It’s the job of your ATV’s primary “drive” clutch to spin on the engine shaft and engage the drive belt. When the clutch’s sheaves compress the belt enough, it starts the transfer of power to the secondary “driven” clutch and the rear transmission shaft. 

When the drive clutch spins, it creates centrifugal force that enacts on the shift weights mounted to its mobile sheave. These oblong rounded wedges push away and out from the center of the clutch, applying force to matching rollers mounted on the immobile “spider.” The shift weights are responsible for the actual movement of the primary clutch sheave. 

So why doesn’t Polaris just build their primary clutch with the correct weight to shift itself as it spins? Why have the weights as separate parts all their own? It all comes down to tuning.

The weights in your drive clutch are key to tuning your vehicle’s peak RPM, shift speed, torque, and horsepower. Without them, also we wouldn’t be able to properly compensate for other adjustments to your ride like aftermarket springs or larger wheels

Many modern clutch weights are adjustable, using a series of holes and weighted magnets or a central mounted screw to balance the different segments. Others may use solid weights, requiring the whole weight to be swapped when tuning. Look for a clutch weight chart in your vehicle’s manual like this one from an older Polaris model.