When Is It Time To Upgrade Your Clutch?

J.G.
By J.G. Pasterjak
Apr 1, 2021 | clutch, transmission, Shop Work | Posted in Shop Work , Drivetrain , Features | From the Nov. 2020 issue | Never miss an article

The traditional manual transmission may be declining in popularity, but for the great majority of enthusiasts attending track days, road races and autocrosses, the old-school stick-and-clutch setup is still the weapon of choice. We may see a day when the desire for true manual transmissions in new cars is fueled more by nostalgia than performance numbers, but they aren’t going away any time soon.

And because of that, lots of effort and technology are still being poured into clutch development by high-performance aftermarket companies and OEMs. So before we dive into this special section covering clutches, let’s review a few basics and discuss why a clutch upgrade may be something to consider.

How Does Clutch Magic Happen?

For your car to move, there needs to be a mechanical connection between the engine and the drive wheels. Sounds simple, right? But for your car to not move–which is something cars do occasionally–that connection needs to be momentarily severed. In manual transmission cars, that decoupling is handled by the clutch assembly.

In a typical single-disc clutch (we’ll deal with multi-plate clutches in another part of this section) there are two main components: The pressure plate assembly is fixed to the flywheel of the engine, and the friction disc is attached to the input shaft of the transmission. 

The friction disc rides on a splined input shaft and is allowed to “float” freely along that shaft. Its travel is constrained by the flywheel on one side and the pressure plate on the other side. When the engine is turning, the flywheel and the pressure plate assembly spin together. When the clutch is fully engaged, the pressure plate presses the friction disc against the flywheel, transferring torque from both the flywheel and the attached pressure plate assembly to the clutch friction disc. This disc is attached to the transmission input shaft, so torque can travel into the transmission and down the line.

Whether the throw-out bearing is actuated by a cable or hydraulic line, it does the same basic job: It actuates the diaphragm spring, which lifts the pressure plate from the floating friction disc and disengages the clutch. This facilitates both starting the car from a standstill and shifting gears. 

When the clutch pedal is depressed to disengage the clutch, the pressure plate lifts off the friction disc, and because the friction disc is allowed to float along the input shaft, neither the flywheel nor the pressure plate assembly can adequately transfer any torque. The pressure plate is lifted from the disc via a diaphragm spring attached to the pressure plate. 

A diaphragm spring is a disc-shaped spring that lifts its outer circumference when the inner part is depressed. That oversimplifies the importance of the diaphragm spring–lots of clutch operation and feel is dependent on the characteristics of this spring–but that’s the basic mode of operation, regardless of what kind of clutch you’re talking about. But there are lots of variables contained within that basic system. First is friction. 

When a clutch is fully engaged, most of the torque is transferred via the clamping force of the mechanical components pressing against each other. But friction between the clutch disc and the flywheel and pressure plate is still a key factor–especially during the engagement and disengagement phases of operation. 

Too much friction and the clutch can be “grabby,” making smooth starts difficult. Too little friction and the clutch may not be able to transfer all of the engine’s torque to the transmission–even at full clamp load.

Since we mentioned friction, you know that friction’s old pal heat is right around the corner. Heat alters the friction characteristics of any system. Anyone who’s ever hit the brakes for Turn 10 at Road Atlanta and gotten, “Doo, doo, dooooooo. We’re sorry, your braking could not be completed as requested. Please hang up and try your braking again later if you don’t die,” knows that heat affects friction. We’ll talk about those ramifications in a minute.

The next variable is driveline lash. When starting from a dead stop, the clutch must be engaged progressively and allowed to slip to get the car moving until the speed of the input shaft matches the speed of the engine. The engine can only slow down so much without shutting off, so that speed imbalance must be taken up through either clutch slip or, if you’re trying to impress your posse, tire slip. 

The drawbacks of constantly spinning the tires to get the car moving are pretty clear to anyone older than 17 or so, so that clutch needs to slip and progressively transfer that torque until speeds match and it can be fully engaged. 

Here’s the actual hardware depicted in the above diagram. The friction disc, located between the flywheel and pressure plate, forms the link between the engine’s torque and thrust at the wheels.

