Dual disk clutch does not shift slower than a single.

MD-LUCKY

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If you thought the plane on the treadmill was bad.. Wait till we dive into this.

The following is entirely my opinion. Feel free to disagree, but I would love an educated explanation with data as a response. If you’ve got a claim otherwise, back it up.

Little background on this post: I’ve been reading for a while that a dual disk clutch will cause your transmission to shift slower than a regular single disk clutch. When I first heard this, I decided to take it at face value and go with it. However, when I decided to purchase my first dual disk clutch, it became a large concern.

Unable to find an explanation as to why a dual disk clutch would shift slower, I let the issue drop and installed a Haisley Street Drag.

A funny thing happened though: I noticed 0, and I mean zip, zero, nadda, difference in shift speed. Why is that?

So, I started to question the folks that claimed a dual disk would shift slower. I wanted specific answers, backed by data. At worse, I got the typical “it just does.” At best, I folks theorized that the increased inertia from added material (and weight) would cause slower shifting. The real question is… Does it?

I figured I could probably gather a little information from google about slow shifting dual disk clutches. Problem is, we diesel folks seem to be the only ones that complain about slower shifting. Most forums and magazines praise the dual disk as a great, yet expensive, upgrade to your vehicle. Lower pedal pressure with higher holding power. But, in all of my searches, I’ve yet to find a non-diesel example of slower shifting after installing a dual disk clutch.

Let me google that for you

There are also examples of high-performance OEM vehicles coming with a dual disk clutch. Most noticeably, the Honda NSX was offered with a dual disk from 1991-1996. These cars redline at over 8,000rpm. This is an important fact that I will get to later, but just keep it noted for now. The goal of the dual disk clutch was to “improve clutch engagement and smoothness.” No mention of slower shifting or reduced 0-60 / ¼ mile times because of it:

Wikimedia Error


I guess it boils down to this: “what changes during a dual disk clutch install, and why would folks believe their transmissions shifted slower after?”

Most folks probably get how a manual transmission works. I love this site for pictures and descriptions of what’s going on:

HowStuffWorks "Fly Wheels, Clutch Plates and Friction"

So in stock form, a single pressure plate pushes a single clutch disk against a single flywheel.
In the dual disk clutch, we are adding an additional clutch disk and a floater plate. Does the pressure plate FUNCTION change in any way? No. It functions exactly the same as a single disk clutch.

So, if we have 2 pressure plates, and they are both rated at 3000 pound plate load, what is the difference between them? Nothing!

Is there a difference if the pressure plate is pushing on 1 clutch disk, or 2 (and a floater plate)? No. We have just doubled the amount of clutch material. Does this increase weight? YES. And THAT is where the changes are occurring.

The flywheel and floater plate are spinning at exactly the same speed. On the Haisley, the float plate was bolted to the flywheel. On the south bend, the floater plate sits inside large grooves cut in the outside rim of the flywheel. In either case, the flywheel and floater plate cannot move independent of each other (if they are, something broke).

The clutch disks are splined to the transmission input shaft. Once again, they cannot spin independent of each other, unless the splines break.

So on a single disk, the pressure plate pushes against the clutch, which pushes against the flywheel. Pretty simple.

Dual disk, the pressure plate pushes against the clutch, which pushes against the floater plate, which pushes against the second clutch, which pushes against the flywheel. Floater plate and flywheel are moving at exactly the same speed (on the Haisley it is bolted in place, on the South Bend the grooves hold it) Clutches are moving at exactly the same speed (splined to the transmission input shaft). There is no change in function.

There are 3 changes we are dealing with:
1: Pressure
2: Material
3: Weight


Lets start with pressure:

Increased clamping force from the pressure plate generally translates to increased pedal pressure. The function is identical, the “feel” is not, whether it is a single disk or a dual disk. Increased pressure plate clamping force causes 2 issues:

1, more “work” for the hydraulics (ie, higher pressures exerted back to the master cylinder.. This is why we blow out the stock hydraulics. They are asked to hold more pressure than originally designed).
2, more “work” for our leg. Once again, on either a single disk or a dual disk clutch, increased clamping force will generally increase pedal pressure. The manufacturers say so right on their websites, usually describing it as “slight increase over stock.”
Do either of these 2 increases in work explain “slower shifting?” No!

