Electric Bikes, Go Green, Not Mean - Drawing The Line

The bikes that left the starting line at the TTXGP, the zero-emissions race held this past June at the Isle of Man, were not spinning their rear tires. Their front wheels were not pawing the sky as they headed for the first turn. Their power was limited by the use of relatively small motors and high gearing, by undeveloped electronics and by all of the small problems that can bedevil new racing projects.

There are companies that are committed to changing that scene, to making electric bikes that enthusiasts will recognize as true sportbikes. What do they have to do to get there? What does electric power look like compared to the internal-combustion engines we're used to?

The dyno charts we've seen for years in magazines paint a familiar picture, showing in a graphic way how internal-combustion engines perform. Since an internal-combustion engine basically doesn't run below its idle speed and makes little power until considerably above that, the typical power/torque curves start at 2000 to 3000 rpm. From that speed to the engine's maximum rpm, torque builds and then declines in a gentle arc (with a few lumps and bumps).

Horsepower is related to torque as a function of rpm. Specifically, horsepower equals torque multiplied by rpm and divided by a constant: 5252. So, at 5252 rpm the torque and horsepower figures are equal and the curves intersect. From 5252 rpm upward, the horsepower curve climbs rapidly from the torque curve.

Power climbs to a maximum and typically falls slightly in the 1000 or so rpm between max power and max revs. The power, torque and rpm levels differ, but the basic shape of the curves for many different internal-combustion engines follow a familiar pattern.

The dyno curve for an electric motor looks very different. The companies working on high-performance electric motorcycles haven't yet released power and torque curves, but we can get a preview of what to expect from Tesla, a company building high-performance electric cars. The graph shown here plots the Tesla roadster's power and torque, and gives us a sense of what an electric motorcycle's power and torque curves might look like.

The Tesla produces maximum torque when the motor starts from a standstill, and almost that same torque until 6000 rpm, when it begins to fall off. But an electric motor can rev far, far beyond its torque and power peaks. Max torque from zero rpm means an electric vehicle doesn't necessarily need a clutch, and its ability to rev far past peak means it can also do without a multi-speed transmission.

The Tesla is a single-speed and so, in addition to rpm at the bottom of the graph, road speed is also charted. With just one gear, rpm and road speed are always directly linked (until you get wheelspin).

With over 200 lb.-ft. of torque until 6000 rpm (about 54 mph), and about 250 bhp at 8000 rpm (72 mph), the little Tesla is very fast (for a car) from zero to about 80 mph. Torque and power fall rapidly from there to top speed, meaning that, as good as midrange is, high-speed acceleration suffers. The car has a kind of split personality: supercar acceleration changing to family car top end. At 120 mph or so, the Tesla is all-in.

Torque and horsepower curves tell the story. Internal-combustion engines are weak on the bottom and strong on top. Electric motors are at their best on the bottom to midrange, and progressively weaker at high revs. Does it have to be this way? Is there a way to improve the electric's high-speed performance?

Looking at the Tesla's graph, you can see that if it were possible to not rev the motor out so far, it would not lose so much power and torque. If you limited the revs to 9000 rpm, for instance, the power curve would look much like that of an internal-combustion engine. But with a one-speed box, that would give you a sub-90-mph top speed.

Solution? Install a multi-speed gearbox. The motor's great bottom-end torque might mean that you would only need two or three gear ratios to keep your good initial acceleration and also have a high top speed. Tesla engineers planned for a two-speed but had problems in its development, opting instead for the simple one-speed and thus asking their buyers to accept compromised high-speed performance. Will future buyers of high-performance electric motorcycles accept that kind of compromise?

A lot will depend on competition in the marketplace, and on the track. High-performance electric motorcycles aren't in the market yet, and early models may, like the Tesla, be single-speed. But as more models come to market and competition intensifies, we're sure to see electric bikes with more power, more refinement and multi-speed gearboxes.