However, friction slip is not really a linear process. Typically, force builds between two surfaces to a point where the force overcomes friction in a fairly sudden fashion. Even the most sophisticated materials will experience this slip-grab-slip-grab cycle, which manifests as what we commonly know as clutch chatter. 

This is where those springs on the clutch disc come in. Most clutch discs are actually multi-part affairs, with a friction disc and a separate hub connected by springs. These springs compress before fully transferring torque through the disc, providing a bit of damping for any surface friction irregularities during the engagement phase.  Additionally, friction materials or rubber or other damping materials can be placed between the hub and the disc to further alter the effects of variable friction on the engagement phases.

So, Why an Upgrade?

Now let’s look at why you may want to upgrade that system and where to focus those upgrades.

Heat Management: You may have noticed the word “friction” popping up in the previous descriptions a lot. Whenever a clutch is disengaged and then reengaged, friction is produced as the clutch components take up those mismatched speeds. Obviously, those friction events will be more intense and more frequent during a track lap that requires 10 high-rpm shifts than during a gentle cruise down the highway. 

So, a high-performance clutch is designed with heat management and resistance as prime characteristics. Usually this manifests as different friction materials on the disc–metallic and ceramic compounds instead of organic materials common in OEM clutches–but more thermal-resistant materials can even be used on the pressure plate assembly as well.

The tradeoff is typically a more aggressive friction characteristic during engagement. Metallic and ceramic compounds have a higher coefficient of friction than organic materials.

More Friction: But that previously mentioned tradeoff with metallic and ceramic compounds can be an upgrade of its own. More friction means more and quicker torque transfer at lighter clamping forces relative to organic friction materials. More friction can also mean less drivability, but modern clutch designers have done amazing work to make high-performance materials and designs extremely livable.

More Clamping Force: Clamping force is important to the total amount of torque a clutch can transfer, so as engine output rises, clamping force may need to rise as well. The clamping force of a clutch is dictated by the strength of the aforementioned diaphragm spring, and although some trickery can be done with arm lengths and leverages, more clamping force typically means that more force is going to be required to actuate that spring. 

That translates to higher required pedal pressures. Again, there are ways to mitigate these effects: through tuning of finger lengths and fulcrum points on the diaphragm spring; through the hydraulic actuation system of the clutch; or even through the rod or cable systems that actuate the clutch on some cars. 

Bottom line: An increase in clamping force does not always mean a fully corresponding increase in pedal effort. In fact, a dramatic increase in clamping force is usually available with only a small increase in pedal effort.

Several individual components make up the clutch disc: The hub and friction plate are joined by springs damped by pliable material that masks small imperfections in engagement.

Less Weight and Polar Moment: You may have noticed that the heaviest part of the clutch–the pressure plate assembly–is attached to the engine. That means whenever you want the engine to accelerate, it has to spin up the mass of that clutch assembly as well. 

Reducing the mass of the clutch assembly reduces the mass that the engine has to accelerate, along with the overall mass of the car. Reducing the overall diameter of the clutch reduces the leverage that its mass has, reducing the force the engine needs to use to accelerate it. 

High-performance clutches typically reduce weight through the use of lighter materials, like aluminum or even titanium. Properly using these materials–which can be costly themselves–may require more expensive manufacturing techniques, pushing up the price as weight decreases.

Puck Clutches: What about puck-style clutches? All of the functional concepts that we’ve discussed about traditional full-disc clutches also apply to puck-style clutches, which replace the full circle of material with multiple pucks of friction material attached to a star-shaped disc. 

Advantages include, obviously, less rotating mass, but the reduced friction area means clamping loads must increase, so friction coefficients must be higher. Because of this, puck-style clutches are typically only for competition applications where shifts happen quickly and drivability is not a concern.

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Comments
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Vigo (Forum Supporter)
Vigo (Forum Supporter) MegaDork
11/24/20 10:45 a.m.

I don't have an actual ton of experience with different clutches but i've installed a ton of stockers as well as some 4 and 6 puck ceramic clutch discs and some stiffer pressure plates. 

