Clutch linkage math - how to

ChargerST

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Posted this on forabodiesonly but this might be of help here as well (pictures are from a B-body anyways)

I'm in the process to convert my car from auto to 4-speed and one of the necessary tasks is to set up the clutch linkage. I think the following lines might help others who are in the same position.

1.) There are several different ways how you can actuate a clutch:

a) mechanical linkage
b) cable
c) hydraulic slave cylinder
d) hydraulic throwout bearing

a) mechanical linkage is found in most late 60s/70s Mopars. A rod is attached to the clutch pedal and Z-bar. Another rod from the Z-bar to the clutch fork transfers the pedal movement to the throwout bearing

b) clutch cables get rid of the Z-bar and directly transfer the pedal movement to the clutch fork

c) The pedal movement is used to move the piston in the clutch master cylinder which then uses hydraulic pressure to move a slave cylinder. The slave cylinder is attached to the clutch fork.

d) Also uses a master cylinder but no clutch fork. The hydraulic pressure from the MC directly moves the hydraulic throwout bearing.

2.) Why all the linkage? A clutch has a very heavy spring pressure to press the clutch plate to the flywheel. The linkage allows you to trade off (pedal) movement for lifting force. E.g. your clutch pedal moves 8 inches but the plate only departs 0.050" from the flywheel - a total ratio of 160. But at the same time the force required to overcome the spring pressure of the clutch is reduced by the factor of 160! The total ratio is divided between several devices (pedal, Z-Bar, fork, clutch levers).

3.) Different clutches require a different linkage.

Basically there are 3 different types of clutches:
- Diaphragm (most modern cars)
- Long style (mostly Ford)
- Borg&Beck (Mopar back in the day)

Each clutch type has a different lever ratio which has an effect on the rest of the linkage from the clutch pedal to the throwout bearing!
Afaik the typical ratios are (unfortunately I can't find the page anymore where I got the info from):

Diaphragm 10:1
Borg&Beck 7:1
Long: 4.5:1

What does that mean? It means that for every inch the throwout bearing moves out the clutch plate departs the corresponding fraction from the pressure plate (I know that the TOB doesn't move an inch ;) ). For example: the throwout bearing moves 0.500" which results in a clutch departure for the diaphragm clutch of 0.050" but the Long style clutch moves much more (0.111").
So if you decide to convert from one clutch style to the other chances are very high that either the force to press the pedal or the clutch disc departure distance will change if you don't adapt the rest of the linkage.

Typically on a Borg&Beck style clutch in our Mopars the pedal has a ratio of 4:1 to 5:1. The Z-bar also has around 2:1 and the clutch fork another 2:1 ratio. If we combine this we get (4 x 2 x 2) 16 ratio for the linkage and 7 for the clutch for a total ratio of 112. Remember, the diaphragm got 160 - the diaphragm would have lighter pedal but a shorter departure distance (0.050" vs. 0.071").

Good thing is that the linkage can be dialed in to achieve the same departure distance and pedal pressure between two types of clutches! You can move the attachment point on the pedal or the Z-bar, use a longer or shorter clutch fork, use a different combination of master and slave cylinder/hydraulic TOB. It's very simple math!

4.) Using a hydraulic master cylinder is a tiny bit more complicated.

In my case I want to use a hydraulic TOB and mount the master cylinder inside the cabin for a stealth look. Things to consider are the relationship between master and slave cylinder diameter and the total travel of the master and slave or hydraulic TOB. A bad pedal ratio cannot be made up by the hydraulic part of the linkage (e.g. use a 5/8" master cylinder with 1 1/4" slave).
Furthermore the direction of the master cylinder pushrod has to be correct - it's best to avoid angles as far as possible (angles are wasted movement and also damage the master cylinder).

20210330-120149.jpg


See the following image for clarification. The blue circle is the (theoretical) swing of the clutch rod attachment point (green dots). A indicates the position of the pedal and pedal stop at rest and point B indicates the depressed pedal. Comparing the distance from the pivot point to a green dot and the distance pivot to pedal tip gives you the pedal ratio. If you draw a line through the two green points (yellow line) you get the ideal direction for the pushrod of the master cylinder. It is no coincidence that this is the same for a mechanical linkage which uses a Z-bar - only this direction doesn't waste any pedal movement!
But as I mentioned earlier a different clutch might require a different linkage. In my case I'm going to use a Borg&Beck/Long style clutch (McLeod) with an internal ratio of 4.8. Since I want to use a hydraulic TOB there is no Z-bar and fork and the master cylinder has a maximum travel of 1.12" - So I'm very limited how I can change the total ratio to make up for the reduces internal linkage of the Long style clutch. The most viable option is to make a new attachment point at the pedal which is what I'm going to do. But a new attachment point is not only going to change the pedal ratio but also the ideal direction for the master cylinder pushrod!
Once I know where which pedal ratio I need I'm going to make another drawing and see how and where to mount the MC.

Here is a picture from Brewers for inspiration but again don't fabricate something similar blindly - the direction of the master cylinder pushrod has to be determined first!

Screenshot-2021-03-18-111245.jpg


Long winded, I know, but maybe it can help someone!
 

rikkitik

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Excellent information, thanks for sharing. I have been curious about this very subject.
My car's previous owner had the clutch changed to a Centerforce diaphragm, for lower pedal effort. But it has an annoyingly narrow engagement band.
The linkage is setup correctly, but honestly, the twin disc in my other car is lots better (though I don't consider it good either). But that's the general nature of multi plates anyway.
I'm not sure if the Centerforce disc just doesn't have much of a Marcel spring, or the linkage ratio is incorrect. But I will soon find out.
 

biomedtechguy

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the twin disc in my other car is lots better (though I don't consider it good either). But that's the general nature of multi plates anyway.
I just had a McLeod RXT dual disc clutch and pressure plate installed last year.
I'm sending you a PM.
 

ChargerST

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One important thing I didn't mention: tolerances in the linkage are not considered in my calculation. A Z-bar has more points where a slop can occur (pedal, Z-bar twice, fork) than a hydraulic TOB (pedal).
 

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