Since the early 1970s, snowmobile clutch systems have used two pulleys with springs, weights, ramps and cams to calibrate shift rate; a flexible, durable belt allows the two clutches to work together. But a new method of transferring engine power to the ground is on the horizon. It will be more efficient and precisely calibrated for varying conditions, but a few questions remain, including which of the new technologies will the sled manufacturers employ and when will it show up under the hood of new snowmobiles.
First, The History Of Clutches
To understand how we’ll go forward, we first need to take a look back at snowmobile clutches. The first snowmobile built by Carl Eliason in 1924 featured an early, direct-drive design. Its little 2.5 hp outboard motor was linked directly to the drive axle of the track drive with sprockets and chains. When the engine was started, the suspended track turned as the engine revved to speed. The track was then slowly lowered to the surface of the snow and away went the sled. Drive trains don’t get any more direct than that!
Eliason’s later designs utilized twin-cylinder Excelsior and Indian motorcycle engines, which had a unit-constructed engine/transmission design. A manual clutch was also incorporated in the designs. In 1941, Four Wheel Drive Auto Company (FWD) took over production of the Eliason snowmobile.
After a few years, FWD designed a new model and sought to overcome one of the problems associated with a geared transmission: drag. When a driver disconnects the engine from a “stepped” transmission with a clutch to change gears, the snow pulls the speed down too quickly, and by the time the gear change is made the sled would need a lower gear to begin to accelerate again.
To solve that problem, FWD designed an all-new, rear-engine snowmobile with variable ratio, centrifugally governed drive and driven pulleys. FWD aimed what would become the snowmobile industry in the right direction when it installed that transmission system on its model K-10 in 1950. Today, we call the system a continuously variable transmission, or CVT.
In 1959, Joseph Armand Bombardier finalized what would become the first Ski-Doo, with one exception. Two hand-built machines made in 1959 utilized a centrifugal clutch similar to what you’d find on a chainsaw or a go-kart. Power from the clutch was fed to a two-speed, planetary transmission that was mounted at the top of the chaincase. You would start the engine and let it idle. Pulling down the lever on the chaincase would engage the low set of planetary gears. As the engine revved up, the centrifugal clutch would engage and the sled would move. As the snowmobile gained speed, the driver simply let the shift lever go back up and the transmission would be in high gear. The entire fledgling snowmobile industry was running centrifugally governed, variable ratio pulleys or CVTs by 1960.
A Giant Leap From Polaris
When I started working with snowmobiles in the late 1960s, the only significant improvement in the transmission system since 1950 was the addition of a cam on the driven pulley to control upshift speed and force a faster downshift. As we worked with increasingly powerful engines for the Halvorson race team out of Duluth, Minnesota, we all knew something big had to be done with the clutches in order to make use of the power out of the engines at increasingly high rpm and narrower power curves.
All of the high performance drive pulleys in use today employ concepts developed by Polaris, which were first used in the drive pulley on its TX models for the 1970 model year. Polaris’ new design allowed the clutch to be calibrated for engagement at any desired rpm and the upshift pattern could be calibrated to follow an engine’s power curve, no matter how “peaky” it was. Polaris’ new drive pulley allowed the use of engine designs that made less torque at low rpm but much greater horsepower at higher rpm.
There have been many improvements in the clutch designs since Polaris first introduced that major step forward in 1970, but the basic concepts of the design remain almost exactly as their original.
What’s Next? Electronics, Hydraulics and Servos
Rather than use flyweights, ramps and springs to control the shift, the next generation of CVTs will include the use of electronics to control the shift pattern of the system and hydraulics or servo motors to do the actual shifting.
There are three types of CVTs in use in various vehicle types. The ones we as snowmobilers are familiar with utilize variable ratio drive and driven pulleys, which are centrifugally governed. These pulley systems can also be controlled by hydraulic or servo motors. Several automotive designs of drive and driven pulley CVTs utilize metal drive belts rather than rubber. These variable ratio pulley systems are the most popular CVTs and show up on all types of equipment today.
Another type of CVT is the toroidal CVT system. This design utilizes two concave shaped discs; one is the drive disc and the other is the driven disc. Between them is a pair of rollers or wheels that, like a drive belt on our snowmobiles, transfers power from one disc to the other. The position of the rollers, which tilt to engage various ratios between the concave discs, is controlled by servos directed by the management computer.
Both the pulley-and-V-belt CVT and the toroidal CVT are frictional CVTs. They rely on friction between the drive belt and pulleys or between the concave discs and rollers to transfer power between the drive and driven aspects of the systems.
A third type of CVT in use is the hydrostatic CVT. This system consists of a variable ratio pump on the input side of the system and a hydrostatic motor on the output side of the system.
What will impact the use of these CVTs is the use of microprocessors to select the optimum gear ratio based on the torque output of the engine and the load on the drive axle so it can select the proper gear ratio based on the needs and power available.
It wouldn’t take many more sensors on our snowmobiles to gather the data required to measure the engine’s torque output at any throttle position and rpm, along with the torque requirement at the drive axle to determine the right gear ratio needed from the transmission in order to provide an efficient and economical ratio for the machine.
These systems will also enable riders to select their preferred performance level. A “high-performance” mode might allow a higher peak rpm and it would keep the engine’s speed near that range no matter how fast the vehicle is traveling. An “efficiency” mode would run at a lower engine speed, and an “anti-slip” mode for icy conditions would gear up faster to prevent track slippage.
Since the CVTs used on today’s sleds continue to work so well in most every application, there isn’t a real rush to adapt new technology to sleds. But once the cost of the new CVT systems comes down, you can bet the Big Four will move quickly to get the technology under their sleds’ hoods.