My favorite 'high torque' clutch that I've personally used on one of my own cars is actually a cobbled 'dual diaphragm' pressure plate on a completely stock clutch disc. Dual diaphragm as in take TWO of the diaphragm springs from stock clutches and put them in one pressure plate, making it twice as stiff as stock. Even with a clutch cable (vs hydraulics), i like this setup. The stiffness of the plate might make smooth driving difficult if it ALSO had a nasty friction material on the clutch disc, but that's the thing.. it doesn't need it because it clamps the hell out of a stock clutch disc and has 'normal' engagement while also holding a ton of torque.

I've also driven pressure plates for the same car that had higher clamp load with less pedal stiffness through altering the lever point of a stiffer diaphragm spring etc, and that's nice too although i'd rather have a stiffer pedal than a grabbier clutch disc if it came down to it. I.e. at this point i'd rather have a clutch pedal that's 100% stiffer but easy to modulate, than a pedal that's 30% stiffer but deal with the annoyance of a grabby puck style clutch disc. 

Of course, there are a bunch of other considerations. Stock rpm? Stock clutch disc weight is fine. Much higher rpm? Reduced mass of the puck style disc shows advantages. Also, if you make the pressure plate stiffer, you're also increasing how hard your left foot is pushing against your crankshaft thrust bearing while you're cranking and have zero oil pressure. You really ought to disable the clutch switch that prevents you from cranking without pushing the clutch pedal if you are going to make the pressure plate stiffer. 

If i could try things for zero dollars right now, I'd actually like to try a full-face organic (stock style) clutch disc, with an unsprung hub, in a reduced diameter from stock, with one of my ridiculously stiff pressure plates, and then try messing with the Marcel spring setup. The idea would be to see if it could have stockish engagement/modulation from the material, stock or better lifespan (mostly through removing the clutch hub springs which are the tiny bombs waiting to ruin your clutch disc), adequate torque capacity with the increased clamp load, and whether a change to the (free height? stiffness? Im not sure?) of the Marcel spring would mostly make up for the removal of the sprung hub in terms of drivability.

That last point is mostly because I'm weak on the roles and possible overlap of the sprung hub vs marcel spring. I suspect the Marcel spring has more to do with a progressive engagement while the sprung hub is mostly to deal with NVH issues of 'lugging' and bad harmonics that might be a durability issue when you have a low-cylinder engine (i.e. 4cyl) operating at low rpm and there is enough time between crank power pulses to let the entire transmission bang back and forth against its gear lash in rythm with the engine. I believe a similar set of issues is why manual transmissions had problems behind 6cyl Cummins diesel engines and were eventually discontinued. 

frenchyd
frenchyd PowerDork
11/24/20 11:32 a.m.

You failed to cover reduced diameter triple disk clutches. Such as 7&1/2 inch AP or Tilton.  That's for when things get really knarly and rotational weight affects aspects of performance.  
No they won't work on Trans  Am sedans. Too heavy. They are more for light sports racers or Formula cars. 
 They tend to be more like on/off switches than something  used with production based syncro's. 

Vigo (Forum Supporter)
Vigo (Forum Supporter) MegaDork
11/24/20 12:53 p.m.

Article says they will cover that stuff in a future installment. yes

fusion66
fusion66 Reader
4/1/21 9:21 a.m.

Advantages include, obviously, less rotating mass, but the reduced friction area means clamping loads must increase, so friction coefficients must be higher. Because of this, puck-style clutches are typically only for competition applications where shifts happen quickly and drivability is not a concern.

This is not correct. Friction coefficient is generally regarded as being independent of friction area. Puck type materials also typically have higher friction coefficients than organic materials (full round).

If a higher clamp force is present with all other parameters being equal (mean radius included), then the purpose of the higher clamp force is to increase the overall clutch torque capacity, not to make up for the lack of friction material surface area.

 

jharry3
jharry3 Dork
4/1/21 1:47 p.m.

When I was a teenager with a '66 Mustang, mid '70's, my dad somehow decided I needed a Puck clutch.  I had no idea what it was so I just installed it.  (ah, the days when kids said "yes sir" to their dads and followed orders)

 I lived with that miserable thing for almost a year driving in stop and go traffic.  My friends were always asking me was wrong with my car since engaging from a stoplight was a clutch chattering experience.

I finally just bought a regular hi-performance street clutch and changed it.

z31maniac
z31maniac MegaDork
4/1/21 2:53 p.m.

