Hydraulic Clutch Advantage: The Rekluse Manual Slave Cylinder

April 8, 2019 / by John Basher

Rekluse is proud to offer direct, bolt-on replacement manual slave cylinders for hydraulic clutch-equipped bikes. Here, we take a look at how the Rekluse slave cylinder improves durability and rider feel. 

Hydraulic clutch systems have been used for years by motorcycle magnates KTM, Husqvarna, Husaberg and Beta. They are also popular in factory racing circles, such as HRC Honda. That’s because hydraulic clutches self-adjust, offer better modulation than cable-actuated clutches, and provide a more consistent feel. However, hydraulic clutches aren’t the silver bullet to all clutch maladies. This design can fail, and when it does, a rider will be going nowhere fast. The chief culprit is a malfunctioning slave cylinder.

Hydraulic clutches rely on slave cylinders for clutch actuation. They provide great performance advtanges, but can also leave you stranded if something malfunctions. It's important to have a reliably-operating part.

What, exactly, is a slave cylinder? In layman’s terms, it is the rider’s connection to the clutch system. The clutch lever controls the fluid that goes to the slave cylinder. When the lever is pulled in, fluid hydraulically pushes the piston inside the slave cylinder, which lifts the pressure plate. It’s similar to what a clutch actuator arm on a cable-equipped motorcycle does. Unfortunately, the slave cylinder internals wear out over time. The good news is that Rekluse recently unveiled their manual slave cylinder, which packs a lot of features and benefits into a modestly priced performance item.  

Rekluse recently added manual slave cylinders to their line up. Read on for the full explanation of the benefits of this hydraulic clutch system upgrade.

Born out of Necessity

Rekluse has been making slave cylinder systems for their automatic clutch systems (RadiusCX, RadiusX, and CoreEXP) for several years. Popular with customers, it was only natural that Rekluse would delve into creating slave cylinders for manual clutches. It was only natural that Rekluse take a closer look at a product that was a few iterations away from being market ready. This began development of a manual slave cylinder. The company quickly discovered that their slave cylinder design made noticeable improvements over the OEM/stock counterparts.

Backing up a bit, the idea for the Rekluse auto clutch was to make the adjustment through the slave cylinder. Engineers designed a dial adjuster on the slave cylinder. It was essentially a bolt that was intended to push the piston against the throw rod to create a gap. It was a smart approach, because it allowed the rider to make quick adjustments (even mid-race if needed). Through trial and error, utilizing different piston O-rings, Rekluse came up with a design that provided better clutch feel for their auto systems.

Initial Rekluse slave cylinder development began with the auto clutch. When the need for manual slave cylinders came about, a lot of knowledge from initial testing and development was carried over.

The hurdle was to carry that research and development over to the manual category. Austin Paden, Rekluse Product Manager/Race Development, explains, “It all came down to closing off the adjustable gap where the bolt fit for the automatic slave cylinder. Once that was accomplished, we focused on making a slave cylinder that its only purpose was to work with a manual clutch, whether it was an OEM/stock or aftermarket clutch system.”

Once developed, the Rekluse manual slave cylinder was put into the field in the spring of 2018. Through rigorous testing by various race partners, including champion off-road racer Destry Abbott, Supercross star Dean Wilson, and AMA National podium finisher Mitchell Harrison, Rekluse learned that the feedback was super positive. Coincidentally, the company began seeing slave cylinder failures from some of the OEM/stock units. Going full steam ahead into production was a no brainer.

Rekluse focused on making a slave cylinder that was purpose-built for manual clutches, then it was field tested in everything from off-road, to Supercross, to outdoors.

Diagnosing a Problem

How can you tell when your slave cylinder internals need replacement? Look at the signs. A major indication of a faulty slave cylinder is when the rider notices a soft feel at the clutch lever. The reason for that sensation is due to fluid pushing around the edge of the piston. When the clutch lever is pulled in, fluid bypasses the piston, resulting in a soft feeling. This is due to the O-ring in the piston.

Another good indicator of a slave cylinder malfunction is when there’s fluid build-up on the engine. However, don’t rely on a quick look-over of the engine to determine the problem. It may take a while to notice fluid pooling up on the engine because there’s a seal between the cases. However, you should start to notice leakage at the bottom of the slave cylinder over time.

Whether trying to diagnose a faulty slave cylinder or installing a new one for the first time (OEM/stock or Rekluse), the first step is to bleed the system. This will ensure that there isn’t any air trapped in the line. However, if you notice that the clutch sensation continually fades through the lever, it’s an indication of a leak in the piston seal.