Let just step away from clutches for a second and compare it to anything else in life.

Lets say we have a machine that requires we push a lever with our legs. The travel distance is exactly the same for every person (we will say, 8 inches), but you can very the weight required to push the lever. I can push 2 pounds in 1 second. Bob can push 2.5 pounds in 1 second. Jim can push 2.2 pounds in 1 second. Frank can push 3 pounds in 1 second.

Which of us is pushing the machine the fastest? None of us (or all of us). It is exactly the same. You see, the machine does not care that we increased pressure. It functions exactly the same. 1 second is 1 second is 1 second.. No matter if it is 1 pound or 100.

The clutch is no different, (unless you believe you can push the clutch so hard that the hydraulic fluid changes properties depending on how much pressure is exerted against it from the pressure plate…) The clutch travel distance does not change (engagement points may, but we will cover that later also.) It does not matter, whether you are using a single disk, a dual disk, a quadruple disk: the travel distance of the clutch pedal does not change. The question is, can you push the clutch pedal to the floor? Yes. If you can push it to the floor, can you push it at the same speed every time? What if we increase the workload required to push the pedal to the floor. What does it change? Only the amount of pressure required to depress the clutch pedal. Distance does not change.

So, if you can press the clutch pedal to the floor in exactly 1 second with 2 pounds of pressure pushing against your foot, are you moving any slower if you can push 2.5 pounds of pressure to the floor in exactly 1 second? No. Does it take an increase in energy? Yes. The increase is in your muscles. You are asking them to do more work at the same speed. Is that humanly possible? At the weights we are dealing with when it comes to pedal pressure, you damn well better believe it is. So knowing that the hydraulic fluid is a constantly, that the distance the fluid travels is a constant, that the distance the pedal travels is a constant, and the speed at which the pedal travels is a constant.. Is there any difference between a single disk and a dual disk? No. There cannot be.

2: Material.

Can material change how fast a clutch shifts? Yes! Does it make a big enough difference that we are going to notice? Doubtful. Now, I’m not speaking for any of the clutch manufacturers here. But, I’m going to theorize. Here is my theory: the 12cb dual disk clutch from South Bend uses different material than the 6cb clutch. I’m also betting the old Haisley Street Drag 6cb dual disk used a different material than their new one. Can this change how “fast” the clutch shifts.. Minimal, minimal amounts. How? A grabbier material clutch is going to shift “quicker” than an organic (or not as grabby) material clutch. It has nothing to do with pressure plate speeds (as described above) or weight (as described later) and everything to do with slip. Or, more correctly, LACK of slip. A clutch with extremely grabby material will slip less than one with more forgiving material. My frustrations with the old street drag was that it allowed almost no slip from a stop. So, knowing that the pressure plate is moving at exactly the same speed, and the clutches, and the flywheel, and the floater plate, regardless of whether it is a single disk clutch or a dual disk, all move at the same damn speed.. How can material change how fast a clutch shifts? It will grab SOONER. When you pop the pressure plate, and it slams against the clutches, they will grab sooner on the floater plate, pressure plate, and flywheel. If we have 2 different materials, A and B. A slips for 2” on the flywheel. B slips for 3”. A is a faster shifting material. Does that end the debate? Unfortunately, not really…

Well damn it, why not? I just said that A shifts faster than B.. That should be the end of the story… and maybe it is. Maybe there are folks out there that are so concerned with 10ths of hundredths of seconds that they will just end the debate there and move on. Professional drag racers (or bench racers). But the truth is, we have friction in another area that can demonstrate if the clutch is catching soon enough.. Tires. If you are breaking traction between shifts (or chirping), the clutch is catching soon enough and with enough force that friction is now lost between the tires and ground… So lets be perfectly honest with ourselves.. How much difference is there in shift times?

