Fighting Friction

The history of rollerizing clutches
Today's rollerized secondary clutches have raised snowmobile performance to a new level. Smooth and consistent power delivery is necessary to keep the snowmobile moving in the very high drag environment it encounters.

The quick reactions to changing load conditions are now taken for granted, as the clutches efficiently up and downshift out of corners and up hills. With the introduction of the new primary roller clutches, the efficiency and response is improved even further.

Past friction
Looking back 30 years, it becomes obvious that most of the improvements in clutch performance have been the result of reducing or eliminating friction in the system.

When I started out as a young project engineer back in the early 70s, one of my first challenges was to analyze the clutch system. The goal was to map out all the functions and then produce a computer program that would predict clutch performance. During the process of determining the forces on all the moving components it became painfully obvious that there was a lot of friction working against the shifting forces.

There were so many components rubbing against each other that it was amazing that those early clutches actually worked at all. The only saving grace at the time was the rather low power and wide torque curves available from industrial based two strokes with low exhaust ports and canister mufflers.

Racing Brings Rubbing
When racing arrived with tuned expansion chambers and narrower power curves, the early clutches were quickly found wanting. Tuning components often consisted of blocks, garter springs or kidney weights sliding against steel surfaces.

This was quickly remedied by introducing flyweights, rollers and cam surfaces to make tuning more accurate. Torque had to be transferred from the drive shaft to the movable sheave. Early designs used a splined shaft, or hex bushing, which produced a notoriously high resistance to the movement of the primary sheave.

This was due to the small radius of the spline, which would in fact multiply up the torque force. The further away from the center the torque transfer point is, the less force is necessary to transfer the same amount of torque and less friction is then generated.
Buttoned Spiders
A large step forward in reducing shifting friction was achieved in moving the torque transfer point as far out from the center of the shaft as possible. The spider tower sliding buttons were the result. There was still sliding friction between the buttons and the moveable sheave surfaces, but the force acting on the buttons at a 4 inch radius was 8 times less than the force acting on the splines at the 1/2 inch radius. The large radius placement of the torque transfer buttons is now universal in all primary clutches.

With the introduction of the torque feedback helix on the secondary clutch, efficiency took a big step forward. The basic design of the snowmobile variable ratio belt-transmission has remained the same since the 70s, but great steps in efficiency have been made in the last 15 years by reducing or eliminating friction forces.

In the primary clutch there are friction forces from the bushings sliding on the shaft, and the spider buttons sliding against the moveable sheave. There is also friction in the rollers and the flyweight pins. When the shaft bushings get worn, the moveable sheave gets thrown out of center by centrifugal forces and this causes binding on the spider buttons, resulting in loss of performance.

In the secondary clutch there is friction on the shaft from the sliding bushings, and also friction between the helix and sliding buttons. Friction is not constant, it varies between static friction and sliding friction. Physics teachers like to show an experiment where a block sits stationary on a tilted plane.

As soon as you push the block and overcome the static friction holding it in place, it slides down the tilted plane. Static friction is considerably higher than sliding friction, and this shows up when the sliding clutch part stops and then changes direction. Because of all these variables, it is difficult to predict all functions with an acceptable accuracy that would give a satisfactory final result in a computer analysis.
Building Better Belts
Although we gave up on the computer program, the analysis gave us a much better understanding of the working functions in the variable ratio belt system. This gave us some definite high priority areas to attack later on when we again were looking to improve clutch performance.

Most people are happy with their clutches until they experience something that is much better, or they are forced to look for new solutions because the transmission is unable to handle new and higher power levels.

This was the case in the mid-80's, when we started racing SCCA sports cars. We chose a class where snowmobile engines and transmissions were allowed (D-Sports). We reasoned that this would be a good way to develop engine and clutch components in the summer season. As it turned out, the clutches and belts could not handle the combination of a 1100lb. car on sticky tires in hot summer temperatures.

Being stubborn rather than reasonanble, we decided to try and solve the problem, rather than switch to a regular gearbox. First we had to de-tune the engine to 125 hp to make the belt last. To be competitive we needed to transfer 185 hp, an improvement of 50 percent in clutch performance. The project was divided into improving clutch performance and to improve the belt design. Belt development was done in partnership with Jim Lewis at Dayco, and resulted in a new top cog belt design that now is used on many snowmobiles.

To handle the power, the belt finally grew to 1.625 inches in width. Although this wider belt can handle 200 hp in a car on sticky tires in the hot summer, it can transfer over 300 hp in a lighter snowmobile in the cold.

If you can improve efficiency you can also transfer more power, because less power is lost in heat to the belt and clutch components. If the belt spins on the sheaves, heat is generated. This not only reduces power, but also heats up the belt. When the belt heats up, it will eventually reach a point when the rubber and cords separate and fail. We kept close watch on the belt temperature when we checked tire temperature at pitstop after sessions. This gave us a good picture of how changes to the transmission components affected the transmission efficiency.

Rolling Advancements
One of the big problems was increasing lap times and falling engine rpm as the race went on. As the secondary helix buttons got hotter, and more dust gathered on the helix cam surface, the transmission performance deteriorated. We would increase as much as 5-10 seconds a lap in lap times from the beginning of the race to the end. By the end of the race we could be over a minute behind, which often meant we got lapped. Reducing the friction on the helix was our first step. We decided to teflon coat the helix, and this resulted in less lap time increase from the beginning to the end of the race.

