Wednesday, June 20, 2012
Hydraulic vs. Cable Operated Clutch Actuation--Clearing Up the Myth:
I received an email asking about hydraulic clutch conversion in which the author asked questions indicating an unclear understanding of what could be accomplished by such a conversion. The most significant being that it could somehow magically reduce the pedal pressure and throw vs. the cable operated clutch--unfortunately it just ain't so.
First so that we can all be on the same page, here is a diagram of the clutch control group used on the 1994 through 2004 Mustangs:
The OEM cable clutch actuation mechanism has an overall mechanical advantage, from the pedal to the throwout bearing, very close to 10:1. I.e. 5 to 6 inches of pedal travel produces the 0.5 to 0.6 inches travel needed to disengage the clutch at the TOB. On my '03 with a RAM HDX clutch I found it requires 45 lbs of pedal pressure to accomplish this, or assuming 6.0 in of pedal travel, some:
45 lbs * 6 in = 270.0 in-lb of work at the pedal
Because of the 10:1 mechanical advantage that 45 lbs of pressure applied to the pedal creates 450 lbs of pressure at the TOB (ignoring friction in the actuating mechanism), and the 6.0" travel makes the TOB move 0.6".
450 lbs * 0.6 in = 270 in-lbs of work at the throwout bearing
Other clutches will of course require different amounts of work to actuate, however that the work applied at the pedal equals the work applied at the TOB is inescapable (again ignoring friction in the actuator).
AND, it doesn't matter how you get that work from your foot to the TOB. You could reduce the pedal travel to 3 in and increase the pressure to 90 lbs, or increase pedal travel to 12 in and reduce the required pressure to 22.5 lbs; it doesn't matter which, it will still take 270 in-lbs of work.
It also doesn't matter what sort of mechanism (I will one last time ignore frictional losses, but we'll get to that in a bit), a cable or hydraulic master/slave cylinders, you use to transfer the work.
OK, now let's look at hydraulic actuation of the clutch in our cars, using this kit from McLeod Racing),
McLeod does not provide full specs for their TOB however typically they provide maximum travel of 0.700", and a piston area of 1.25 in². It will still take the same 450 lbs of force to disengage the clutch, so we can calculate that we'll need the hydraulic pressure applied to the TOB to be:
450 lbs / 1.25 in² = 360 psi.
To make things simple for now let's work with a master cylinder having a 1.00 in² piston. Obviously with 1 in² pistion it would take 360 lbs of force to create 360 psi, and we would need a pedal with a 8:1 mechanical advantage to get back to the 45 lbs of force the cable clutch required:
360 lbs / 8 = 45 lbs
Now let's look at how far that pedal will need to travel. The same 0.6 in TOB movement is needed so we will need to move 1.25 in² * 0.6 in = 0.75 cubic inches of fluid. To move 0.75 in³ of fluid the 1 in² master cylinder piston will have to move 0.75 in. So, the pedal will move:
0.75" * 8 = 6 in I.e. right back where we started.
Pedal pressure could be decreased by increasing the pedal's mechanical advantage, at the obvious expense of having to move the pedal a longer distance, or by using a master cylinder wait a smaller pistion--let's take a look at this using a 0.75 in² piston (which is what McLoed includes in their kit shown above. This means that to create 360 psi will require a force of only 270 lbs be applied to the (smaller) piston:
270 lbs / 0.75 in² = 360 psi
However, to move the 0.75 in³ of fluid needed to move the TOB piston 0.6 in (which it has to to throw out the clutch), the 0.75 in² master cylinder piston would have to move 1.0 in. See what's happening here? Our 8:1 ratio pedal selected above will now only require 270 lbs / 8 = 33.75 lbs of force, but it will have to move 8 in!
But wait, we only want it to move 6 in so it will have to have a 6:1 ratio. Therefore to create the 270 lbs of force needed at the master cylinder piston will require 270 lbs / 6 in = ?
Yup, 45 lbs same as the cable...
OK with that settled let's look at the frictional losses of cable vs. hydraulic actuation. The cable is at a disadvantage in this regard, and at an increasing disadvantage as the operating force increases. However for all practical purposes with most Mustang clutches and modern cable construction (self-lubricating polymer liners and coatings) this disadvantage is negligible.
The only real advantage offer by a hydraulic actuator lies in the flexibility provided by not have to route and align a cable. The hydraulic tubing can turn corners, go over and around the block, headers, steering shafts and etc., without hassle. You could run it to the back bumper and back without measurably affecting pedal effort...