Once again, we are NOT talking about how long it took to change gears.. We are only talking about the difference in time between when you released the clutch (pressure plate) to when it fully seated and stopped slipping. So, is an aggressive dual disk that catches within 3” slower shifting than a soft material single disk that catches in 5”? Nope. So, does the fact that the clutch is a dual disk make it take longer to shift? No. The factor is MATERIAL, not the amount of disks in the clutch, and it has nothing to do with how quick you change gears.

Can we make a blanket statement of “dual disk clutches shift slower than single disk clutches?” Absolutely not. We can only say that depending on the material used in the clutch, shift speeds may very, REGARDLESS of whether is it a single disk of dual disk. How much? Fractions, fractions of a second. Going even further than that, we generally WANT a little slip between gears for various reasons.. An entire thread, all together.


That leaves us with 1, single, final variable that could make a dual disk shift slower than a single.

Weight.

Where is the weight increased? On the engine side, the flywheel weight is increased (by bolting a floater plate to it, or allowing it to ride in grooves on the flywheel). On the transmission side, the clutch weight is increased (by adding an entire additional clutch).

Could this finally be what causes the transmission to shift slower? At face value, the answer seems to be “yes.” Why? Pretty simple: you are putting additional load on the transmission synchros by adding weight. (You could, in theory, also make it more difficult for the engine to reduce engine speed due to the increased weight in the flywheel, which could make it more difficult to match engine speed to gear ratio in your shifting. I’m not going to bother with the math involved in that theory, but I’m willing to bet it is minimal at best.. Much like I’ll show the additional clutch is minimal, at best.)

Lets start with how additional weight could cause the transmission to shift slower (at least in theory).

Since your foot is moving at the same speed, the hydraulics are moving at the same speed, and clutch material doesn’t become a factor until you are in gear and releasing the pressure plate against the clutch… The only other factor of how a dual disk clutch could shift slower is if you are unable to move your hand into the next gear at the same speed. That is the true “shifting speed.”

I wanted to do this with the nv5600, but I can’t find a gas engine application. So I’m going to use the nv4500 instead.

Assuming you can move your hand at the same speed before you installed the manual transmission, the only factor in gear selection speed should be if the synchros can handle the additional weight and allow you to change gears at the same speed as before. They are functioning in the exact same way, but are required to do more work with the added weight.

So, if the synchros are capable of handling the increased work load, they will function in the same way, correct? Much like if you push 2.5 pounds at exactly the same speed as 2 pounds, you are not moving any quicker or slower.

How do we know if they are capable of handling the additional weight? Measure the centrifugal force, compare how much additional force the second clutch adds, and see if there is a real life example of the synchros managing more force than what the additional clutch generates.

Check out this handy calculator to show exactly what I’m talking about:

CalcTool: Centrifugal force calculator

We are using made-up numbers since I do not have the exact numbers available.. It makes little difference in the demonstration though. Watch:

13” radius (approx size of clutch), Speed 3200 rpm (engine/transmission speed), weight 5 pounds (approx the weight of a clutch disk) = 18905.3 lbf.

These are not exact numbers, but they don’t need to be for the demonstration here. You’re clutch is exerting18905.4 lbf at 3200rpm.. Or, when many are shifting. That means that the synchros need to deal with 18905.4 lbf in stock form. Can they handle that? Yes.

Lets double the weight, just like a dual disk clutch: 13” radius, 3200 rpm, 10 pounds weight = 37810.6. Pretty darn significant change, right? Well.. Lets think about this for a moment.

The NV4500 has many gas engine applications. Same transmission, but 1 huge difference.. RPM baby!

Lets plug all of our numbers in for a GAS engine this time.

13” radius, 5200 rpm, 5 pounds weight… …. …. …. … = 50886.5 lbf.

Whoa doggy! Big difference.

