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speed vs. distance? Answered

Hello All, I am considering building a electric motorcycle in the next year, all the Instructables I have read on the subject boast top speeds of 70 mph and a 10 - 20 mile range. I live in a small city in Canada where the maximum speed limit is 50KMh(~30mph) and the city is not even 7 miles across, also there is nothing outside the city that is close enough to get to with a ev such as this. My question is how does the speed and distance connect, for example, if I was to travel at half the speed would I be able to expect to get double the distance?



9 years ago

There are two main factors to consider: - Travelling fast is less efficient (uses more energy per mile) because your rolling resistance and air resistance is higher. For petrol engines this is balanced by the fact that they become inefficient when run too slowly, but AFAIK electric motors don't have this problem until you are going very slowly. - Batteries don't always give the same amount of energy. If you discharge a battery at double the rate, it will last less then half as long because higher current discharges are less efficient. I believe the effect is most pronounced with lead-acid batteries. If you check the spec for your battery online, it should give the amount of energy it will give when discharged constantly over 20 hours and constantly over one or two hours- the first will be greater. These two coupled together mean that a bike that could do 15 miles at 70mph could probably do something like 25 at 30mph. The relationship is a lot more complicated than just "double the speed halves the range", every electric vehicle is different in this respect. Be wary of figures like that though, they may mean the maximum range is 20 miles only if you travel at walking speed and it will only go 5 miles at 70mph. Goodhart makes a good point, an electric motor has a "preferred" speed at which it has the best efficiency, so ideally your bike should be geared to hit that motor speed at your preferred cruising speed- it's no good riding a bike at 30 that is designed to cruise at 70.

It would depend largely on how it is geared.
Unlike an internal combustion engine, sitting idle (if nothing is drawing power) does not lower one's mileage. BUT, traveling at higher rates of speed IF it is not geared properly, will use a lot of extra charge. I don't have figures in front of me though to prove this.

I'm sure others can prove something with hard numbers but I think in reality you only get a better miles/km per gallon/battery charge but probably not double if you halve the speed. There are a lot of factors that come into play but most of the energy is used to start the vehicle moving up to speed. Like regular gas vehicles, you get better mileage when cruising on the highway instead of stop and go. So the trick is to pick the most efficient engine that is the right size for your vehicle and maybe use some regenerative power tricks. You don't want it underpowered for performance nor do you want it too powerful that you are just dragging around extra weight and taking extra juice to power the motor.