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Bihili
04-28-2015, 01:03 PM
I am debating to modify my clutch linkage.
My clutch adjustment rod is extended to the end and I have more free play than I want so I need to lengthen the adjustment rod or modify my pivot arm.


While I am doing this I was thinking about modifying the linkage.
With my horsepower and size of tire the single clutch is near its limits. The pressure plate springs are holding the clutch as it should but my leg does get a workout from the heavy springs..

To change the geometry to reduce the amount of leg pressure would you recommend lengthening the arm under the dash or lengthen the lower arm of the pivot shaft?

http://pic100.picturetrail.com:80/VOL470/4002687/8306581/147590885.jpg
http://i686.photobucket.com/albums/vv226/bihili57/Clutch%20linkage.gif (http://s686.photobucket.com/user/bihili57/media/Clutch%20linkage.gif.html)

http://www.55-57chevys.com/tech/57/6-9.gif (http://www.55-57chevys.com/tech/57/6-9.gif)

chevynut
04-28-2015, 01:41 PM
I'd go hydraulic ;)

How much HP and torque are you running?


To change the geometry to reduce the amount of leg pressure would you recommend lengthening the arm under the dash or lengthen the lower arm of the pivot shaft?

If you want to make the clutch easier to depress, you'd actually want to SHORTEN the arm on the pedal or SHORTEN the lower arm on the bellcrank (pivot shaft). You could also LENGTHEN the upper arm on the bellcrank. The problem is you'll lose travel.

chevynut
04-28-2015, 01:43 PM
Do you know what the overall clutch pedal ratio is on the stock setup? In other words, when you move the clutch pedal 1" how far does the pushrod on the fork move?

Rick_L
04-28-2015, 02:21 PM
The problem you're going to have is that to reduce the force required at the pedal, you will need to increase the travel at the pedal. That means you will have to raise the pedal, moving it up and toward the dash. Do you really think that's what you want?

What sort of pressure plate are you running? There are diaphragm pressure plates that will hold most anything you'd have under the hood. With a diaphragm clutch, the stock linkage and geometry should work fine. If you already have a diaphragm clutch, look for what might be wrong with the linkage to add friction. The other thing you can do is add rod end bearings to all the links. There's also ways of providing a better bearing for the z-bar pivots (or repairing them).

Bihili
04-29-2015, 07:38 AM
When I purchased the six speed years ago it had a hydraulic setup and I sold it.
HP is about 535 according to a desktop dyno program and the the clutch and pressure plate were redone at a clutch shop.
Pressure plate is a Hays, Borg and Beck style, 10.5 in, 3000 Static Pressure,
I am not going to make any major changes as everything is OK.
Just thinking about this situation and trying to see if I can improve it.

So if the arm under the dash is shorten and the arm under the pivot shaft is lengthen at the same time have I accomplished anything or is it a wash?

chevynut
04-29-2015, 08:17 AM
Personally I don't think you're anywhere the limits of a single disc clutch, but I'm no expert. I just know that before dual-disc clutches came out lots of drag racers used single disc clutches and I'm betting they had more horsepower. I'm curious why you didn't go with an 11" clutch instead ofthe 10.5"?

You can't get something for nothing (unless Obama's involved ;)). To disengage the clutch you have to push X force over Y distance at the throwout bearing. Force times distance is work. The fork and all the other linkage just change your leverage, but you still have to do the same amount of work.

So let's say you need to push 500 pounds at the throwout bearing and it needs to move 1". That's 500 in-lb. Let's say you only want to push 50 pounds at the pedal....you need to move the pedal 10". All the linkage is used for is to get that leverage, and to change directions.

So if you want to lower the force, you have to increase the distance and vice versa.

Any idea what force you're pushing now? Is your inside pedal and lever stock? It doesn't look like you're using a stock z-bar. I think there are different length forks available too, but if you're modifying the linkage you don't need to mess with that.

chevynut
04-29-2015, 08:29 AM
So if the arm under the dash is shorten and the arm under the pivot shaft is lengthen at the same time have I accomplished anything or is it a wash?

It depends on how much you shorten and lengthen each arm. Without knowing how much pedal force you have now, and how much you want it's hard to give you much input. If we knew the lengths of the levers, we could probably come up with some recommendations. The problem with the mechanical linkage is that it has to go through that hole in the firewall. So I wouldn't mess with anything but the lever that the clutch fork pushrod goes to. There's no need to mess with other levers unless there's an alignment issue. Shortening that lever (the bottom of the z-bar) will decrease the pedal force, and the distance the fork is pushed.

Bihili
04-29-2015, 09:19 AM
I'm curious why you didn't go with an 11" clutch instead of the 10.5"?
a long time ago my crank and flywheel were balanced together and I did not want to change flywheel.


It has stock pedal and linkage with shorten Z-bar width.

I will get some measurements of the different arm lengths.

Also it is obvious the hydraulic setup would have been the solution but this is what I have.

Thinking about the big picture it seems to me the geometry GM used fifty years ago was for a pressure plate with much less static pressure.
If I can find the amount of force it takes to depress the pressure plate maybe I can calculate a way to improve the situation.

Rick_L
04-29-2015, 09:32 AM
Again, to get less force at the pedal by modifying the linkage, it's going to require more travel at the pedal. I don't think you will find that acceptable.

On the other hand, if you chunk that B&B pressure plate and get a diaphragm or say a dual disc with metallic facings, you won't need as much pedal pressure either.

I don't know if a hydraulic setup will change anything either. Not knowing specifics, it could easily have exactly the same problems.

chevynut
04-29-2015, 09:48 AM
Again, to get less force at the pedal by modifying the linkage, it's going to require more travel at the pedal.

