TEN THINGS ABOUT THE SECRETS OF HYDRAULIC CLUTCHES

(1) What is it? The hydraulic clutch has the same plates, fibers and clutch basket as a standard cable-activated clutch. The only difference is in the system that disengages the clutch plates. When the clutch lever is pulled in on a hydraulic clutch, fluid is compressed down through a line to separate the clutch. With a standard cable clutch, the lever pulls on a cable that is connected to an arm that is used to separate the clutch plates. 

(2) Purpose. KTM originally instituted hydraulic clutches because they were easier to pull in than cable-actuated clutches. The easier pull allowed them to use stiffer clutch springs, making the clutch stronger and giving it more torque. They also provide a more consistent feel than cable-actuated clutches while at the same time simplifying the manufacturing process. Instead of a cable pulling on an arm to actuate the clutch, the hydraulic system uses a clutch slave cylinder and a push rod, which are inside of the engine and out of harm’s way. 

(3) How does it work? The clutch lever is connected to a master cylinder reservoir full of fluid. Once the lever is pulled in, the fluid is compressed and pressure builds up. The increased pressure sends a pulse of fluid down the hose to the clutch slave unit where the fluid pushes against a piston inside of the clutch slave unit, which in turn pushes up against a rod. The rod extends from the left side of the engine (shift lever side) to the right side where the clutch basket is. The rod is pushed into a top-hat-like fitting that disengages the clutch by pulling the plates apart.

(4) Self-adjusting. When clutches get hot, the fiber plates expand, which lessens the distance between the plates and alters the clutch’s point of release. Cable-actuated clutches need to be adjusted as the clutch plates heat up and cool down. With hydraulic clutches, the fluid is constantly self-adjusting to compensate for heat and wear. This keeps the clutch running optimally while giving you a consistent feel at the lever.

(5) History. KTM first introduced hydraulic clutches on its motocross models in 1998, and by 2000 all of its full-size motocross bikes were using the system. Husqvarna has used Magura hydraulic clutches since 2014, when KTM bought the company, and Kawasaki is the only Japanese brand to switch to a hydraulic clutch when they debuted it on the 2019 KX450. 

(6) How to go hydraulic. If your bike didn’t come with a hydraulic clutch, you might be able to upgrade by using Magura’s aftermarket Hymec hydraulic clutch kit. Hymec stands for “hydraulic from mechanical.” If your bike didn’t come with an OEM hydraulic clutch, there will be no master slave units on the engine cases to activate the push rod. The Magura Hymec system overcomes this by including a hydraulic slave cylinder that bolts outside the cases to move the actuating arm that your clutch cable used to pull. 

(7) Clutch slave issues. The Magura master slave units on the Husqvarna FC450s before 2019 suffered seal failures. The seal on the hydraulically activated plunger would wear out, allowing fluid to escape, causing a loss of pressure inside the slave unit. When the seal leaked, the clutch would go completely away without warning. Prior to Magura redesigning the slave unit seal in late 2019, the solution was to install a Brembo plunger piston, which fit perfectly and had a better seal design. 

(8) Maintenance. If your bike wants to creep forward, even when the clutch is pulled in, that is a sign that the hydraulic fluid in your clutch’s hydraulic system is either worn out or has air bubbles in it. This problem is fixed by bleeding the hydraulic clutch fluid and replacing it with fresh fluid in the line between the clutch lever and the master slave cylinder. Follow the directions in your owner’s manual to bleed the clutch.

(9) Mineral oil. Be forewarned that some older Magura hydraulic clutch systems require mineral oil instead of brake fluid in the system. If brake fluid is used in a system that was designed for mineral oil, the brake fluid will cause the seals to swell and leak. Different seal materials are required depending on which fluid is used. Magura’s Hymec kits require Magura’s proprietary Blood Hydraulic mineral oil.

(10) Brake fluid. The type of hydraulic fluid your clutch system requires is printed on the cap of your clutch master cylinder reservoir. If your bike requires brake fluid, we recommend Dot 4 or Dot 5.1 fluid with high boiling points. The dry boiling point, which is the higher number, is taken when the fluid is in perfect condition. The wet boiling point is taken when the fluid is fully hydrated and at equilibrium. Brake fluids are hygroscopic, meaning they attract water. When water contaminates the fluid, it causes the fluid to boil at lower temperatures, which reduces the effectiveness of the clutch.

 

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