This is one of the fundamental principles of Solex. What is the connection between the engine and the tire? This is the roller! We offer rollers for Solex 3800 5000 European made. The clutch him, used to stop your VeloSolex on red except having to stop your engine. The acceleration out of it departs and transmits rotation of the crankshaft is the principle of the centrifugal clutch.