Maybe, but maybe not. It's possible that the throwout bearing is traveling more than it needs to to disengage the clutch. I know you have to have some (small) clearance between the TO bearing and the clutch fingers, but I don't know how far the TO bearing needs to push the clutch to disengage it. I went through these calculations when I designed my hydraulic setup. Somewhere I found the TO bearing travel needed to disengage the clutch and worked back from there to the pedal. You can only do so much but I wanted to try to optimize the setup. I have an 11" diaphragm style clutch.

chevynut
04-29-2015, 09:53 AM
I found it:

"Release Distance Required?

Our research shows that a typical GM clutch pressure plate, whether three-finger style or diaphram, 10-1/2" or 11" generally requires 0.550" of travel to release the clutch disc. "

http://www.novak-adapt.com/knowledge/clutches_etc.htm

"The "softest" clutch is the diaphragm type. It also requires the least amount of travel to release. The diaphragm type clutch works good in lightweight, low geared vehicles. It is not the best clutch for high RPM use as the diaphragm spring will stay "flat" or released from the centrifugal force generated by the RPM. A variation of the diaphragm type was used for a while by GM, that to some extent helped this problem. This was called the Hi-Cone diaphragm type and was designed so the spring - instead of being flat when released - still had a slight bevel. These Hi-Cone units were not bad but still won't hold like the Borg and Beck coil spring type. Aftermarket units like the CenterforceŽ, use centrifugal weights to counteract this high-rpm flattening and subsequent loosening."

So you may be right, Rick, that his options are limited. I still think it's worth looking at the travel and trying to optimize that.

chevynut
04-29-2015, 10:05 AM
Here's some more good stuff:

"For Reference
The pressure plate must move about .100 to .120 of an inch to RELEASE THE DISC and provide .030 to .050 air gap between the disc and the flywheel.




A 9" clutch has about a 4.5-to-1 arm or diaphragm ratio.
A 10.5" clutch has about a 6-to-1 arm or diaphragm ratio.
An 11" clutch has about a 6.6-to-1 arm or diaphragm ratio.



The release bearing must move away from the fingers or diaphragm sping at least 1/16"
(.0625 rounded off to .06) for freeplay.
The release fork ratio is determined as described in the release fork section.


Example: A 10.5" clutch and a #RAGM GM release arm (http://www.novak-adapt.com/catalog/clutch_arms.htm) (2.17 ratio).
So, 0.120" required movement multiplied by the ratio of a 10.5" clutch equals .72" plus .06" movement of release bearing for freeplay equals .78 of an inch. Multiply .78" by the ratio of the release fork (2.17") equals 1.69" (or 1-11/16") of travel required where the linkage attaches. It doesn't matter if the linkage is mechanical, cable, or hydraulic, it must be able to move the end of this arm with this pressure plate the above indicated amount in order to properly release the clutch disc."

Rick_L
04-29-2015, 02:08 PM
My statements were based on the assumption that bihili's clutch linkage is of stock dimensions as far as lengths and angles.

But this may not be true. He did say he "had a lot of freeplay". I wonder if the linkage was modified at some time in the past. Maybe it's just the length of the lever on the z-bar where the rod to the clutch fork is connected. A little too long and it's a problem. Or maybe the lever arm where the pedal rod attaches is too short.

Bihili
06-22-2015, 08:20 AM
Example: A 10.5" clutch and a #RAGM GM release arm (http://www.novak-adapt.com/catalog/clutch_arms.htm) (2.17 ratio).
So, 0.120" required movement multiplied by the ratio of a 10.5" clutch equals .72" plus .06" movement of release bearing for freeplay equals .78 of an inch. Multiply .78" by the ratio of the release fork (2.17") equals 1.69" (or 1-11/16") of travel required where the linkage attaches. It doesn't matter if the linkage is mechanical, cable, or hydraulic, it must be able to move the end of this arm with this pressure plate the above indicated amount in order to properly release the clutch disc."

Follow up:
I measured the amount of movement of the clutch fork arm where the linkage attaches. According to Novak it should be 1-11/16 for a 10-1/2 inch clutch.
After removing all of my free play from worn components the factory linkage still does not come close to 1-11/16 of movement.
I had drilled a hole in the lower arm of the Z-bar about one inch higher to give me more leverage.
But this would reduce the amount of movement at the clutch fork arm even further so I did not raise it.
So the linkage is now tighter but I still have a stiff clutch pedal to live with.

chevynut
06-22-2015, 10:00 AM
Bill, I had a discussion with Novak about this since I still have to set up my clutch slave. Here's what they say on their website:

"Our research shows that a typical GM clutch pressure plate, whether three-finger style or diaphram, 10-1/2" or 11" generally requires 0.550" of travel to release the clutch disc."

Eric at Novak told me you want .55" plus .120" for disc clearance. This is at the fingers. Then he said you want 10-15% margin on that. So the total the pressure plate fingers need to move is .737-.770". That may be a little conservative as I found out.

Now you need to multiply that by the fork ratio which they say is 2.17 to 1 so you get 1.67" needed at the fork. I mentioned that my Wilwood MC only has 1.4" of travel and I was worried about that not being enough. He replied with this:

"When we set up our 3/4" slave with a 3/4" master with the GM hydraulic release arm (2:1) that comes with our kit, it usually nails it every time, with an average of 25% margin travel.
I suspect the difference here is that our tests used virgin clutch kits that had not been burned in, and there are quite a few feathers on new friction discs. I wonder if we should retest with a broken-in clutch and see if we come in closer to .475" to .500" of travel for release."

If I were you I'd go ahead and try the rod in the upper hole and see how it works. 1" higher seems like a lot to me, but maybe you could raise it a little and still have enough travel. If that lever is 4" long and you raise the hole .4" you should decrease the pedal effort by 10%. You still might have enough travel.