When it starts slipping or you've dramatically increased HP over stock. 

That doesn't make for a very good article though.

noddaz
noddaz UltraDork
4/1/21 3:56 p.m.
z31maniac said:

When it starts slipping or you've dramatically increased HP over stock. 

That doesn't make for a very good article though.

Nailed it!

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
4/1/21 4:07 p.m.
jharry3 said:

When I was a teenager with a '66 Mustang, mid '70's, my dad somehow decided I needed a Puck clutch.  I had no idea what it was so I just installed it.  (ah, the days when kids said "yes sir" to their dads and followed orders)

 I lived with that miserable thing for almost a year driving in stop and go traffic.  My friends were always asking me was wrong with my car since engaging from a stoplight was a clutch chattering experience.

I finally just bought a regular hi-performance street clutch and changed it.

I've only driven a puck clutch at one (long) track day and it was a miserable experience. Fine on track, undriveable in the paddock. Never again.

weedburner
weedburner Reader
4/2/21 11:03 a.m.

Nothing wrong with puck style disc drivability when paired with the proper clamp load. Let's say one has a typical 10.4" organic/organic disc paired with a 2800lb pressure plate. At that clamp pressure, the organic disc would have about 509ftlbs of torque capacity. Simply swap that organic disc for a ceramic puck, now you have about 768ftlbs of torque capacity. Just by changing the disc alone, torque capacity increases around 50%. For an organic to ceramic puck disc swap to have close to the same drivability, you would have to also reduce the pressure plate's clamp load from 2800lbs for the organic, to around 1720lbs for the ceramic.

Friction materials behave pretty much the same brakes vs clutch. Lets imagine if your brake pedal worked like the clutch, push the brake pedal down to release the brakes, then releasing the pedal allows a spring to apply the brakes. If you were to adjust that application spring's maximum pressure to where it could just barely lock up the tires, overall brake force would relatively easy to modulate with your foot. Adjust the application spring's maximum pressure to 150% of what's required to lock up the tires, all that extra capacity does for you is narrow the sweet spot for modulation. Same with a clutch, capacity beyond what is needed only serves to narrow the modulation sweet spot.

Grant

 

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
4/2/21 12:15 p.m.
Keith Tanner said:
jharry3 said:

When I was a teenager with a '66 Mustang, mid '70's, my dad somehow decided I needed a Puck clutch.  I had no idea what it was so I just installed it.  (ah, the days when kids said "yes sir" to their dads and followed orders)

 I lived with that miserable thing for almost a year driving in stop and go traffic.  My friends were always asking me was wrong with my car since engaging from a stoplight was a clutch chattering experience.

I finally just bought a regular hi-performance street clutch and changed it.

I've only driven a puck clutch at one (long) track day and it was a miserable experience. Fine on track, undriveable in the paddock. Never again.

There is a lot of "it depends".  My favorite setup is a puck clutch with a stock pressure plate.  Light feel and very short engagement/disengagement.  I don't have an easy time getting full clutch pedal stroke and it isn't getting any easier, so the short travel needed is a godsend.

My car came with an ACT 6 puck with a really REALLY stiff pressure plate (on a stock 12A!) that had a very long travel to boot.  That was heinous.  I ditched the plate for a stocker and fell in love with it.

fusion66
fusion66 Reader
4/2/21 12:16 p.m.

Adding to the organic versus ceramic (puck) discussion. Typically an organic driven disc has a "cushion element" between the two sides of the friction material resulting in improved modulation. I have not seen a performance automotive puck type driven disc that use a similar feature. 

The "ceramic" materials typically found in a puck type clutch often have different behavior when compared to organic friction materials. As the ceramic friction material approaches a lower differential speed (near lock-up), the coefficient of friction increases resulting in increased judder. Organic materials typically have a flatter coefficient of friction curve as it approaches zero differential speed resulting in a smoother engagement and less judder. This is of course painting with a broad brush but representative of the 10-12 mixes of each material type that I have experience with. 

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
4/2/21 12:18 p.m.

In reply to fusion66 :

Ceramic?  Copper, man.  All the grinding noises when it is slipping smiley

fusion66
fusion66 Reader
4/2/21 12:24 p.m.

