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novel chain drive for electric bicycles

28 November 2011

German transmission chain manufacturer, iwis has introduced a chain drive system for electric bicycles, which it developed in co-operation with its customer and close neighbour, Clean Mobile. The hybrid drive comprises a 1.2kW motor, delivering a maximum torque of 150Nm, and an overall efficiency of 80 percent. Michael Frank reports

Only three years ago iwis chains were rarely seen on electric bicycles. At that time, iwis’ customer and close neighbour Clean Mobile was just a three-man start-up with a bright idea to develop a new drive system for an electric bicycle and through its previous contact with the company, iwis became involved in the project.

The design specifications for the new drive included an exceptionally small but powerful electric motor running at high speed - even at low riding speeds. The motor was to be mounted near the pedals and its power transmitted to the rear wheel to maintain optimum efficiency. The target application was a cargo three-wheeled cycle with a total weight of 400kg, and the drive was required to comply with Pedelec (Pedal Electric Cycle) legislation, which specifies the requirements for electric power assisted bicycles.

It quickly became clear that this goal could not be achieved with conventional reduction gearing, so iwis technicians and Clean Mobile decided to split the gearing into primary and secondary transmissions. They also decided that the primary (reduction) gearing should take care of the speed reduction to pedal crank and allow standard components to be used for secondary power transmission.

Clean Mobile finally implemented the system that iwis proposed in a direct dual drive (DDD) system where the pedal crank and adjacent motor shaft were connected with three chain arrangements, together yielding a reduction ratio of 1:38. The effect of the teeth of each sprocket on each chain, and the resulting overall transmission ratio were calculated in several stages, allowing suitable chains to be specified from iwis’ range.

The combination of the three-chain arrangement reduces the motor speed from 3,600rpm down to pedalling speed. A freewheel hub ensures that the pedal force exerted by the rider is transmitted only to the wheel and not to the motor should the battery ever fail. A second idle arrangement disengages the pedals from the rear wheel, as on a conventional bicycle.

To the surprise of all project partners, tests by the department of drive control and actuator technology at the University of the German Armed Forces in Neubiberg near Munich reported an exceptional efficiency of approximately 80 percent across a broad operating range. To verify this figure, iwis repeated all measurements and was able to confirm the findings.
A major contribution to the drive’s overall efficiency is made by the chains, through which the tensile forces are transmitted only in the direction of travel. A spur wheel with helical gearing, used for noise reduction, would have induced additional, lateral forces and thereby reduced overall efficiency.

An electric bicycle equipped with the DDD system won the first official E-bike world championships, held in October 2010 at Intermot in Cologne, Germany – the world’s largest bicycle and motorcycle show. The eSpire-branded hybrid bicycle of Munich cycleworks Third Element won the competition with a massive six lap lead!

Following their successful co-operation, iwis and Clean Mobile formed a strategic alliance in 2011, covering mainly industrial parts production and assembly for both the entire DDD system and a harmonic chain drive (HCD). The partners are currently developing more powerful units and ‘de-tuned’ versions that use plastic parts, with customer inquiries from the two-, three- and four-wheel vehicle industries showing a keen interest in these new drives.

Michael Frank is new business development project manager at iwis, based in Munich, Germany

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