Should you upgrade your clutch?

J.G.
By J.G. Pasterjak
Jan 26, 2024 | 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 …

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Comments
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 GRM+ Memberand 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 GRM+ Memberand 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) GRM+ Memberand 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.

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