If you feel you're having problems with the slave cylinder, the first step of troubleshooting would be to bleed it. However, if performance issues persist, there could be additional seal problems.

Rings Matter

On the whole, OEMs typically use a single O-ring with a bladder to prevent oil from mixing with the rear brake fluid or hydraulic fluid. Rubber reacts differently, depending on what oil it is subjected to. That’s why it’s important to keep engine oil separate from brake fluid (generally a Magura system) or hydraulic fluid (Brembo). Otherwise, the seals can expand and fail.  

Rekluse initially relied on an X-ring, but they reevaluated their decision after more testing. Paden explains why Rekluse moved away from the X-ring. “On the engineering side of things, an X-ring is a top-of-the-line seal. However, we discovered that an X-ring can only hold up for so long. A standard O-ring has a larger surface area that contacts the piston cylinder. Ultimately, a standard O-ring is a better solution, so that’s what we moved to.”  

Finding the Right Rubber

Instead of using a single O-ring design and using the bladder for protection (what’s found on OEM/stock systems), Rekluse incorporated a dual-seal design. There are two separate O-rings in the piston itself. One is designed for sealing out engine oil; the other is for keeping hydraulic or brake fluid in the slave cylinder. There are two advantages to that design. The first is that it creates a very good seal surface. The other being that the O-rings are easier to install and replace, as there’s less threat of rolling them over or tearing the rubber. This is a common failure when people are replacing the OEM/stock O-ring and bladder.

Slave Cylinder Exploded View

Rekluse Slave Cylinder Installation

With such an intricate design, the idea of installing a Rekluse manual slave cylinder may sound daunting. Fear not, as it’s as simple as bolting it on and bleeding the system. Regardless of your mechanical aptitude, the install should take anywhere from 15-30 minutes. The KTM and Husqvarna slave cylinders are pretty straightforward. The 2019 Kawasaki KX450 design is more complicated with how the lines go into the slave cylinder, so that may take additional time.

Paden offers several helpful tips for installing a Rekluse slave cylinder. “Pay close attention to the orientation of the slave cylinder during installation. Make sure that the piston is going in straight. The good news is that the piston will come already installed into the slave cylinder. Make sure that all bolts are tightened down evenly, because the new slave cylinder will be putting pressure on the throw-out rod. You don’t want to create a see-saw type of tightening situation that could wreak havoc on the internals.”

Find specific installation instructions for your product here.

Make sure the piston is oriented straight in the housing when installing the slave cylinder. This is important for even pressure on the throw-out rod.

Features and Benefits

While being easier to replace than an OEM/stock unit, the Rekluse manual slave cylinder is also full of features and benefits. One happens to be a very strong return spring. Typically, there’s a spring that pushes the piston back. Normally it’s a light spring. Its job is to maintain fluid pressure so that every time the clutch lever is pulled, the return spring is keeping pressure on the clutch side. Rekluse uses a wave spring that’s much stiffer. It creates a more connected feel by guaranteeing that the piston is always engaged to the throw rod.

Manual-Slave-Cylinder-Graphics-square

Understand, too, that metal plays a major part in performance. The characteristics of metals and how something is machined makes a big difference in feel. A lot of OEM/stock slave cylinders are constructed out of cast aluminum. The housing flexes, which creates a different feel. Rekluse slave cylinders are crafted out of hard anodized billet 6061 aluminum, which is more rigid. Combined with a stronger return spring, the Rekluse design makes the hydraulic clutch feel more like a cable-actuated clutch. In testing, riders noticed a smoother transition during engagement and disengagement.

Manual-Slave-Cylinder-Graphics-squre

Across Several Applications

The Rekluse manual slave cylinder is available for most motocross and off-road models with hydraulic clutches. That includes KTM, Husqvarna, Husaberg, Beta, the Cobra CX65, and the new 2019 Kawasaki KX450. Oddly enough, the same slave cylinder that’s used on the KTM 65SX mini-cycle is also used on the KTM 690 Duke. Regardless of your hydraulic clutch-equipped motorcycle, pairing the manual slave cylinder with a Rekluse manual clutch is not necessary. The OEM/stock clutch is also sufficient.

The Rekluse manual slave cylinder is available for all popular off-road models that come with a hydraulic clutch from the factory.

The Rekluse manual slave cylinder retails for $169. Check out the Rekluse website to learn more and find Rekluse products for your machine. The website also has a very helpful bike selection search engine that will list the appropriate Rekluse products for your motorcycle.

Topics: NEWS, NEW PRODUCTS, featured, Powersports, Clutch Tech, Manual Clutch

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Written by John Basher

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