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Inside Ski-Doo's New Primary Roller Clutch

OEM reaches out into uncharted territory with its new race sled
2016 Ski-Doo racer snowmobile primary roller clutch
Ski-Doo’s 2016 racer got many upgrades (including to the track and engine), but the big news is the new primary roller clutch.
Let’s go racing! That was the reaction from many in the crowd when Ski-Doo unveiled its new clutch at Hay Days 2015. This new roller clutch is only going to be used on Ski-Doo race sleds this year, but we all know that what wins on Sunday usually sells on Monday! If all goes smoothly, we expect to see this clutch on consumer model sleds very soon.

The roller revolution continues with this new primary roller clutch for the Ski-Doo snocross racers. It’s nothing unheard of, or all that new, as rollers were first introduced in the 1990s to reduce friction in the secondary clutches. The advantage of quicker reacting clutches on both belt life and power transmission output quickly led to it being adopted for most production snowmobiles.

Polaris, Arctic Cat and Ski-Doo have all used secondary roller clutches on their race sleds for years, but Ski-Doo is the first to trade its venerable TRA for a primary roller clutch.

What’s it good for?
The secondary roller clutch cuts down on friction by eliminating the torque buttons of a button-style clutch, and it improves efficiency by allowing more consistent side force on the belt. This reduces power-robbing belt slippage and lowers heat input into both the belt and the clutch sheaves. The same advantages can be had by introducing rollers instead of buttons on the primary clutch, but this has proven to be a much more difficult engineering challenge.

Rollers on primary clutches must live in a much harsher environment. In the primary, rollers are subject to not only the firing pulses from the piston, but also several orders of engine vibrations, including torsional nodes from the crankshaft. Rollers need side clearance to work smoothly, and this means tight tolerances to prevent rattle noises at idle. Primary roller clutches have been available from the aftermarket since the late 1990s, and they went thru many stages of development before becoming reliable for high-performance engines like large triples and turbo sleds. It quickly became clear that reducing friction from the torque buttons on large, high-torque engines improved belt life and shift patterns.
2016 Ski-Doo racer snowmobile primary roller clutch
The new roller clutch uses three traditional flyweights, with five positions of clicker adjustments.
Ski-Doo does it!
Looking at Ski-Doo’s new primary clutch, it’s obvious that the OEM has spent many years addressing many of the problems associated with roller primaries, and as a result it introduced a number of new and innovative items. The first item is the twin roller setup. The top conical roller transfers the driving torque when the belt system is under power, while the thinner bottom roller takes up slack when the engine decelerates or reverses. This prevents any “rattling” at idle, as both sides are in contact at the same time, but in different directions.

The clutch operates with three flyweights, but these are wide and thin for better contact with the rollers. Built in is an eccentric pivot, which provides five “clicker” positions that can be adjusted by loosening a bolt on the flyweight. Additional washers can be placed on this bolt to fine-tune the weight. Each “clicker” position changes the shift speed by approximately 150 RPM. The larger roller runs on a needle bearing to further reduce shifting friction.

The clutch has only a movable sheave and an outside cover like Ski-Doo’s new eDrive clutch, but a large rubber dampening donut on the driver cover undoubtedly dampens the engine pulses and vibration inputs before they reach the rollers, allowing the use of smaller rollers.

It will be interesting to see how this works out for the racing teams. Snocross is one of the toughest environments for testing products, and any shortcomings will quickly be found and corrected before this new clutch becomes available on production models.

INSIDE SCOOP: Dissecting Ski-Doo's Primary Roller Clutch
There are double rollers, one large for acceleration, one smaller for reverse.
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