Gear: June 2010 Archives

I heard back from the people at Mission Motors. They say that the bike will have about 136 hp and weigh around 480 lbs. For comparison, the Honda CBR1000RR, weighs about 440 lbs wet (gased up) and develops 178 bhp and 82 ft-lbs of torque. [*] (range is approximately 190 miles). So, these bikes are actually not that incomparable. Except for the estimated price of the Mission Motors superbike being upwards of 60k, that is...

Now that hybrid cars are a commodity item and there's starting to be more interest in electric cars, you've started to also hear about electric motorcycles. All the models I've seen had prohibitive performance, principally because the energy density of batteries is much worse than that of gasoline. Wikipedia says that LiOn cells have a density of about 500-700 kJ/kg compared to about 46000 kJ/kg. Since the weight of the gasoline and the engine is such a large fraction of the weight of the motorcycle, the weight of the battery is correspondingly more important.

Yesterday evening, I heard the CEO of Mission Motors on the radio, so I thought I would check it out. They're quoting a range of 150 miles for their sport bike with a quote of 100 foot-pounds of torque. (for comparison, the range of a typical sport bike is something like 200 miles, with about 40-80 foot-pounds of torque). Unfortunately, they don't list any of the other specifications, including vehicle weight or horsepower, which makes it hard to do a straight-up comparison with a gasoline-powered bike. I suppose one might argue that a straight-up horsepower comparison would be misleading, but weight is a pretty important consideration, so it's hard to know what to think of this bike without knowing that.

I've got mail into the Mission Motors guys asking for this stuff, (they're apparently claiming in public 30-40 lbs heavier than a comparable sport bike). I figured I'd try some back of the envelope calculations. Here are two attempts:

• The battery pack that powers the Tesla Roadster weighs around 1000 lbs and stores about 53 kw/h of power. The Roadster is based on the Lotus Elise platform and weighs about 800 lbs more than the Elise. The Elise gets about 25 mpg, so you'd need about 8 gallons of gas (64 lbs). You're looking at about a factor of about 15X in battery versus gas. Even if we assumed that the required power scales with mass (which is not even close to true, you'd be looking at a factor of over 10X.
• Wikipedia claims that a gasoline motor has about 20% efficiency at turning energy into fuel into power output at the back wheel. If we assume 100% efficiency of the electric motor, then given the 100:1 mass-energy density ratio of gasoline versus batteries, we would expect that batteries would weigh about 20X as much for the same amount of power delivered to the wheels. Sport bikes get about 35-45 mpg, so you're looking at about 3.5 gallons of gas (28 lbs), so we'd expect the battery to weigh 280-420 lbs.

At 35-45 mpg, 150 miles is about 3.5 gallons, weighing 28 lbs, so you're looking at between 280 and 560 lbs of batteries (with 280 being optimistic). The actual electric engine is pretty light, but you still have the chassis, wheels, etc., which probably account for about half the dry weight of the bike. So, my guesstimate is that you're looking at a bike which weighs between 450 and 700 lbs. 450 would be about average for a sport bike. 700 would be really heavy.