Scooter CVT

Open the throttle on an Activa or a Jupiter and the scooter just goes, with no gear changes. The transmission inside the long cover on the left side is a belt CVT with a centrifugal clutch, driven straight off the crankshaft.

• The drive pulley on the crank, called the variator, has one fixed half and one that can slide. Behind the sliding half sit several roller weights on a ramp plate.
• At idle the rollers sit inboard and the belt rides low in the variator, so the ratio is low. The centrifugal clutch at the other end is retracted and the rear wheel is free.
• Twist the throttle: revs rise, the clutch shoes fly outward and grip the clutch bell, and the wheel starts to turn. The rubber V-belt now carries drive to the rear pulley.
• As revs climb further, the rollers are flung outward along the ramps, pushing the sliding half inward and squeezing the belt to a larger radius. At the same time a torque spring in the rear pulley lets the belt drop lower there. The ratio rises smoothly.
• The clutch bell drives a small set of reduction gears in the hub, which turns the rear wheel.

Close the throttle and the process reverses; at walking pace the clutch lets go and the scooter freewheels.

What you notice: nothing to do but throttle and brakes, a slight engine flare before the clutch bites, and a flat engine note as the ratio does the work. Ratio behaviour is tuned by roller weight and spring stiffness, which is why scooters of the same engine size can feel very different. The belt, rollers and clutch shoes are scheduled service items.