Vapaarullajärjestelmät

Vapaarullajärjestelmät, often translated as freewheel systems or freehub systems, are bicycle components that allow the bicycle wheel to rotate independently of the pedals. This is achieved through a mechanism housed within the rear hub. When the rider stops pedaling, the freewheel disengages, allowing the wheel to spin freely without the drivetrain turning. Conversely, when the rider begins to pedal again, the freewheel engages, and the pedals are once again connected to the rear wheel.

The primary function of a freewheel system is to enable coasting. Without it, the pedals would continue to spin whenever the rear wheel was moving, making it impossible to stop pedaling or to control the bicycle's speed by simply easing off the pedal stroke. This is crucial for efficiency and safety, especially when descending hills or navigating varied terrain.

Freewheel systems are typically integrated into the rear hub. Early designs were often separate units screwed onto the hub shell, known as freewheel hubs. Modern bicycles predominantly use freehub systems, where the freewheel mechanism is built into the hub body itself. This design is generally lighter and allows for a wider range of gear ratios to be fitted to the cassette. The engagement of the freewheel is usually accomplished by a set of pawls that mesh with a set of teeth, allowing rotation in one direction but preventing it in the other.