Delivering very positive drive
How Zero Motorcycles cut out the middleman to get power from the motor to the rear wheel
Zero’s SR/F naked streetfighter produces a remarkable 140lb. ft of torque from zero rpm. But the SR/F makes use of the firm’s Direct Drive system, meaning a direct connection from motor to rear wheel. How do they do that?
The missing link
The lack of an intermediate gearbox between the electric motor and the rear wheel allows the SR/F’s motor to propel you forwards with as much gusto as the rear tyre can handle. Minimising frictional losses by removing the need for a clutch and gears, this system also helps improve the bike’s effective range, thanks to it using all its expended energy for propulsion, and reaping the extra benefit of the reduced mass of the powertrain. By comparison, the average petrol motorcycle with a conventional transmission will lose 10-15% of its power between the crankshaft and the rear tyre.
Converting the charge
The motor controller converts the DC power of the battery into the AC source for the motor. The battery casing is a stressed part of the trellis chassis, while the exposed 110bhp motor is then bolted to the aluminium forged and cast rear chassis plates. The swingarm is then mounted on large plain bearings seated in the frame plates outboard of the motor, with final drive running directly from the output shaft. The 75-10 brushless motor is passively air-cooled using an aluminium heat-sink, further minimisng weight and servicing complexity. Its interior permanent magnet (IPM) is wrapped in a series of copper coils, creating a stator motor. Once energised, they generate magnetic flux, which interacts with the magnets sandwiched within the rotor inside a metal lattice, the effect of that polarity spins the output shaft to create drive to the back wheel. To tackle the torque, Zero use a motor controller to smoothe out the delivery. The Cypher III electronics package constantly evaluates, with support from Bosch’s MSC system, the available grip and rider throttle inputs, and regulates the delivery to provide a facsimile of the naturally building power and torque delivery you’d experience from a petrol engine and gearbox. Without that control, the SR/F’s delivery would overwhelm the back tyre effortlessly.