Toroidal CVTs

Another version of the CVT -- the toroidal CVT system -- replaces the belts and pulleys with discs and power rollers.

Photo courtesy Nissan Global Nissan Extroid toroidal CVT

Although such a system seems drastically different, all of the components are analogous to a belt-and-pulley system and lead to the same results -- a continuously variable transmission. Here's how it works:

• One disc connects to the engine. This is equivalent to the driving pulley.
• Another disc connects to the drive shaft. This is equivalent to the driven pulley.
• Rollers, or wheels, located between the discs act like the belt, transmitting power from one disc to the other.

The wheels can rotate along two axes. They spin around the horizontal axis and tilt in or out around the vertical axis, which allows the wheels to touch the discs in different areas. When the wheels are in contact with the driving disc near the center, they must contact the driven disc near the rim, resulting in a reduction in speed and an increase in torque (i.e., low gear). When the wheels touch the driving disc near the rim, they must contact the driven disc near the center, resulting in an increase in speed and a decrease in torque (i.e., overdrive gear). A simple tilt of the wheels, then, incrementally changes the gear ratio, providing for smooth, nearly instantaneous ratio changes.