modifying clutch linkage

[Bihili]

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?




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[chevynut]

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]

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]

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]

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]

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]

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]

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]

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]

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.