and then selecting the clutch for it rather than buying a pre-emptive clutch ahead of time that is overkill.
your purchase is based on a known quantity and not a guess. I hate guessing. It usually causes more problems that you have to fix afterwards (more $ & TIME).

If you are on a race team and have little time in between setting up the car and the race and have to buy a clutch in a rush ahead of time because you under a time pinch, then you choose a clutch for the power goal because being an optimist, what you are going for = your power goal.

The older I get (it must be an age thing) and the longer I do this, the less tolerance I have for imprecision and guessing and glossing over the details. I've found in this hobby that the more detail you pay attention to with the least amount of guessing saves you a lot of grief and headaches and brings more smiles. As you know, Murphy's Law pops up any time and bites you with the things that you should have paid attention to but missed. Because you rush, Murphy pulls the surprise party and gives you the frustration grief. I try to avoid that these days.

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Discussion Starter · #11 ·
In the friction of carbon materials, it has been found that the coefficient of friction is dependent on the total energy absorbed by a given mass and the rate at which the energy is dissipated. Usually, low energy absorption and low rate of energy dissipation produces relatively high coefficients of friction. Conversely, high energy absorption and high rate of energy dissipation leads to lower coefficients of friction. In addition, testing at low energy input in a moist environment produces a low coefficient of friction, which may increase significantly during the course of the test. Similar friction transitions occur during high energy tests. However, these do not appear related to moisture effects....

Both friction transitions and hot spots are produced because of mechanical or thermal disturbances. Our published studies show that any type of failure (mechanical or thermal fatigue) of the friction film or bulk material was sufficient to generate either frictional transition or thermal instabilities.

coefficient of friction database website

in a nutshell, the above quote says that the higher you go on the coefficient of friction (cf) , the less heat the disc will hold and give off or shed more heat the harder you bite.
I find that the number one cause of clutch failure is thermal fatigue moreso than mechanical fatigue.

This is why a higher cf for carbon kevlar over organic disc material is preferred.

carbon composite cf from the database ranges from 0.5-1.2. The material used in our discs must range from 0.5-0.8 cf. but I'm guessing here.

it'd be nice to know the exact cf for say an ACT street organic material clutch disc over an Action kevlar racing disc.

Secondly the other thing that I got from reading that quote was that it says in paraphrasing: if moisture in the form of condensation builds up on the disc surface it can affect the ability to shed heat off the disc once it bites.

Any time you people want to throw in your research into our database in this thread on what the clamping forces and coefficient of friction specs are for the various clutch brands out there so that we can start using this equation to help us pick the right clutch for our particular setup (it's engine tq), go right ahead....don't hesitate to help eveyone here....

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When you say thrust bearing, you're talking about the clutch thrust bearing (also called the clutch release bearing or the clutch throwout bearing, according to this) and not the crankshaft thrust bearings, right?

If so, why would you worry about a Honda bearing failing with higher loads with a new PP? Wouldn't you put in the new, aftermarket bearing that came with it?

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Discussion Starter · #13 ·


More clamping force on the plate requires a stronger bearing with also a release fork and retention spring to load the fingers on the plate enough and activate the pressure plate's diaphragm.
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