Clutch Pedal Pressure: what do I want?

[Seymour Snerd]

In setting up my SPF GT40 the two ends of the clutch chain (pedal ratio, M/C diameter, and clutch pressure plate) are set, and I know the pressure plate disengagement force. I have control over both slave cylinder diameter and clutch lever length. What I do not know is what force at the pedal I should shoot for. If I had a running manual-transmission car I'd measure it and extrapolate from there, but my daily drivers are all automatic.

Anyone know what range of clutch pedal force would be considered in this community to be appropriate for my beast? IOW, what range of force is considered "moderate" and what force would people like us (fans of hi-po automobiles) call "heavy", given the GT40 context.

 

[Randy V]

Hi Al,

Totally non-technical answer here....

Consider climbing a flight of stairs with 6" risers.. The amount of force exerted on your left foot would be considered to be a fairly heavy pedal..

I think you would want something to be less than half of your weight.

 

[Seymour Snerd]

I think you would want something to be less than half of your weight.

Good thought, thank you; and empirically supported in that in the meantime I found a post here where Wanni asserted that 18-22 kg is the target (40-48 lb). Elsewhere on the web someone measured his '93 Corvette at 45-50 lb because someone told him it was too heavy.

I certainly wouldn't want to have to stand very long with my weight on the balls of my feet, which is the scenario you describe, so if I (arbitrarily) take half that figure as a target it's a little under 50.

McLeod says a good rule of thumb for overall clutch pedal mechanical advantage is 12:1, and they say my McLeod pressure plate has a clutch fork force of 500 lb, so that's pointing to 42 lb.

 

[Ian Clark]

Alan,

Sounds like you answered your own question Good choice with McLeod Clutches, never had one fail.

Cheers

 

[EGLITOM]

Alan

This is a calculation i have done for my clutch seutp ( excel file attached).

I ended up with about 50 lbs with the 3/4 inch main and the stock porsche slave. Let´s put it this way, this is not a ladies clutch it needs some real man.
I don´t know if you have spent any time in a gym and especially there in the "leg press". Imagine 100lbs of weight in there, How many repititions can you do ?

I decided that i will leave it that way and see how it is to drive it.

TOM

 

[Seymour Snerd]

Alan

This is a calculation i have done for my clutch seutp ( excel file attached).

I ended up with about 50 lbs with the 3/4 inch main and the stock porsche slave. Let´s put it this way, this is not a ladies clutch it needs some real man.
I don´t know if you have spent any time in a gym and especially there in the "leg press". Imagine 100lbs of weight in there, How many repititions can you do ?

I decided that i will leave it that way and see how it is to drive it.

TOM

Good, thanks. That's what I'm going to do.

 

[Bob Putnam]

My approach would be entirely different...

You need to have the clutch release completely at 50%-70% movement at the pedal face, no matter what the pressure required. If the clutch doesn't release properly, all this talk about pedal pressure is moot.

You can calculate the pressure after figuring the necessary (release) stroke, and if it's very low, you can change the movement ratios to raise the engagement position for convenience.

 

[Seymour Snerd]

My approach would be entirely different...

You need to have the clutch release completely at 50%-70% movement at the pedal face, no matter what the pressure required. If the clutch doesn't release properly, all this talk about pedal pressure is moot.

I know, but I have fairly good reason to assume I don't have that problem in the first place. For one thing, the clutch, the pedals, the cylinder diameters and overall system stiffness are not particularly different from any other street car.

So, within a reasonable range of mechanical advantage I still need to determine where to operate, and that has nothing to do with the point of engagement. My interest is more "where is the boundary between moderate and heavy from a human factors point of view?" and not at all "how light a clutch can I have in my 60's replica race car?"

 

[Dimi Terleckyj]

Hi All

I wouldn't go any bigger than 5/8ths for the clutch master which will give a reasonably light pedal but still with feel.

Dimi

 

[Seymour Snerd]

what range of force is considered "moderate" and what force would people like us (fans of hi-po automobiles) call "heavy", given the GT40 context.

Just to close the loop, after all that I did get this reply directly from McLeod:

"40# at the pedal is considered moderate. 80# would be considered heavy."

And on a related note in designing the clutch release system:

"We like to see between .100" to .150" freeplay between the face of the bearing and the fingers on the pressure plate. From there you need .500" of travel once the bearing touches the fingers for a clean release."

 

[Ian Clark]

Hi Alan,

It's a question of pedal ratio and cylinder diameters, the SPF pedal box is modelled after the original but what is the pedal ratio on your clutch pedal? Then there's the stroke, most masters are designed for 1" stroke with a little extra before hitting the internal stop.

Also the pedal stroke must be centered so that the mid point of start of the clutch action and end is also perpendicular to the pedal pivot. Otherwise you'll have an over center humpy feeling clutch. This is adjusted by push rod length. Do not attempt to change leg room with the pedal push rods, proper geometry must be retained.

Then there's the master cylinder diameter, with McLeod asking for .100>.125 clearance and .500 for lift, you're using at least .625 travel in the clutch slave cylinder. You need to know how much fluid is displaced at that amount of lift to be sure a small master cylinder (which gives lightest pedal) doesn't run out of travel before you've completely lifted the clutch pack.

Sooooo, I can only advise on what to watch out for, don't have one here to say definatively what you need. I know what works on CAVs with either the stock or upgraded pedal boxes in ZF applications. The theory is the same no matter what brand. Hope this helps.

Cheers

Ian