Hydrolic Experts needed

Dan, here's what I understand you have: slave cylinder, 0.88" diam. master cylinder, 0.75" diam. This means that your slave cylinder will move 0.726" (roughly 3/4") for each inch of movement of the master cylinder. If you go to a 1" bore master cyl, your slave cyl will move 1.29" (about 1 1/4"") for each inch of master cyl movement. Is this what you're asking? If the slave cylinder diameter is smaller than the master cylinder, the movement is increased and the force is reduced. The opposite happens if the slave is larger than the master.

The ratio of distances moved is fixed by the diameters of the two cylinders. You can't change the relative amount of movement unless you change the cylinder diameters.

 

from your drawing, it appears that you could screw the helm joint out. I don't know if that would increase your travel or not.

 

You could also change the attachment points. For more movement at the throwout bearing, you'd move the MC attachment point down on the pedal arm (away from the pivot), or move the fork attachment point inward (toward the pivot).

 

Adjusting the pushrod doesn't change the action. It only changes the overall length of the cylinder.

You don't push directly on each cylinder. Instead, the force is applied and delivered by levers (the clutch pedal arm and the clutch fork). Besides changing the diameters of the hydraulic cylinders, you can change the geometry of these levers.

To increase the movement at the clutch pedal, you either have to move the master cylinder more (by moving the attachment point down on the clutch pedal arm) or you can change the length or pivot point of the clutch fork. These geometric changes can happen in addition to the change in relative distances due to changes in cylinder diameters.

 

Well, yeah, if you can arrange to move the master cylinder more, then the slave will move more too. The most movement will happen when the cylinder axis is at 90 degrees to the clutch pedal arm.

 

Hmm... yeah... 10 degrees isn't that much, but you'd get both an increase in distance by pushing straight on to the cylinder, as well as an increase by moving the load away from the lever's fulcrum.

 

Dan --

Sorry I'm only coming in so late in your discussion, but I have some input. I converted to a hydraulic clutch when I did my Buick 350 engine swap, and I had a similar issue finding a slave cylinder with enough throw for the clutch. I ended up finding this one:

http://www.colemanracing.com/section/index.htp?id=314

It's a slave cylinder with 1-3/8" throw. I bought the kit that came with the mounting bracket (for Chevy 350) and adapted it to my bellhousing. I ended up modifying the pushrod so that it fit my clutch fork better than the heim it comes with. If you run out of options with your current setup, you might try looking into this.

Hope it helps,
Jeff

 

Just remember that going to a larger master will give you more slave travel it will also increase pedal effort as force=pressure x area.. you do the math..



BUBBA

 

Bubba is right Dan, though I think you are focusing too much on the cylinders and not on the levers, where the actual work is done. The hydraulics are just a way to transfer the work from the pedal arm to the clutch fork, although you can design the hydraulics to give you more or less mechanical advantage.

Look at this simple drawing;


There are two things you can't change 1) the distance the clutch fork must move in order to disengage the clutch, and 2) the distance the pedal can move. All of your mechanical advantage comes from these two distances. Let's say the fork must move 1" and the pedal moves 4", then your mechanical advantage is 4:1. You apply 20lb of force at the pedal to get 80lb of force at the fork.

If you design the system properly, when you move the clutch pedal "enough", the clutch fork will move "enough." Let's say that you want the clutch to disengage at half the pedal throw, then you must combine the efffect of the distance of the load from the fulcrum and the ratio between the areas of the two cylinders, as Bubba pointed out.

 

Dan, the angle doesn't make that much difference, as long as the difference from 90 degrees is small. We can calculate the meaning of small:
sin(85 degrees) = 0.99, or 99% of the movement we'd have at 90 degrees.
sin(80 degrees) = .98
sin(70 degrees) = .94
sin(60 degrees) = .86
sin(45 degrees) = .70

So less than about 20 degrees difference doesn't significantly change the MC stroke length.

The distance of the load from the fulcrum has a much stronger effect though. Let's say the pedal moves 4" and the load is 5" from the fulcrum, and the pedal arm is 20" long. Then the movement of the piston will be 1" (5"/20" * 4" = 1"). The further the load is from the fulcrum, the longer the MC stroke is going to be.

Get out your tape measure and measure the length of the pedal arm, from the fulcrum to the pedal. Measure the distance of the MC from the fulcrum, and divide that by the length of the pedal arm. You'll get some number smaller than 1. Measure the distance traveled by the pedal from its stop to the floor, and multiply that by the small number. That's the maximum distance for the MC stroke. For the 0.75 MC and 0.88 SC, multiply the MC stroke by .75 to get the SC stroke. If that's not the stroke you are getting, than either the measurements are wrong, you have differnt parts than you thought, or something is broken (like air in the cylinders).