By: Larry Kelly
The force required to pull the clutch lever is ultimately controlled by the engine's power output. A Ducati superbike with a maximum torque of 65 ft-lbs. being transmitted though it's clutch needs to have around 430 pounds of preload in the clutch spring(s) to prevent the plates from slipping.
This means that the hydraulic pressure on a typical 28mm slave cylinder needs to be about 425 psi to overcome the 430 lb. spring preload and disengage the clutch. The distance that the slave cylinder needs to move (the pushrod that in turn separates the plates) has to be at least the thickness of the 2mm dished plate in the clutch pack. Say 3mm tops.
About 94 lbs. of force is needed to be applied to a typical OEM 13mm diameter master cylinder piston to create 425 psi of pressure in the incompressible hydraulic fluid that in turn moves the slave cylinder. For every 1mm that the slave cylinder moves the pushrod, the OEM master cylinder has to move about 4.6mm. This requires the displacement of about 1.2 cc of hydraulic fluid. But only in the ideal world.
In the real world, the master and slave cylinders are connected by a rubber clutch actuation line that expands a little under this 425 psi pressure. This undesirable expansion can be reduced in two ways. The rubber can be reinforced with Kevlar fiber or steel braid, or the line internal diameter can be reduced (by choosing a 2 or 2.5 mm I.D. line rather than a regular 3mm) thereby increasing the line wall thickness (and it's strength.) The net result of having a real-world line is that it increases somewhat the volume of fluid that the master cylinder has to displace to get the same internal line pressure. This is often described as a "spongy" lever.
Continuing on. In order to apply the required 94 lbs. of force to the clutch master cylinder piston, the clutch control lever needs to be pulled, and here's where the lever's mechanical advantage comes into play.
The human hand can't repeatedly (without fatigue) apply 94 lbs. of force to the lever so the master cylinder lever is designed to provide a mechanical advantage - to amplify the hand's force. The OEM design for example provides between a 4:1 (two finger) to a 9:1 force reduction (end of lever.) This means that you have to normally apply between 23 and 11 lbs. respectively with your hand to release the clutch.
The lever's mechanical advantage has it's consequences, however. Instead of having to move the master cylinder only 9mm to disengage the clutch 2mm, the end of the clutch lever now has to move more than 3 inches. In a racing situation this is undesirable so there's replacement radial master cylinders offered that reduce this movement, but at the sacrifice of higher clutch pull forces. For the street it's desirable, since the longer lever travel makes it easier to launch the bike from a stop by increasing the range of the "friction zone."
That said, the only practical way to reduce the clutch lever pull force is to change the diameter of the slave cylinder. The force-reduction aftermarket slave cylinder replacements offer around a 20% reduction but at the penalty of needing a longer clutch lever pull to get full disengagement. Another solutions would be to just reduce the distance between the clutch lever pivot point and the master cylinder in combination with decreasing the diameter of the master cylinder. These options are not currently offered in the aftermarket.