YAMAHA LINGYING 125

The primary pulley in a motorcycle automatic transmission is a core component, especially in continuously variable transmission (CVT) systems, where it automatically adjusts the gear ratio by varying its diameter. Below is an analysis covering six aspects: structure, operating principle, technical features, application scenarios, advantages and disadvantages, and maintenance recommendations.

I. The Core Structure of the Drive Wheel
The driving pulley of a motorcycle's CVT system typically consists of the following components:

Fixed half-wheel: Directly connected to the engine crankshaft, providing essential support.
Movable half-wheel: Designed to slide axially relative to the fixed half-wheel via bearings, allowing adjustment of the wheel diameter.
Centrifugal roller (Pulizhu): Installed in a groove on the movable half-wheel, it uses centrifugal force to drive the movement of the half-wheel.
Spring assembly: Provides preload, controlling the initial position of the roller and ensuring smooth gear-shifting responsiveness.
Example: Scooters such as the Honda PCX and Yamaha NMAX use this structure, where the drive wheel is connected to the driven wheel via a belt, creating a continuously variable transmission system.

II. Working Principle: Centrifugal Speed Control and Diameter Variation
Low-speed/Starting phase:
The engine speed is low, resulting in small centrifugal force from the centrifugal rollers. As a result, the movable half-wheel moves closer to the fixed half-wheel, and the diameter of the driving wheel is relatively small.
The driven wheel maintains a large diameter under spring pressure, resulting in a high gear ratio (similar to a low gear), which delivers high torque output—perfect for effortless starts and hill climbing.
Acceleration/Medium-Speed Phase:
As the rotational speed increases, the centrifugal force on the rollers grows, pushing the movable half-wheel outward and causing the diameter of the driving wheel to expand.
The belt was forced to move outward, reducing the diameter of the driven pulley and lowering the transmission ratio, which smoothly increased the vehicle's speed.
High-speed cruising phase:
When the engine reaches its maximum RPM, the driving pulley has the largest diameter, while the driven pulley has the smallest—resulting in the lowest transmission ratio (similar to a high gear), which enables efficient cruising.
When decelerating to reduce oil flow, the centrifugal force on the roller weakens, allowing the spring to return to its original position, which in turn increases the transmission ratio again—preparing the system for the next acceleration phase.
III. Technical Features: The Core Advantage of Stepless Transmission
Continuously Variable Transmission Ratio:
Unlike the fixed gears of a conventional transmission, a CVT achieves smooth adjustment of the transmission ratio by continuously varying the diameter of the drive pulley, resulting in seamless power delivery without any jerks or interruptions.
Adaptive road conditions:
The system automatically adjusts the transmission ratio based on engine speed, throttle position, and vehicle speed, eliminating the need for the driver to manually shift gears—making it especially ideal for congested urban driving conditions.
Structure simplified:
By eliminating complex components like the clutch and gearshift mechanism, it reduces failure rates and keeps maintenance costs lower than those of traditional manual transmissions.
4. Application Scenario: Standard Equipment for Scooters
The CVT system has become the mainstream choice for scooter motorcycles due to its ease of operation and high level of comfort.

Urban commuting: In scenarios involving frequent starts and stops, continuously variable transmission can reduce driver effort and enhance the riding experience.
Beginner-friendly: No need to master shifting timing—lowering the learning curve.
Comfort requirement: Linear power delivery to eliminate the jerky sensation typically associated with shifting in conventional transmissions.