Why new Zero tech could make electric bikes more appealing
Zero’s Direct Drive system means maximum efficiency between your wrist and the rear wheel
Santa Cruz-based electric motorcycle firm, Zero, pulled the covers off their new SR/F naked streetfighter last week, a bike that produces a stonking 140ftlb 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?
No interference
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 bikes’ 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 1015% of its power between the crankshaft and the rear tyre. Zero’s Brian Wismann, revealed that: “The motor controller converts the DC power of the battery into the AC source for the motor. Ten years ago, people built DC systems for motorcycles, but no consumerfocused electric vehicle is using just a DC motor anymore.”
Converting the charge
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 exiting direct from the motor’s output shaft.
The 75-10 is Zero’s latest Z-Force brushless motor and 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 turn the belt drive to the rear wheel. To tackle the mountain of torque (it kicks out 50ftlb more than a Panigale V4 R, all of which is technically available immediately in a flat torque curve) Zero use a motor controller to protect the rider from unintentional exuberance. 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 natural 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.