SnoWest

22 SNOWEST JANUARY 2017 Continuous Variable Transmission systems are used on everything from cars to side-by-sides these days, but they evolved primarily on snowmobiles. CVTs that are used in cars are very different than those on your sled. Car CVTs typically use a steel belt and run in an oil bath with electronic controls on the actuation. There would be a big weight and cost penalty if we used that kind of CVT on snowmobiles. Snowmobile CVT systems are com- prised of three main components, the primary or drive clutch, the secondary or driven clutch and the belt. Belts have evolved from a polyester fiber, reinforced rubber construction to the expensive aramid fiber with rubber compounds and top cogs that you see today. These modern belts can take more heat and transfer the big horse- power that machines can produce these days. Belts can withstand temperatures of 180 degrees F for extended periods; in the past this was not possible. Drive or primary clutches are an rpm-sensing device consisting of a sta- tionary sheave (closest to the motor) and a movable sheave. The movable sheave acts initially as a clutch like in a manual transmission when it is engaging. There is clearance between the sheaves and the belt until the rpm of the motor is high enough to make the movable sheave close onto the belt. This is engagement rpm. There are components called weights in the drive clutch that operate by centrifugal force to close the sheaves together. The drive spring opposes this force. Once the belt is engaged, the moveable sheave continues to try to close and push the drive belt away from the center of the clutch. By mak- ing the drive pulley "bigger" and thus the driven pulley "smaller," the system is effectively changing gearing. DRIVE SPRINGS Drive springs are a critical tuning component of the drive clutch. By changing the preload of the drive spring, you can change the engagement rpm. This is the first number on a drive spring rate. The second number on a drive spring rate is the finish rate. This number tunes the top rpm by control- ling the upshift rate of the drive clutch or how fast it changes gears. Because this spring operates in a very hostile environment with motor vibration and harmonics, it is important to keep fresh drive springs in your snowmobile to ensure constant performance. The centrifugal member of the drive clutch may be a flyweight, a slider puck or a lever arm with a roller on it. The faster a drive clutch spins, the more force this component applies to close the movable sheave. Most drive clutch- es use flyweights of one configuration or another. The traditional design is the Polaris P-85. Flyweights can be tuned by changing them for heavier or lighter weights or by adding or sub- tracting mass in them such as in the Venom Products Rooster adjustable shift weights. These weights allow tun- ers to adjust the location of the mass in the flyweight as well, to add belt pinch at low rpm or increase shift ratio at high rpm. INNER WORKINGS OF A CVT BY GLENN DOHRER Ski-Doo drive clutches have had a novel feature in that the force that the movable sheave exerts on the belt can be changed quickly in the field. The TRA clutches use an eccentric bolt that moves the ramp that the lever arm acts on. By changing a TRA clicker setting, you can change the running rpm quickly without disassembling the clutch. The new pDrive from Ski-Doo is a flyweight clutch and uses a mecha- nism that moves the pivot point of the flyweight to adjust rpm. Driven clutches are a load-sensing rather than rpm-sensing component of

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