modifying clutch linkage

[chevynut]

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 Centerforce®, 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]

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 (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]

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]

Example: A 10.5" clutch and a #RAGM GM release arm (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]

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.