I keep hearing that hydraulic clutches are more powerful than cable ones but is it so?
Essentially a hydraulic system is similar to a lever system. You trade in distance for force. So it should’n have any force advantage compared to a cable system.
Am I mistaken?
It’s easier to design a bigger “lever,” and fit it into a smaller space, with hydraulics than with cables. There comes a point where with very stiff clutches you would need a long pedal throw, or compound pulleys, to get the force you need.
Imagine you have two clutch actuating system: one is cable, one is hydraulic.
Both have the same ratio, X pedal travel = Y clutch fork travel.
The hydraulic system is still lower effort as it doesn’t have the friction of the cable system. The cable system needs some form of pulley to change direction (= friction).
With a cable system it is also likely the geometry will not be perfect due to packaging needs. A hydraulic system can run the hose whichever way needed without adding friction/effort.
Friction is present in all hydraulic systems. Whether it is lower than that of a mechanical system will depend on the design of each.
FWIW, I replaced the clutch in my Mustang with a high-performance model. I destroyed several clutch cables until I realized that stock cable couldn’t take the force required (the cable tore the lining). So, I had to periodically lubricate the supposedly maintenance-free cable or it would break. OTOH, my F-350 has a monster clutch, and the hydraulic clutch release has never had an issue (although the actual pedal-to-cylinder mechanism is a piece of junk, which I replaced with my own design).
You forgot the third kind of clutch activation device, the mechanical clutch. My 66 GTO had a mechanical clutch and was much harder on the left leg than any hydraulic or cable clutch. Though harder to use than the others, it had much better feel. You could feel the clutch disc touching the flywheel. I could never do this with the other types of clutches.
I don’t think any cable systems use pulleys, just a lever at the transmission end. The biggest difference between a cable system vs. hydraulic is a hydraulic clutch is noticeably smoother.
That depends on how the cable routing is laid out. The great majority of cable systems do not use pulleys. Nevertheless, depending on design a pulley may add only negligible friction.
A few systems had pulleys, but as you figure most did not. Other than clutches that use the relatively new integral release bearing slave cylinders, all systems that I can recall, whether mechanical, cable, or hydraulic, had a release lever in the bellhousing. A well-engineered cable setup can be quite smooth, but sadly not all were.
Yes, it’s essentially an application of leverage regardless of how it’s routed and applied. “More powerful” really doesn’t make sense in this context. As mentioned by others, the significant difference will be in friction. It’s probably generally easier to have less friction with a hydraulic system than with a cable, but I find it hard to believe that there’s a significant, or even noticeable, difference between typical hydraulic systems and modern, well-designed cable systems. Some of the older cable systems, and especially the mechanical systems with their mess of linkage, pivots, bellcranks and such, did have noticeable friction losses.
Are there hydraulic clutches with vacuum booster? Like in the brake system?
I was told that some Porsche 911 and Ferrari models had a power clutch, but Google is not allowing me to find any further evidence.
In my (limited) experience this is one of the main reasons for hydraulic clutches; underhood packaging is so tight on modern vehicles that it’s easier to run a hydraulic hose than it is to run a cable and maintain the correct bend radii to prevent cable binding.
I was under the impress that big-rig trucks use some kind of assist on the clutch actuation. Can someone confirm?
If pulleys are used in a cable-actuated system, they can be made nearly frictionless, if one cares to go to the length of using needle bearings on its hub. Even without that though, the friction will be minimal; most of the friction will come from the cable sliding back and forth through its housing.
BMW boxer motorcycles from 1996-2001 used a cable-actuated clutch. In 2002 they made minor changes to the bikes. The clutch itself was pretty much identical, but the actuation had gone to hydraulic. Friction was notably less than the cable-actuated version.
Not a clutch system, but there are both cable- and hydraulic-actuated braked commonly in use on mountain bike. These days, it’s mostly disc brakes, but there were hydraulic brakes back when it was all rim brakes too. I’ve used both types and my personal experience is that the hydraulic brakes are much smoother and feel more responsive to small changes in input.
It’d be hard to compare ‘power’ since other details of the design differ, but I think it’s comparable between cable and hydraulic. At least in a bicycle rim-brake context, I do think it’s easier to design the hydraulic brakes to have more ‘leverage’ due to all the physical constraints. My hydraulic rim-brakes would easily twist the mounting bosses away from the frame with just two-finger pressure (until I installed reinforcers that is!), while the cable brakes could not. With disc brakes, I’m not sure how the maximum leverage potential stacks up, but I feel that hydraulic is still superior from the standpoint of the hard-to-quantify “feel”.