Not Pulling Your Weight

Little Change in Clutch Technology

Published in the September 2008 Issue White Out & Wide Open—The Blog Steve Janes
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When it comes to the driving force of a snowmobile, what converts the engine power to the track is the drive system, with a major focus on the clutches. And what can be said about the clutches?

Basically, you have two pulleys (drive and driven) linked together by a belt. "Torque is increased and output shaft speed decreased as the relative diameter of the drive sheaves decreases and/or driven sheave increases. Belt traction is achieved by contact of the sides of the belt in V of sheave, not by bottom contact. Traction portion of belt expands outward as belt curves around the sheaves, thus increasing contact pressure. If the taper face V-Belt sheaves are properly engineered, the effective diameter of the sheave can be changed while the unit is in operation. By combining two such sheaves in a drive unit, infinitely variable power transmission over a wide operating range can be obtained."

What's really interesting is that this description used for snowmobile clutches came out of a 1978 snowmobile service manual. And in the 30 years since that manual has been sitting in a shop collecting dust, not a lot of the basic concept has changed. "

Most torque converter units also act as the drive clutch, automatically disconnecting engine and drive train at a pre-determined engine speed. Most drive sheave units use the governor weights as the clutch engaging mechanism. When engine is at slow idle speed or stopped, the distance between flanges is slightly greater than the width of belt, and sheave is free to turn around the belt which rests on an idler bearing in bottom of sheave. As engine speed increases, governor weights are thrown outward by centrifugal forces, causing belt engagement and transmission of power to the driven sheave.

"Spring pressure on driven sheave must be great enough to maintain face contact with belt under all operating conditions, and in units which are primarily speed sensitive, must apply enough pressure to prevent belt slippage throughout the operating range."

Basically, you have a primary engine pulley (attached to the crank shaft), secondary pulley (attached to the drive shaft) and the belt. As the engine spins the crank shaft, weights inside the drive pulley are thrown outward, which forces the pulley's sheaves to pinch together, squeezing the belt and forcing it to spin. And at the same time, the belt is forced to ride higher on the sheaves, which in turn changes its egg shape from small-to-big to big-to-small. (Keep in mind, before the sheaves where pinched together, the belt rested in the bottom of the primary pulley, slipping between the spinning sheaves while the other end of the belt was pinched between the top of the sheaves of the driven pulley.)

As the belt spins and its shape goes from small-to-big in the drive pulley, it is pulled deeper into the driven pulley. With the tension on the belt now on both pulleys, the driven pulley must spin to match the drive pulley . and this causes the jackshaft to spin and the gearing reductions to come into play, which ultimately determines how fast the drive shaft turns the track.

Throughout the process, there are variable ratios changing-initially with the pulleys changing from big-to-small to small-to-big.

This is still the same. Clutches still function the same way now as 30 or even 50 years ago. Although there have been some variations to the drive system between the clutches and the track, such as direct drives (in the 1980s) and more recently the Diamond Drive system used by Arctic Cat, the only difference in the clutches has been refinements in design and materials.

"I can't believe snowmobilers are still using centrifugal drive clutches with belts," explained Max Maxedon at Tri-City Performance in Centerville, UT. "We've seen vast improvements in engines, ignition systems, fuel systems . but the clutching is basically the same. We've gone from the days of 55-horsepower snowmobiles to 160-horsepower snowmobiles with basically the same clutch system."

Maxedon is quick to point out the clutch systems of today are better than they were 30 years ago . it's just that the technology hasn't changed.

"There have been improvements, but basically, it's the same stuff," he explained. "We have better backshifting. Clutches control engine rpm a lot better. They are more dependable. We have improved the rollers and the bushings. The belts are better. Helixes are better. Clutching system parts are all improved. But it's still all the same."

Maxedon also pointed out that the changes are basic "Band-aids" to a system that is becoming antiquated. He said he's surprised with all the advancements in snowmobile technology that the clutches are virtually the same now as they were 30 years ago.

Maxedon said that just like most other snowmobile enthusiasts, he tries every new gizmo that comes out for clutches. "And they all tend to have their place and they all tend to improve the system. But it's still the same basic technology . just better ways to try to harness the horsepower."

So whether you're reading the latest on clutch tuning or an old service manual from the 70s, it probably doesn't matter. Until there's a major breakthrough in drive system technology, we'll still have two pulleys connected by a belt that make the snowmobile go.

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