MotoMojo: Hydraulically Actuated Clutches

Dec 17, 2014 View Comments by

MotoMojo: Hydraulically Actuated ClutchesEarly motorcycles often had a foot-operated clutch and a hand-operated shifting mechanism that followed automotive practice. It no doubt made for some interesting scenes as new riders tried to master the technique, wobbling on one foot while starting out on a steep, off-camber hill while 
carrying a passenger, likely a girlfriend they were trying to impress.

Use the type of fluid indicated for the specific motorcycle you are working on. Reservoir lids are often marked with type.Thankfully, modern motorcycle clutch controls are almost universally mounted on the left handlebar and actuated in one of two ways. Either a cable connects the clutch lever to the actual clutch assembly, or a hydraulic “master” cylinder (with the clutch lever attached) is connected to the clutch assembly via tubing and hoses to the “slave” cylinder. The master cylinder is a type of pump and sends fluid under pressure to the slave unit. The slave unit in turn pushes a piston down a bore to operate the clutch.

In this tiny reservoir on a KTM, fluid is being added with a plastic syringe to avoid spillageBoth cable and hydraulic systems have advantages, which is why both are found on various motorcycle models, sometimes within the same brand.

Cable-operated clutches are less expensive to manufacture, a little lighter in weight, and generally simpler to repair, as just a cable has to be replaced. However, with bigger and more powerful engines, the pressure plate springs, which clamp clutch discs together, must have more force to prevent the clutches from slipping when they are supposed to be fully engaged. A hydraulically-operated clutch can overcome the higher forces required to disengage larger, heavier clutches with less lever effort.

The bleeder fitting is opened, fluid is pumped out, and then the fitting is closed. Repeat until fluid comes out solid with no bubbles.The hydraulic clutch fluid level should be checked regularly. The fluid used in clutch systems is the same as brake fluid. Check the recommended type in the owner’s manual or on the reservoir lid, typically DOT 3 or 4. Many bikes have sight glasses on the side of the reservoir, or a see-through reservoir, which makes it easy to check. Others may have a solid-metal reservoir that requires the lid to be removed to check the level. There should be marks indicating the minimum and maximum levels. Fluid should be added to bring it up to the maximum level. If the level starts to go down, it indicates there is a leak somewhere in the system, typically at the master cylinder seal, slave cylinder seal, or hoses and fittings. Find the leak and correct it as soon as possible.

Before removing a lid, position the handlebar so the top of the reservoir is level, not tilted. Otherwise fluid may run out and get on paint and other delicate surfaces. Fluid can quickly ruin paint if it is spilled, so wash it off immediately with lots of water.

Clutch and brake fluid absorb moisture from the air via the cap vent, and after a while becomes contaminated and can corrode internal parts. Most manufacturers recommend that clutch (and brake) fluid be changed, flushed, and air bled about every two years.

The back of this clutch slave cylinder shows the piston. It must be pushed out with pressure to replace seal.If the fluid looks dark or dirty or is more than two years old, remove the fluid from the reservoir with a suction device (such as an old turkey baster). Refill the reservoir with fresh fluid and bleed air from the system. This can be done with a simple clear bleeder hose and a wrench, or you can use a suction pump such as a Mityvac with a brake fluid-collecting container. Pump the clutch lever until some resistance is felt, then open the bleeder screw to let air escape. Close the bleeder and repeat until no air is left and only fluid comes out.

Sooner or later (sooner if you’ve never flushed the fluid) the master and slave cylinders will need to be rebuilt or replaced. If the pistons and bores are in good condition, you can save money by removing and disassembling the slave and master cylinders and installing new seals. Most cylinders have a circlip or snap ring, which holds the piston in the bore against a spring. Once the clip or ring is removed, the internal parts should come out. Lay them on a shop rag in the order they were taken out.

Inspect all parts for damage or corrosion. Following the shop manual for the specific model, carefully replace the seals, wet them with brake fluid, and reinstall the parts in the exact order they were removed. Reinstall the snap ring or circlip, while an assistant compresses the spring and parts using a tool such as a Phillips screwdriver in the center hole. Install the master or slave cylinder along with the hose connection. Bleed the system to get trapped air out. Test operation and be sure the clutch is working normally before riding the motorcycle on the street.

Text and photography by Ken Freund

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