Think the engineers thought this one over when they designed the transmission? You bet your sweet ass they did. The synchros aren’t even breaking a sweat at our RPM levels, even with a big, heavy, dual disk clutch. The biggest issue you may encounter by going with a dual disk clutch is increased bearing and synchro wear from the increase in weight. However, the synchros are more than capable of handling those loads… They were engineered to do so from the factory!


So what is the only plausible explanation for a dual disk clutch “shifting slower” than a single?







It’s called the Placebo Effect, and only us diesel heads seem to experience it.

I’ve got tough skin, so all you nay sayers, let’s hear why I’m wrong. :Cheer: :Cheer:
 
Ok, it was all in my head. My dual disk shifts just as fast as the stocker.
 
I can't find something to argue with your case, but I know I cannot shift my DD as fast as I could the stock single.
 
Wow, that's the longest post I've ever read...

Here's my reason why it shifts slower: I think the transmission just gets alittle scared with all that spinning weight up front...$.02
 
Good post, I've always wondered the same thing. We just put a dual disk in my buds 09 Viper, granted its only 2 8" clutch disks lol but still shifts as good as stock.
 
I know that my syncros don't catch up as fast, but that could be a pilot bearing problem or something dragging. Other than that, I can make it shift just as fast.
 
clutch

i dont disagree with your post but i could tell a differnce with my south bend!!!! it was slower
 
McLeod Double Disk between a LT1 and a T56? Shifted the same as the stock clutch.

The first run of the SBC 3800 pulling clutches behind a nv4500 in 2002? Much slower in 3-4-5 than the stock clutch. Placebo shouldn't be an effect since I hadn't heard of dual disks being slower before it was installed. :shake:
 
I think my SBC seems to shift just as fast... If not, really close. .02
 
Ok, this is a case where the desk jockey wasnt in the field first.

You are assumine the gasser has the same gear ratios as the diesel. There are two different versions of the NV4500. One is for a gasser.

The gasser:
has a smaller flywheel than the diesel. (12" disc)
engine returns to idle far more quickly than a diesel.
gasser NV4500 has smaller internals giving it less intertia
Those allow it to match the revs to the synco speed much better.

Another thing you need to take into account is that the floater plate is still gonna put some drag on the input shaft as the only thing to make it separate from the 1st disc nothing really. It just skips off of the disc, there are no levers to force it away.

I know the flywheel and floater plate in my valair weigh over 130#. The entire stock clutch barely weighed that.

So now the engine has that much more inertia to slow down to match the syncos rpm.

The guts of and NV4500 are heavy and they take time to slow down to match the engine too.

The input shaft is getting a bit of parasitic drag from the floater.

It seems to me the lower plate load street and drag disc's are quicker, therefore allowing smaller springs in the pressure plate. That gives the pressure plate more room to move, which gives the floater more to move, then couple that with disc's that weigh less because they have less friction material. Hense the street DD's having a lower power rating.

If your theory is correct, then explain why no matter how hard you yanked on the stick it would not shift any faster than when floating the gears. And yes the clutch was fully disengaged.

If you truley wanted the fastest shif possible, take the trans apart, file off every other tooth on the syncro and dog rings. May grind a bit but, you made it shift like a stocker or better.

Also those small clutches race cars use are made with far lighter materials.

And I base this on no scientific evedence. :hehe:
 
Ok, this is a case where the desk jockey wasnt in the field first.

You are assumine the gasser has the same gear ratios as the diesel. There are two different versions of the NV4500. One is for a gasser.

The gasser:
has a smaller flywheel than the diesel. (12" disc)
engine returns to idle far more quickly than a diesel.
gasser NV4500 has smaller internals giving it less intertia
Those allow it to match the revs to the synco speed much better.

You may want to re-check those facts.

NV4500 Transmissions - Adapters - NV4500 Conversions

I couldn't find anything about a smaller flywheel and clutch.. Would make sense I suppose. I just can't find anything on it.

