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Yamaha's New YSRC Clutch

Evolution of an expertly engineered transmission
2017 Yamaha Sidewinder L-TX YSRC clutch
It is the transmission for many newly colored Yamaha Sidewinder snowmobiles, and the YSRC clutch helps set this sled apart from previous Yamahas.
Kort Duce photo
Yamaha sleds move fast, but changing major components for any manufacturer is not often a quick process. It’s been a while since Yamaha produced an extensive upgrade of their CVT clutches, perhaps for good reason. Yamaha clutches have always boasted solid design with few problems and lots of tuning options for their varied models.

Let’s examine the history of Yamaha clutching, and the OEM’s philosophy of durability combined with tuning options. With more than four decades of snowmobile CVT experience, Yamaha made all the right moves with its new YSRC clutch! It’s a solid piece of engineering that undoubtedly has progressed through several years of design and testing.

History is clutch
Yamaha clutches have been in gradual development since the 1970s, always in response to new demands from more powerful engines. There have been a number of innovations along the way that made them easy to tune. In the late 1970s, Yamaha came out with a set of racing weights with several locations on the back of the flyweights where extra mass could be screwed on. This was long before the now-popular aftermarket weights offered this feature.

Yamaha engineers, however, felt that screwing extra mass onto the flyweight was not reliable enough, so this design evolved into the current flyweights, which have several holes through them where rivets of different material and weight can be riveted solidly in place in order to fine-tune each flyweight’s mass and center of gravity.
new 2017 Yamaha YSRC clutch
new 2017 Yamaha YSRC clutch
New spider and new secondary, as well as new sheaves on the two clutches and an increased shift ratio, are just a few of the updates with the new YSRC clutch.
The second feature, which is appreciated by more advanced tuners, is the ability to use different diameter clutch rollers. Changing roller diameters results in more aggressive acceleration. There are three roller diameters available, and they can be changed without dismantling the clutch and unscrewing the spider because they sit on separate arms from the slider shoes, which have their own spider towers. As a result, the Yamaha spider has six arms instead of the normal three found on other brands.

The next step was a redesign of the secondary into a reverse helix, where the helix is mounted on the outside, much like earlier Arctic Cat models. An update of the primary in the ’90s saw a deeper cover, allowing longer and stronger springs.

Two other features also made the CVT system more durable. On the newer 4-strokes, a rubber cushion drive was included in the reduction gearbox to lessen a lot of the harsher 4-stroke power pulses. Although top cog belts were already available on the aftermarket, Yamaha was first to introduce a top cog belt on a production model, beating the other manufacturers to the bell by more than a year. In comparison tests, the Yamaha Mitsuboshi belts have shown themselves to be both stronger and more durable than competitive brands.

With all this development, it’s been puzzling to many why Yamaha had not introduced a secondary roller clutch 15 years ago like other brands. Perhaps the answer lies in their preference for 4-stroke engines with a wider power band than the 2-strokes, and therefore not so dependent on quick-reacting downshifts.

The new YSRC CVT system for the turbo sleds introduces a much upgraded primary clutch and a new secondary roller clutch. Both are based on many changed components from their YVXC models.

Change is good
Although it may not be obvious at first glance, there are major changes to this new YSRC design that improves both strength and efficiency. Brand new sheaves on both the primary and secondary are of larger diameters, increasing the overall shift ratio from 3.8 on the YVXC clutch to 4.1 on the YSRC. This is a 10% increase in overall ratio, allowing a powerful turbo sled to reach a 15mph-higher top speed without losing any low-ratio pulling power.
2017 Yamaha Sidewinder L-TX YSRC clutch
Kort Duce photo
The new belt needed to be wider to transfer more power. A wide belt means a larger sheave angle to better support its width, and prevent it from being pushed out of the clutch at full shift. Sheave angles were increased a couple degrees to fit the wider belt. This in itself is not a bad idea, because tests in the early days of CVT design showed the wider angles to be more efficient because of the better belt support at top speed.

Yamaha engineers also took the opportunity to strengthen the sheaves with ribs that are now deeper and curved to act as fans, but with a very efficient blade design. The new rearward curving blades cool better while still reducing power loss when moving the air compared to earlier designs. Updated, stronger bolts and bushings hold the rollers and flyweights.

All the old flyweights, rollers and pressure springs from the YVXC unit can still be installed in the new YSRC primary clutch, giving tuners great range in custom tuning this new clutch. The final changes on the primary clutch moves from the rather large slider shoes to conventional slider buttons. This is claimed to reduce friction and improve both life and consistency in shift patterns.
new 2017 Yamaha YSRC clutch detail
new 2017 Yamaha YSRC clutch detail
Updates like new flyweight bolts, bushings, rollers and weights all add up to a clutch that can handle the strains of a big 200+ horsepower 4-stroke engine. This is still a torsion spring clutch too.
Roll with it
So why did Yamaha finally decide to upgrade to a roller secondary? It’s well understood that rollers give a smoother shift, but perhaps not so easily realized that they also improve efficiency and the ability to transfer more power. This is because a roller clutch reacts more quickly to changes in side forces, and that makes it superior over buttons in keeping a more constant side pressure on the belt. In turn, this prevents the belt from slipping and overheating at higher torque loads.

Higher torque loads are exactly what Yamaha faces with the new turbo sleds, and a roller secondary matches this application well. Yamaha wisely decided to also keep the torsion spring design, which makes it even quicker on the downshift. We feel this is a better setup than the straight spring rate design in some other clutches, which unfortunately reintroduces more friction and slows down the backshift reaction.

All the present YVXC torsion springs will fit in the secondary, but the YVXC secondary helixes will not. The YSRC helix has a larger diameter and a different contour to work with the new roller design. Otherwise, the design follows the basic YVXC reverse cam design except with rollers replacing the buttons, and the sheaves are larger in diameter with a wider angle and are stronger with larger and more aerodynamic ribs for cooling.

Quick impressions
So how does the YSRC work on the turbo sled? AmSnow test rider Butch Veltum, who took numerous early rides on the new sled last spring, was very impressed with the overall performance. He liked the smooth shift action, but he was most impressed with the very quick downshifts, which always held the turbo sled in its powerband. Also impressive was that nothing was abrupt in nature; shifts were smooth and predictable. This made it an impressive high-powered, top-of-the-line performance sled, and it was actually pretty darn good in the corners for having such a big motor.
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