physics of cable vs hydraulic clutch activation

You are correct on every point. Leverage is leverage the pedal moves X distance and the clutch moves Y distance and you have the same force answer cable or hydraulic. There is a little less friction in a hydraulic system, but it's not going to make a lot of difference... 5% reduction in pedal force at most.

Exactly how the system is design can change the feel and new stuff tends to be designed better than older stuff so that can change things too....often new stuff has a progressive linkage arrangement that makes it easier to hold the pedal down by making it harder to push at the top. A lot of thought goes into simple things like the clutch linkage.

 

Afraid not guys. Take a look at any textbook on hydraulic principles. By varying sizes for pistons and lines you can have a force multiplier effect that would take impossibly long levers to duplicate mechanically. Hydraulic brakes were way more efficient than mechanical brakes, even before vacuum assist (power brakes) was introduced.

Hydraulic clutches are not just a fluid equivalent of a cable with one to one force and travel.

Hydraulic pumps can increase the effectiveness of hydraulic systems, but are not necessary in simple systems involving master and slave cylinders, like a hydraulic clutch.

Taz
Terry Phillips

 

Taz, Taz, Taz.......

Force1 x distance1 = Force2 x Distance2

It matters not how you get there.

 

Mikes formula is correct Taz. Or to put it another way if you go to a very small diameter cylinder to lower effort you may wind up needing a pedal stroke 2 feet long to displace enough fluid to operate the clutch.

 

Hydraulic system helps to implement a solution but it cannot lighten the clutch pedal.
Just think in term of conservation of energy : to release the cluch mechanism you have to provide a certain amount of energy which is basically the product of the force by the displacement. The same (or roughtly the same) amont of energy must be applied at the pedal where it is also the product of force time the displacement, and that whatever you have in between, a cable or hydraulic.
There is no mistery, if you want a very light pedal (low force) you will need a long displacement. For the same pedal displacement, you will have the same effort.
This is of course without considering any other device, like springs, which can change the feeling on some parts of the travel of the pedal, but without bringing external energy, or some assistance (pump) which take some energy on the engine to add it on the command.

PS: computation of the energy is a bit more complex because the force is not constant and varies according of the pedal position, so the mathematical computation is a bit more sophisticated, but it is anyway a product of force time displacement

PS2: I was a bit late on that one

 

Apparently I am not making myself too clear on this. By varying piston size, you can vary displacement or force. I understand all the math and basic physics involved because I do that for a living.

Hydraulic systems this simple rely on some very basic physics. A displacement of 1" on a 3" piston can result in a displacement or force of a ratio of the squares of piston radius (piston area), or 2.25, to 0.25 for a 1" piston (9:1). This can either be an increase in displacement or an increase in force, or a combination of the two, depending on how you set up the system. So all your equations are correct, but your understanding of what is happening is lacking. Play with mechanical advantage as well, and you end up with as light a clutch pedal as you like.

Taz
Terry Phillips

 

OK I am with mke from the beginning on this one.

Yes, but f1 x d1 still equals f2 x d2, same as a mechanical lever.

Forget about the formulas for a minute and just think about this...

There is a piston at each end of the hydraulic line. Each piston needs to displace the same volume of fluid. If the piston at the clutch pedal end has an area of 1 square unit and it displaces a length of 5 units then 5 cubic units of fluid is being displaced. If the piston at the clutch end has an area of 10 square units it will feel 10 times the force, but that 5 cubic units of fluid will only displace it 5/10 of a unit, 1/10 of the travel of the clutch pedal. So f1 x d1 = f2 x d2.

If you require very light clutch pedal pressure you will have to have a very long clutch pedal stroke because you are pushing hydraulic fluid into a short fat cylinder from a long skinny cylinder. If you try to use a lever to fix this you will end up back where you started.

 

I’m not say hydraulics are not a very convenient way to go, they are. With hydraulic you can get most any ratio you like in 1 step, you can’t do that with a mechanical system……the point is you don’t have to do it in 1 step. The second point is you cannot achieve any pedal force you like because the lower the pedal force you want, the longer the pedal travel you end up with and realistically you can’t have more than about 4” of pedal travel.

In the hydraulic system you normally have:

Pedal to master – mechanical ratio
times
Master to slave – hydraulic ratio
Times
Slave to throw-out – mechanical ratio

In a 308 (I think dino is the same) mechanical you have:

Pedal to cable – mechanical
Times
Cable to pivot arm – mechanical
Times
Pivot arm to throw-out – mechanical

Each of the 3 mechanical steps can be up to about 4:1 giving a max practical of 64:1 in the pure mechanical set-up. A stock type clutch want about 0.4” of throw-out travel so you could get the pedal as high as 25”…..way more leverage than you want. For a clutch, the choice comes down to what is cheaper in production (normally mechanical) or more reliable (normally hydraulic) but performance would be the same.

 

Exactly.

The way way you fix it is a multi-disc clutch pack that requires less spring force because there is more active area. I run a 3 disc clutch that will handle 800 ft-lb torque and have a honda civic feeling clutch pedal, almost too soft, with a mechanicl linkage. I am switching to hydraulic with my V12 project to save space in the bellhousing (the throw-out bearling is built into the slave cylinder, very compact set-up), it has nothing to do with fixing any pedal force problem.