Another thing you need to take into account is that the floater plate is still gonna put some drag on the input shaft as the only thing to make it separate from the 1st disc nothing really. It just skips off of the disc, there are no levers to force it away.

Nothing makes the first (or a single disk) clutch seperate from the flywheel either, right? Same situation.

I know the flywheel and floater plate in my valair weigh over 130#. The entire stock clutch barely weighed that.

So now the engine has that much more inertia to slow down to match the syncos rpm.

Yeah.. Right thought track there, but its not the syncho rpm they are matching. The synchros bring the transmission up to engine speed, not the engine down to transmission speed.. There is a difference.

The guts of and NV4500 are heavy and they take time to slow down to match the engine too.

The input shaft is getting a bit of parasitic drag from the floater.

It seems to me the lower plate load street and drag disc's are quicker, therefore allowing smaller springs in the pressure plate. That gives the pressure plate more room to move, which gives the floater more to move, then couple that with disc's that weigh less because they have less friction material. Hense the street DD's having a lower power rating.

If the pressure plate is moving further, wouldn't it be shifting slower? ;) It only needs to move far enough to disengage the clutch. But, your foot is moving the same speed and distance, it shouldn't matter. Unless you are only pushing the clutch half way down during the shifts.. Interesting idea though.


If your theory is correct, then explain why no matter how hard you yanked on the stick it would not shift any faster than when floating the gears. And yes the clutch was fully disengaged.

If you truley wanted the fastest shif possible, take the trans apart, file off every other tooth on the syncro and dog rings. May grind a bit but, you made it shift like a stocker or better.

You lost me here. Removing synchros would kill your shift times. You would have to double clutch and try to match revs.. Way, way slower than a good synchroed transmission.

Also those small clutches race cars use are made with far lighter materials.

And I base this on no scientific evedence. :hehe:

Damn it if you didn't give more time and effort than anyone else so far. I think you are on the right track, but the evidence just isn't there. The only difference is a slight increase in weight..

To REALLY boggle the mind, I would love to weigh a factory stock single disc clutch, then 2 aftermarket dual disk clutches.. I don't think the weight doubles, but I decided to guess that route anyway. I would love to know what a stocker weighs vs what the 2 clutches weigh..

I'm glad at least someone put the effort into it though. :)
 
I always was told that a dual disc slows down slower once disengaged, because of weight and overall slippage, thats the reason it's hard to get into reverse sometimes!

Mine can be stubborn to get into gear, but it's merely the stock hydraulics I figure! It will shift as fast as a single as long as the truck is still in the power band, but it seems if I over rev or shift too late it wants to be stubborn!

once again no evidence to back this up, just what it feels like! but overall I am very impressed and can't wait to get the hydraulics
 
You may want to re-check those facts.

NV4500 Transmissions - Adapters - NV4500 Conversions

I cant find the source now. There is a differance internaly between the HD and regular NV4500. Bigger shafts or something.

I couldn't find anything about a smaller flywheel and clutch.. Would make sense I suppose. I just can't find anything on it.

Parts store lists the gasser having a 12", not 12.25.

Nothing makes the first (or a single disk) clutch seperate from the flywheel either, right? Same situation.

Correct, however, the floater isnt gliding on rollers it takes some energy to move it. I only assume the heat generated is what moves them apart.

Yeah.. Right thought track there, but its not the syncho rpm they are matching. The synchros bring the transmission up to engine speed, not the engine down to transmission speed.. There is a difference.

I didnt mean to imply they did. Having the engine fall down to the rpm of the next taller gear makes the syncro's job so much easier.

If the pressure plate is moving further, wouldn't it be shifting slower? ;) It only needs to move far enough to disengage the clutch. But, your foot is moving the same speed and distance, it shouldn't matter. Unless you are only pushing the clutch half way down during the shifts.. Interesting idea though.


Since the plate is moving more it gives the floater more room for it and the first disc to get away from each other. A lesser plate load will also let you stomp the clutch a hair faster. It dont seem like much but it is.


