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Belt drive

Full automation instead of full employment, just step on the gas instead of laboriously sorting gears: a variomatic transmission takes over the shifting work on scooters.

Rainer Baumel

07/02/1996

Instead of gears and claws, flyweights and conical disks work in the scooter gears – for the benefit of people, because they free them from annoying switching and coupling work. Especially beginners on two-wheelers with a class 3 or 1b driver’s license appreciate this service, as the unfamiliar subset requires sufficient attention. Especially in dense city traffic, where you have to constantly blink, brake, change lanes or accelerate.

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A scooter with a variomatic gearbox and centrifugal clutch is the quickest means of transport, not least because of its ease of use (see MOTORRAD 11/1996: The fastest way to the workplace). One is firmly seated on the drive shaft, the other is rotatably mounted on the output shaft together with the anchor plate of the coupling. One of the two conical pulleys can be moved, a wide toothed V-belt transfers the engine power from one pair of conical pulleys to the other. When idling, the pair of pulleys on the output shaft is pressed together by a helical spring, so the belt engages the largest cone diameter there. Its length is dimensioned in such a way that it simultaneously pushes the pair of conical disks on the drive side apart and engages the smallest diameter there – the shortest gear ratio is applied.

Just above idle speed, the clutch anchor plate rotates so fast that the clutch jaws on it are pressed outward against the clutch drum by centrifugal force. This is firmly connected to the output shaft, the frictional connection between the input and output shaft is thus established. If the speed continues to increase, the flyweights of the variator gradually move outwards and push the conical disks together on the engine side. This forces the belt to have a larger diameter; the belt automatically pushes the rear conical pulleys further apart against the spring pressure – the gear ratio becomes longer and longer as the speed increases.

When braking, this chain reaction takes place in exactly the opposite way: the speed drops, the rollers of the variator go inwards, the coil spring pushes the rear pair of conical disks together, the belt pushes the front one apart – the gear ratio becomes shorter again. Finally, the coupling anchor plate rotates so slowly that the coupling jaws detach from the drum and the frictional connection between the input and output shafts is interrupted.

With the right choice of roller weights and spring stiffness, the right gear ratio is automatically and continuously set for every driving situation. Unfortunately, this simple type of power transmission has so far been reserved for scooters, which – admittedly – with their drive train swing arm (the motor sits on the rear wheel swing arm) offer the best conditions for this. But even with small-displacement motorcycles, such an automatic would be a realizable and sensible contribution to full employment and, moreover, cheaper to manufacture than the usual manual transmissions.

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