 

OK Mark, I have it now. There is no advantage to hydraulic systems except there is no way to make a mechanical system that matches the force or displacement of a hydraulic system in the limited space available in most cars. I think that is pretty much what I said originally. Mechanical and hydraulic clutches are apples and oranges because of the differences in the way leverage is applied.

Taz
Terry Phillips

 

The early one did.

 

Taz,
You can build a mechanical system that matches the force and displacement of the hydraulic system in every way. It's also not generally a packaging problem to go mechanicalreally. I think the main push to hydraulic has been for mostly about a self-adjusting being much easier to do with a hydraulic component in the setup.

 

correct with a couple slight additions.

You can make a single disc clutch for a 400 hp engine Honda civic light by going to a large diameter, not good for a sports car because it requires a large diameter flywheel but it works fine for trucks.

You could also add some kind of assist to lighten the pedal force. The nissan TT had a vacuum assisted clutch just like what is normally used on brakes but there are other ways it could be done like ferrari's electrically operated F1 clutch.

 

Well, there are still some parameters to play with :
- a larger diameter of the cluch would allow to reduce the pressure on the clutch for the same amount of torque ... but that is not very practical for a high rev engine.
- changing the friction material, for a higher friction coefficient, would also allow to reduce the pressure for the same cluch dimensions.

It is all a matter of trade-offs ....

Late again ...

 

It looks like you posted while I was busy editing

 

yes! It takes too long for me to find the english words

 

You can get a clutch assembly to hold more power with a more aggressive friction material.

The best way to build a nice streetable clutch is to go to multiple discs rather than more aggressive material.

And yes, assume you need 2.0in travel at the pressure plate to completely disengage the clutch. Whether you attain that 2.0 inches of travel through the full range of pedal motion through a cable or hydraulics the effort will be identical with the exception of higher friction with the cable setup.

If for a given clutch you lighten the pedal (with either mechanical or hyd) you have also reduced the distance the throwout bearing travels.

 

No- Not correct. You are still assuming the hydraulics are just a tube with fluid in them like a fluid cable. This is only true if the master and slave cylinders are exactly the same size, which is what I have been saying all along. Mark's listing of "hydraulic advantage" is what you are not taking into account. In addition to hydraulic advantage, all the mechanical lever tricks can also be used on a hydraulic clutch, although hydraulic advantage can lessen the need for mechanical leverage.

Also the premise that high hp cars cannot use a single plate clutch is incorrect. My 7 liter Z06 with 470 ft lbs of torque and manual shifter 575Ms with 434 ft lbs of torque both have light, easy to use, single disc, hydraulically actuated clutches. Quite a change from the bear traps we had for mechanical clutch linkages fitted to my 67 427 Corvette, which was also a single disc clutch.

Taz
Terry Phillips

 

It takes 'X' amount of energy to move the throwout bearing 'X' distance.

Master cyl piston diameter vs stroke and slave cyl diameter is working out leverage ratios just the same as a cables distance from centerline and clutch release fork from pivot to throwout bearing.

If it takes the pedal 4 inches to move the throwout bearing 1 inch in both hyd or mechanical then the input energy required is identical for both, minus frictional differences as overall ratios are identical.

"Hydraulic advantage" is the same as mechanical advantage, it's just another form of transferring the energy. If the end ratios are the same than effort is the same regardless of if it is hydraulic, cable, gear, levers or otherwise. (not considering friction)

Think of it in simpler terms, such as a balance scale. A simple arm with a centered fulcrum takes the same amount of weight on each side to be equal. Such would be the case with 2 vertical hydraulic cylinders of the same volume connected to each other.

Now, move the fulcrum on the mechanical scale to make a 2-1 ratio. Now one side needs twice as much weight as the other to remain equal.

Same thing with hydraulics. Make one cylinder half as much displacement also achieving a 2-1 ratio and you still need double the weight on one side to remain at equal heights.

The end ratio is the end ratio. No means of achieving that ratio is going to be able to extract more energy than what is inputted or accomplish the same amount of work with less input energy.

Or, answer me this. Lets say this particular car has a pressure plate that needs to move 1 inch for disengagement and it takes 200lbs of force to do it.

Both cable and hydraulic clutch setups are at a 4-1 overall ratio. That means it will take 50lbs force and 4 inches of travel in the pedal to move the throwout bearing 1 inch with 200lbs force.

Please explain how the hydraulic setup could defy the laws of physics and take less input force and movement to achieve the same work.

 

I am no engineer but I stayed at a holiday inn last night.

The problem with a lever is it exerts a rotational force, not a linear one. Quite a bit of mechanical advantage is lost with the lever because it essentially rotates around a fulcrum. A hydraulic piston on the other hand can exert an unlimited amount of linear force.

Please forgive my drawing.

Dave

Having said that, most automotive hydraulic clutches still get their mechanical advantage using levers (Clutch pedal and bell housing arm)
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Will the plane take off if the treadmill is a mechanical or hydraulic one ?

 

LOL, what a maddening thread. Amazing how many people can't get their mind around that scenario.