Electric motorcycles

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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.

5 Comments

I've been reading about these too, and my response is always confusion. I'm confused as to why they're aiming for sportbikes first - why not start with something where weight and performance are not the defining characteristics for the bike, like a cruiser or a scooter?

When calculating your comparison, you have to remember that regenerative braking/coasting is also a factor for most electric vehicles in generating their mileage estimates, and therefore also in how they size the batteries, so it may not make sense to do a straight stored power vs weight comparison.
I've seen a few electric cars that actually have an adjustable setting for how much engine braking they produce, and I would think that on a motorcycle that would be roughly equivalent to having or not having a slipper clutch. It might even be possible to simulate a downshift by having a spike in the engine braking threshold that gradually decreases as the vehicle slows.

Also, the difference in the impact of accessories in the overall power drain is probably significant in something the size of a motorcycle when determining the raw power requirements - motorcycles don't have: heater/ac (both of which require a compressor if you have no hot coolant to generate heat), radio, satnav, power windows and door locks, more headlights and tail lights, brake booster, power steering, etc. so the baseline power drain (independent of your speed and acceleration) also would be a factor.

I would think that motorcycles would be a good candidate for the hybrid tech that they're testing in auto racing, which is usually some sort of large capacitor or inertial storage device to provide a temporary boost in power for a comparatively small increase in weight. If nothing else, it might be a way to reduce the size of the batteries.

The state of the art electric motorcycles raced at the Isle of Man last week, the winner (MotoCzysz E1PC) running the 37 mile public road course at average speed of 98 mph. I think Mission entered, but didn't show.
An interesting interview with the builder here discussing the present and future of electric bikes here - http://hellforleathermagazine.com/frontlines/Czysz2010/czysz-lyt.html

I've thought for a long time that once we get good enough tech, it is a lot more likely we'll synthesize octane from CO2 to run vehicles than that we'll use batteries. Battery technology just isn't going to get very far compared to the energy density of octane.

If I had a nickel for each time I came to www.educatedguesswork.org.. Superb post!

If only more than 34 people would hear about this!

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