The continuously variable transmission, or CVT, is a common component in the powersports industry. This is the transmission that has given scooters and snowmobiles the ability to be “gas and go” for decades. The CVT has also found a home in ATVs where a simple automatic transmission is desired.
The vast majority of CVTs, on the road and off, are of the variable diameter pulley type. This consists of two sets of pulleys connected by a stiff rubber belt. The inside of the pulley faces form a V-shape. The gear ratio is changed by moving the two faces of one pulley closer together, and the two faces of the other pulley farther apart.
Due to the V-shaped cross section of the belt, this causes the belt to ride higher on one pulley and lower on the other. This changes the effective diameters of the pulleys, which in turn changes the ratio. The distance between the pulleys does not change, and neither does the length of the belt, so changing the ratio means both pulleys must be adjusted (one bigger, the other smaller) simultaneously in order to maintain the proper amount of tension on the belt.
In every system of this type there will be a drive and a driven pulley. The drive pulley is the pulley that receives power from the crankshaft. The power is then transmitted via the rubber belt to the driven pulley. The driven pulley delivers this power to a final drive in a scooter or a sub-transmission in an ATV. There will be a centrifugal clutch either before the drive pulley or after the driven pulley to allow the spinning engine to be disconnected from the wheels when stopped.
At idle, the drive pulley will be held open with a spring, creating the minimum diameter for the belt. At the same time, a spring will hold the driven pulley closed, creating the maximum diameter for the belt. This is the “low gear” of the CVT. As power is applied, a variator mechanism, often just a ramp with roller weights, will cause the drive pulley faces to close. This increases the diameter of the drive pulley and forces the driven pulley to open and reduce its diameter. This lowers the ratio simulating a higher “gear.”
Some variable diameter pulley CVTs are more complicated than others, with engine brake controllers and electronically controlled ratios, but there are some basic maintenance tips that apply to all.
The belt condition is important. If the belt is fraying, it must be replaced. The slack or deflection of the belt cannot be adjusted like a motorcycle drive chain. The belt must be replaced if the deflection is out of specification.
Most belts are unidirectional. The belt must be installed to rotate in the correct direction.
The pulley faces and belt must be clean. If any substance, especially oil or grease, has contaminated the belt or pulley faces, the CVT will not function correctly. Wear clean gloves when handling the belt and pulley faces.
The pulley faces must be free of damage and excessive wear. If the pulleys are scratched or warped they should be replaced. MPN