Electric Motorcycle Wish List

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Ask Kevin Cameron (Cycle World/)

In response to my remarks about Kawasaki’s two around-town electric motorcycles, reader Sprchkn deplores their low top speed, hankering for something more like Team Green’s Z125, or just over 60 mph.

What Sprchkn is seeing here is design compromise. Clearly, Kawasaki wanted something with the appearance of a proper motorcycle, but at a weight easy for beginners to manage: 300 pounds. To make the removable batteries possible to carry to an indoor charger, they were kept to a weight of 12 kilograms each, or about 26 pounds. Their total energy storage capacity is quoted as 3kWh, which translates to 4 horsepower-hours. In order to provide a useful range of 41 miles, top speed had to be governed to prevent operator exuberance from exhausting the batteries in a much shorter distance.

More range and speed means more weight in an electric motorcycle. Kawasaki gave its new electrics limited range and speed for a beginner-friendly weight. (Kawasaki/)

Reader Hmmcycleworld understandably wants more power—say, 20 hp—and a 300-mile range.

Kawasaki could have provided a higher governed maximum power for these electrics, but that would have considerably shortened their range (stock power is about a third of this).

As for the 300-mile range, let’s do the arithmetic. If we stay with the present governed power but want to travel 7.3 times farther (300 ÷ 41) we will need 7.3 times greater battery weight. Since the two 26-pound removable batteries weigh a total of 52 pounds, a rough estimate of a battery storing 7.3 times more energy is 7.3 x 52 = 380 pounds.

OK, that’s pretty breathtaking, so let’s imagine that the removable batteries contain a bunch of excess weight in the form of an attractive structural outer case, electrical contacts, and other parts, such that the weight of the actual lithium-ion cells is only 80 percent of that, giving us 0.8 x 380 = 304 pounds. Since the quoted bike weight is around 300 pounds including the two removable 26-pound batteries, we must subtract their 52-pound total weight from the above 304 pounds, giving us an estimated battery weight of 304 - 52 = 252 pounds of battery to give our bike the desired 300-mile range at the original governed power level. Now our electric bike’s weight rises from 300 plus 252 to a total of 552 pounds. Will that be OK for beginners?

Or we could work backward from the 1,200-pound weight of a Tesla 85kWh battery. The two batteries in these Kawasakis total 3kWh of energy storage, and we found above that we’d need 7.3 times more energy for a 300-mile range, or 3 x 7.3 = 21.9kWh. As that is 25.8 percent of the energy in the Tesla 85kWh battery, we multiply 0.258 times 1,200 to get 309 pounds as our rough estimate of battery weight.

How can I be sure of the numbers for the Kawasaki batteries? They quote the batteries as 50.4 volts each, and “30 x 2″ ampere-hours. OK, we learned in high school that volts times amps equal watts, so 30 x 50.4 = 1,512 watt-hours, or 1.512kWh for each battery, totaling 3.024kWh.

For a further crude check we could consider the Energica Ego electric racebikes built for Dorna’s electric roadracing class. Their claimed weight was 589 pounds and they were tasked to go a distance of 35 kilometers, or 21.7 miles. A MotoGP bike without its 145-pound engine weighs about 200 pounds, so the difference between 589 and 200, or 389, would be approximately the combined weight of battery, traction motor, and power supply. This is a compromise made in the direction of high power—170 hp.

Kawasaki’s new electrics are designed to be light enough for first-time riders, and to allow the batteries to be carried indoors for remote charging (instead of trying to drop an extension cord out of a third-story apartment window to a bike parked on the sidewalk below). Yes, you could double battery weight to 104 pounds to achieve an 82-mile range at the 6.7 hp governed power level. But then indoor charging becomes quite a chore.

If indoor charging in a multistoried apartment building is the goal, batteries can’t be prohibitively heavy. (Kawasaki/)

Bear in mind that around-town power consumption is strongly dependent upon weight, as vehicles are constantly starting and stopping. The more battery weight you choose to add, the greater the amount of its energy will be consumed in just moving the batteries. Kawasaki chose a compromise aimed at first-time motorcyclists.

Pick the compromise that pleases you best, and build your own winning product.

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