Ceramic?  Copper, man.  All the grinding noises when it is slipping

Yep, I don't know why the term "ceramic" is used when it is mostly copper powder that is mixed with other goodies, pressed together under extreme force, and then sintered in a furnace to essentially melt it all together.

 

 

 

Keith Tanner
Keith Tanner GRM+ Memberand MegaDork
4/2/21 12:58 p.m.
Pete. (l33t FS) said:
Keith Tanner said:
jharry3 said:

When I was a teenager with a '66 Mustang, mid '70's, my dad somehow decided I needed a Puck clutch.  I had no idea what it was so I just installed it.  (ah, the days when kids said "yes sir" to their dads and followed orders)

 I lived with that miserable thing for almost a year driving in stop and go traffic.  My friends were always asking me was wrong with my car since engaging from a stoplight was a clutch chattering experience.

I finally just bought a regular hi-performance street clutch and changed it.

I've only driven a puck clutch at one (long) track day and it was a miserable experience. Fine on track, undriveable in the paddock. Never again.

There is a lot of "it depends".  My favorite setup is a puck clutch with a stock pressure plate.  Light feel and very short engagement/disengagement.  I don't have an easy time getting full clutch pedal stroke and it isn't getting any easier, so the short travel needed is a godsend.

My car came with an ACT 6 puck with a really REALLY stiff pressure plate (on a stock 12A!) that had a very long travel to boot.  That was heinous.  I ditched the plate for a stocker and fell in love with it.

This clutch came from our usual manufacturer (I think, this was 2003 or so) so I trust it was set up at least reasonably properly. It sucked for driveability, and since you can make an organic full disc clutch that will hold up to 400+ in a Miata, there's just no point in trying. My LS3 car runs a stock LS7 clutch and has never twitched. Basically, I have yet to come across an application in my admittedly limited experience where it was needed.

I've had a car with a dual plate kevlar setup. Super-sexy. Installed in a 1200 lb car with a 150 hp 1.6, so it may have been slight overkill :D 

If I understand Grant's math, you get 50% more torque capacity with the ceramic puck. But you have to decrease the pressure plate force by 40% to get the driveability back. So you're at about the same torque handling capability but you have a lighter pedal. I can see that appeal. We've been getting higher clamping forces with lighter pedal forces by using more stroke instead.

weedburner
weedburner Reader
4/2/21 4:08 p.m.

More torque capacity in a clutch isn't necessarily better, you only want enough to get the job done. Having more than you need only makes the car harder to drive at it's limits.

If your goal is to maintain traction thru the shift, you want to reduce overall clutch clamp pressure until you have just enough to hold the engine going into high gear at WOT. With a diaphragm PP you typically don't lose clamp pressure over the life of the disc, so more clamp pressure than you need is just going to make the car harder to drive without shocking the tires loose during clutch release. If you have a Long or Borg & Beck with coil springs that lose clamp as the disc wears, you will need extra clamp pressure at the beginning of the disc's life so that you will still have enough clamp at the end of it's life.

Grant

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
4/2/21 4:42 p.m.
fusion66 said:

Ceramic?  Copper, man.  All the grinding noises when it is slipping

Yep, I don't know why the term "ceramic" is used when it is mostly copper powder that is mixed with other goodies, pressed together under extreme force, and then sintered in a furnace to essentially melt it all together.

 

 

 

Someone should write an article about it or something wink

 

I have installed "ceramic" faced clutch disks that were full faced, that were puck style, and that were a weird friction material shape like an inverse puck type.  None of them looked or felt like a copper (sintered, I'm sure) unit like I've been using from ACT or Competition Clutch.

Pete. (l33t FS)
Pete. (l33t FS) MegaDork
4/2/21 4:45 p.m.

In reply to Keith Tanner :

The "Happy Meal" clutch does feel really, really nice.  I also personally think Mazda did something for the better when re-engineering the smoothcase's clutch fork layout for the B6/BP engines.  Even a lighter duty ACT clutch could reliably crack clutch forks or break pivot balls, and RX-7 racers just shrug and say, yeah that happens, you need to reinforce them.  I haven't heard of that happening in Miataland.

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