Reducing friction definitely was a step in the right direction. How about if we could eliminate friction altogether? To do this we would have to eliminate the sliding button and install a bearing instead. We remachined a Polaris secondary and installed three cam follower needle-bearing rollers to run on the helix surface. This did not meet with immediate success as the rollers chewed up the aluminum cam.

We had a steel cam available for use in desert races with dune buggies, and a case hardened version turned out to do the trick. Eliminating the friction by using rollers was a great improvement. Acceleration out of corners was harder and downshifts much quicker. Belt temperatures dropped, the clutches ran much cooler, and the laptimes stayed constant through the race. By eliminating a 15 percent friction source, efficiency had improved considerably.

We were not sure if this would transfer to an improvement on snow, but when we tried it in the following winter, improvements were considerable. Since then secondary roller clutches have seen increasing acceptance in snowmobile applications.

Rolling Spiders
Since rollerizing the secondary was such a big improvement in performance, how about rollerizing the primary and getting rid of the sliding buttons on the spider tower? Would this result in too little friction, and would the two clutches then fight each other? A lot of questions needed to be answered when we started on this project back in 1995. Our first prototype was a modified Ski-Doo TRA clutch. We removed the button towers and welded in place plates to mount three rollers running in the button channels. Plastic rollers with ball bearings inside were used. The roller bearings would insure that there was no friction, and the plastic outside would absorb engine vibrations.

The test was successful and showed great improvement in acceleration and downshift. An extra bonus was a very smooth engagement, which we had not experienced before with any clutch system. Unfortunately, we had to abandon the TRA project, because our welded up assembly constantly broke.

Making a new casting would limit us to only Ski-Doo applications. Instead we chose the Comet 4 Pro clutch for our second try. Roller passages were machined in the moveable sheave, and rollers mounted on the spider.

The rollers have a small clearance with the passage, so they can only touch on one side at a time. This proved to be a successful combination and the unit has been sold to snowmobilers for four seasons to date. Comet is continually updating the 4 Pro and in 2000 they redesigned and strengthened the movable sheave by adding a crossrib under the flyweight pin and increased the material thickness in this area.

Customers using the roller primaries have made several interesting observations. One eastern driver drag racing a 1000cc triple claims to have gained two sled lengths off the line by lowering the engagement because the roller clutch engaged so smoothly.

Data recording of the engine rpm during shifting has shown that a regular button clutch has rpm variations of 100 rpm in a sawtooth fashion during the upshifting due to friction varying between static and sliding values. The rollers eliminated this and showed only a small 20 rpm variation probably due to engine vibration.

The smooth engagement and quick upshift of the primary roller clutch is a new feeling of quality not available before. How much friction force is there really between the buttons and the moveable sheave? By comparing the roller clutch to a similar tuned button clutch, we had to use 46-gram weights in the button clutch and only 42-gram weights in the roller clutch. This is a 4-gram reduction, and these extra grams were needed just to overcome the friction at the spider button surface.

Eliminating friction in the moveable parts of the primary or secondary clutches means the sheave pressure on the belt is constant, and preventing the belt from slipping. When the belt doesn't slip on the sheave, more power is transferred and less heat is developed. A friend built a large 1200cc triple, and had belt problems until he installed both a primary and secondary roller clutch. With both clutches rollerized, performance was excellent and belt life was good.

In the Real World
So how does the combination of roller clutches work together? Does it really pay to have both a primary and secondary roller clutch, or if you only can afford one, which one should you get?

We have done extensive testing with all combinations and here is what we found. The rollerized primary offers a unique smooth engagement, and the rpm changes on upshift are in the range of two to three times quicker than the button clutch. The secondary roller offers hard acceleration and quick downshift.

By running the two together you get both a hard shift out from the secondary roller, and since you rev much quicker to the power peak with the primary roller, this also improves acceleration. This is a powerful combination for a snocrosser or trail machine. A second good combination is a front roller with a teflon coated helix in the secondary.

This is a good combination for drag racing where the racers don't like rollerized secondaries because they feel they downshift sometimes when they meet resistance instead of continuously upshifting. This is also a good combination for trail riding if you can only afford one rollerized clutch. We found no disadvantages by combining front and rear rollerized clutches, and no tendency for the two to fight each other. They both add advantages in different and distinct areas.

More Challenges

So have we now eliminated all friction or are there still problem areas to be conquered? The biggest problem area remaining is the sliding bushing on the moveable sheave. This component has a hard life, constantly thrown around by belt and centrifugal loads.

When it wears, conditions get worse and the moveable sheave starts wobbling and may pinch the buttons. With rollers there is no pinching so there tends to be better performance for a longer period. All kinds of material has been tried for this bushing, and presently a fiber-backed bushing providing some cushioning seems to work best.

A correctly designed billet cover which adds stiffness seems to reduce the load on the movable sheave bushing by transferring more load to the cover bushing, and this is enhancing the performance of the moveable sheave and cover assembly.

More work is probably needed in this area as power demands increase.
Clutch development has gone from reducing friction by design, to eliminating it completely with rollerized primary and secondary clutches. Rollerized secondary clutches are already standard on most brands and we predict that rollerized primaries will be available as standard in the near future.

In the meantime, the aftermarket is there to fill your needs and advance the technology.
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