Torque in an Electric Motorcycle | Drawing the Line


If you’ve been following the development of electric motorcycles, you’re certainly aware that torque is a main topic of discussion. But torque is torque, whether it’s produced by your wrist turning a wrench, an internal-combustion engine or an electric motor. What’s different, then, about torque in an electric motorcycle?

An internal combustion (IC) engine doesn’t run much below its idle speed of 1000 rpm or so, and its torque builds to a maximum several thousand rpm later. An electric motor can make its maximum torque the instant power from the battery is applied, and that torque is essentially constant for several thousand rpm before it starts to taper off.

The IC bike needs a clutch of some kind to begin moving, because the engine shafts are rotating but the wheels are not—there has to be slippage of some kind to connect rotating parts to stationary parts. The eBike, however, with its full torque at zero rpm, doesn’t need a clutch. The motor begins to move at the same time as the wheels, and there is no slippage.

So the difference is not in the torque itself, but in when and how it is applied. The electric motor provides beautiful simplicity by eliminating the need for a clutch and a multi-speed transmission.

Some confusion, however, has been introduced lately about torque figures for eBikes. An IC engine’s output is listed as crankshaft torque, because the various gears and sprockets in the drive system multiply crankshaft torque; we’d need six torque figures for a bike with a six-speed gearbox. A typical liter-bike has an approximately 1.5:1 primary, a 2.6:1 first gear and a 2.5:1 final drive. Multiply 1.5 x 2.6 x 2.5 to get the overall first-gear ratio of 9.75:1. If our liter-bike has 80 lb.-ft. of torque at the crank, the torque turning the rear wheel in first gear is 780 lb.-ft., and there are different (lower) torque figures for each of the other five gears.

Since the eBike has just one overall drive ratio, perhaps it would make sense to use the rear-wheel figure since it won’t change as the IC engine’s figure would. Say the motor makes a motor shaft torque of 90 lb.-ft. and there is a 6:1 reduction to the rear wheel. Multiply 6 x 90 and we see a rear wheel figure of 540 lb.-ft.

The problem here, of course, is that if we are trying to compare the IC bike to the eBike, we might look at the data where the torque of the IC bike was listed at 80 lb.-ft. and the torque of the electric bike was listed as 540 lb.-ft. and get a totally distorted impression. "The electric bike has more than six times the torque of the IC bike!" That’s obviously not true.

The way the data has been reported for IC bikes is standardized, and we can make comparisons with some confidence that they’re valid. If we are going to avoid confusion about the figures quoted for eBikes, we need to also standardize their figures and use motor torque as the baseline.

There's more confusion about what maximum torque at zero rpm means for acceleration. Since torque's relation to horsepower is described by a simple equation (Horsepower = Torque x RPM / 5252), you can see that even though the eBike has max torque at low revs, there isn't much power at low revs. Our eBike described earlier with 90 lb.-ft. of torque is only making 17 bhp at 1000 rpm. Since horsepower represents the work that the motor can do, there isn't much work done at low revs.

This is not to criticize the eBike, as an IC bike wouldn’t be doing much at 1000 rpm either. We need a lot more data from eBikes to really see how they compare to IC bikes. An important reference will be acceleration times and speeds, and we’ve seen few of these so far. There have been some lap-time comparisons from the TTXGP races, but we’ll need to break down those laps into segment times to see where improvements in acceleration, cornering speed and top speed play their respective roles.

IC bikes excite us through their high level of development, but eBikes excite us because of their potential and the amazing pace of their advances. In weeks and months, we’ll see more change in electric bikes than we’ll see in years in the internal-combustion world.

Electric motorcycles are simpler mechanically than conventional internal-combustion bikes, but the nuances of comparing power output are complicated by the fact that they don’t have gearboxes.