Tech: Why gears are old hat
Electric bikes turn torque into forward motion without the need for weighty cogs and clutches
‘There’s no need for the extra bulk’
Electric bikes might still only sell in fairly small numbers in the UK, but it’s a market that’s growing fast. So if you haven’t had the chance to ride one yet there’s a pretty strong chance that you will in the next few years. When that time comes there are a few readjustments to make, none more significant than the loss of the traditional clutch and gearchange as the vast majority of electric bikes have one, fixed gear, which gives a seamless spread of power and torque from very low speeds.
To understand how they do this, and why petrol-powered bikes can’t, you only need to take a glance at their torque curves. An electric motor, whether it’s in a bike or your washing machine, is capable of its maximum torque at minimal revs. It’s the marketing line that virtually every electric bike firm uses: “Instant torque and power from 0rpm,” says Zero Motorcycles’ marketing flannel. And it’s true, of course, but with a constant input of electric energy; defined by the battery, an electric motor’s torque drops away in a linear fashion as revs rise. The torque curve looks more like a diagonal downward line, with the peak at zero rpm and a steady drop as revs rise. In contrast, a petrol engine’s torque starts low and bulges to a peak as revs increase, dropping off again at the highest reaches of the rev range as torque is sacrificed for engine speed and outright power. Cast your mind back to the physics classroom and you might recall that power is force multiplied by velocity. An electric motor’s power remains fairly constant (governed by the energy it’s fed from the battery and speed controller) so as its velocity (rpm) increases, its force (measured as torque, which is rotational force at a defined distance) decreases. Multi-speed gearboxes operate differently but do the same job – converting input power into appropriate torque or rotational speed at the rear wheel. They’re torque multipliers; low gears trade rear-wheel speed for more rear-wheel torque, high gears move the balance in the other direction, giving more speed but reducing torque at the tyre. That’s why acceleration is blunted in higher gears. Since electric motors already make the same trade-off, there’s simply no need for the extra weight, bulk, cost and complexity of a superfluous multi-ratio transmission and clutch.
Not that there are any rules, of course. Electric bikes with conventional gearboxes do exist, most memorably the Brammo models (which were briefly rebranded as Victory). However, in most instances it’s a move that does more to provide a comfort blanket of familiarity for petrol bike riders than one that actually improves performance.