Why is it that hydraulic clutches are easier to use than cable clutches? By my reasoning

Pedal force * pedal travel = throwout bearing force * throwout bearing travel

should apply to either type of clutch (work in = work out). So assuming pedal travel and the pressure plates are the same, the pedal effort should be the same. Am I missing something? Or are cable clutches limited to a shorter pedal travel for some mechanical reason? Something doesn't add up.

 

It's not as simple as your equation states. You develop a lot higher forces with hydraulics than simple mechanical advantage. Pedal travel is still the same between the two.

 

The main difference is that modern, axial-slave hydraulic clutches are designed to move the pressure plate a smaller distance than older, mechanical actuator clutches. Greater precision makes a better clutch but proper installation is more critical.

 

Well I guess the full equation for a hydraulic clutch would be

pedal force = throwout bearing force/slave cylinder piston area * master cylinder piston area * master cylinder moment arm / pedal moment arm

and

pedal travel = throwout bearing travel * slave cylinder piston area/master cylinder piston area * pedal moment arm/ master cylinder moment arm

multiply the two equations together and you get

pedal force * distance = throwout force * distance

How is it not as simple as that? The fact that you're using hydraulic advantage (is that a term?) instead of leverage doesn't change the "work in = work out" relationship.

 

Interesting...does the precision you're talking about have to do with pressing all the clutch fingers evenly? Or if not, what part the system has more precision?

 

AFAIK, the big difference is linkage friction and hysterisis.

 

If it was friction, wouldn't the clutch only feel stiff when you're pushing it in, not when you're holding it in or letting it out?

 

Didn't understand this either...since both press the same or similar throwout bearing against the clutch.

 

If you check the force to push in a mechanical linkage clutch vs. let it out, the latter is usually ~ 30% less. For a direct hydraulic (no shafts, arms, ball pivots, etc.) the difference is ~5%.

 

That's more friction than I would have thought. I'd still say that even the 30% less force releasing the mechanical clutch is still more force than it takes to push the hydraulic in. I guess it's a combination of friction and more pressure plate travel.

 

gary'sright about them being more precise. nowadays if you surface a flywheel more than .025" you have to shim the flywheel. with mechanical linkage you didnt have to do that because they traveled farther. and since hydrolic (i.e. modern) throwout bearing only require 10 thousandths to engage a clutch, the leverage is much greater.if you dont need as much pedal travel for example you can set you pivot point higher= less force required to engage. also the size of master cylinder piston versus slave piston, has an effect on leverage also, so there are a lot of factors involved that all change depending on the car.