When Is It Time To Upgrade Your Clutch?

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 …

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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. 

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. 

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

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.

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.

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

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

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!

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.

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

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.

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. 

In reply to fusion66 :

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

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.

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.

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

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 

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.

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.

In reply to jharry3 :

The puck clutch is getting no love here.

That is all I have had in my RX-7 autox car for several years. My preference is the unsprung variety.

Though it is rarely driven on the street, it is no more difficult than the OEM variety, it just takes a little getting used to.

Rallykar had a Kevlar clutch in it once. I didn't know that the break-in period is several hundred miles of light touch driving. Protip: Kevlar clutches require several hundred miles of light driving to break in, cuz if you don't do that, it'll slip. A lot. 

Well it depends on driver ability and car weight and use. There are a whole lot of drivers out there that can trash a clutch in record time by abuse and then claim that the clutch was no good.

I will go out on a limb and say that unless you are a drag racer (or act like one on the street) and do burn outs for fun, you should be able to get away with a pressure plate with a tad more holding force.  There will always be cars with under-designed clutches for the weight, engine and use they will be put to, but in general I've found the pressure plate to be sufficient.

I did go to a Centerforce dual friction clutch on a Fiero I built with about 300 bhp,  but on my race cars (MG powered) simply used a competition clutch cover after I oiled a disc and couldn't get another one before the next race  and I only had a stock factory disc to replace it with.  It never wore out in several years of racing, but I'd qualify that by stating that we did not do standing starts and I am very easy on shifts so very little spin on shifts.

I used to sell car parts to pay for university and would see customers that drove like idiots and fried a new clutch in a week or two on the street.  I guess the expensive fancy clutches are to suit that sort of use.  In other words, much cheaper if you don't have to pay the 'idiot tax'.

There are a few cars that have been designed with clutches that are inadequate for anything approaching hard use, but really not that many.

Don't get me started on other bad driving habits - people complaining that the release bearing they bought with the new clutch only lasted a few miles and such (I went out for a drive with a couple of those people and watched them sit at stop lights with the transmission in gear and the clutch held down - bear in mind that many of the British cars we served used carbon faced release bearings and that sort of abuse was guaranteed to trash them in short order.