You lost me here. Removing synchros would kill your shift times. You would have to double clutch and try to match revs.. Way, way slower than a good synchroed transmission.

Thats actualy an OLD drag racers trick. It works best on transmissions that have gears with an almost flat angle. (M22 rock crushers, 883 Hemi boxes, ect.

Damn it if you didn't give more time and effort than anyone else so far. I think you are on the right track, but the evidence just isn't there. The only difference is a slight increase in weight..

To REALLY boggle the mind, I would love to weigh a factory stock single disc clutch, then 2 aftermarket dual disk clutches.. I don't think the weight doubles, but I decided to guess that route anyway. I would love to know what a stocker weighs vs what the 2 clutches weigh..

I'm glad at least someone put the effort into it though. :)

Why dont we ask one of the clutch companies? They could tell you the differance in them.

Dan or Peter would both know.
 
dude i'm with roachie on this one about the drag on the disc between the flywheel and the floater. it seams to me and maybe im wrong but on a single disc when you release the cluch the fingers pull the pressure plate back, and the disc would have to slide a little on the trans shaft, by it self, to take all the drag off the one side of the disc and the flywheel. now on a dd when the pressure plate releases you still have the drag of the one side of the first disc on the floater and then both sides of the second disc between the floater and the flywheel. so it seams that you are increasing the drag by 3 times. meaning one side of a disc draging vs. three sides of the discs draging. dont know if that made any sense. does to me. :blahblah1:
 
I cant find the source now. There is a differance internaly between the HD and regular NV4500. Bigger shafts or something.

The input and output shaft are different. No internal difference (that I know of).

Parts store lists the gasser having a 12", not 12.25.
I will plug that into the equation.. think it will make a huge difference? ;)

Correct, however, the floater isnt gliding on rollers it takes some energy to move it. I only assume the heat generated is what moves them apart.

The floater cannot move independent of the flywheel. It is either bolted in place (haisely) or held by 3 large grooves (SB), but it cannot move at a different speed. The only "drag" it is adding is the additional weight on the flywheel. The clutches cannot move independent of eachother. If 1 clutch is slipping past the floater, the other clutch is slipping past at the same speed. There is no other option.

I didnt mean to imply they did. Having the engine fall down to the rpm of the next taller gear makes the syncro's job so much easier.

Bingo. But if the transmission can shift at 5250 RPM in a gas engine with more centrifugal force, why can't it shift at 3250 with less force? Or, if the synchros are capable of handling greater forces at 5250 than a dual disk clutch can exhert at 3250, why would it shift slower?

Since the plate is moving more it gives the floater more room for it and the first disc to get away from each other. A lesser plate load will also let you stomp the clutch a hair faster. It dont seem like much but it is.

The movement of the clutch is a mute point at best. I went through this in pretty long detail above. If you move 2 pounds 8 inches in 1 second, and I move 1 pound 8 inches in 1 second, you are not moving any quicker than I am.. I am not moving any quicker than you are. The difference, if there is any at all, is so tiny it would make no difference what-so-ever in shift speed.

If you can't push an upgraded clutch in as fast as the factory clutch, buy a stair master!

now on a dd when the pressure plate releases you still have the drag of the one side of the first disc on the floater and then both sides of the second disc between the floater and the flywheel. so it seams that you are increasing the drag by 3 times

If you still have drag, your clutch hasn't released. If we where talking about old cable clutches, we would say your clutch was out of adjustment. Modern hydraulics, this just isn't the case. The clutches cannot move at different speeds from eachother, but they can move at different speeds from the floater plate. If you remove pressure and rev the engine, the flywheel and floater plate will speed up at exactly the same speed relative to eachother. The clutches will remain at exactly the same speed relative to eachother. There is no drag between them.
 
i drove a buddys truck and it shifted as fast as a stock clutch and was very smooth truck is a 2001 6speed with a SBC DD 4200 with upgraded hydros
my truck in the other hand is very slow and hard to get in so i need new hydors
 
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