# How to estimate max speed of EV

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Winston
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How to estimate max speed of EV

Hello I was wondering if anyone could tell me how to estimate the max (straightaway) speed of an EV based on things like rpm of motor, gear ratio, tire diameter, weight of vehicle+operator. Say I'm looking at this motor, which is rated at 2600 rpm. So obviously it only spins at 2600 rpm when it's under no load, right? If I could just figure out how fast it would spin under like 200 lbs then I could do a simple gear ratio conversion to get the rpm of the rear wheel and then use the circumference to get a speed. But I feel like torque has to be involved. Do you have to consider the distance between the motor and the rearwheel sprocket or something? And I'm sure wind resistance makes a whole mess of everything too. I'm thinking of converting a crappy Schwinn mountain bike into an EV: replace pedals with motor, swap out rearwheel sprocket with a custom 25 chain one, use some u-bolts and wood to make a battery rack. I like physics and math so please explain things and be detailed.

Thanks

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Re: How to estimate max speed of EV

There are a couple of website that do that for you. Including one that provides a speadsheet. Good luck.

Don't buy any futher away than you can drive to and return it.

Winston
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Re: How to estimate max speed of EV

8ping8
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Re: How to estimate max speed of EV

These two website is quite easy to use, and pretty much accurate.

http://www.angelfire.com/fl/procrastination/rear.html

http://www.bgsoflex.com/rpmmph.html

Hope this helps

Wai

Winston
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Re: How to estimate max speed of EV

These websites only offer simple calculations involving tire circumference... nothing I can't do on my own. I would say they are inaccurate because the max speed value is only theoretical. How do you know if enough torque can be delivered to the wheel to overcome resistive forces (air resistance, rolling resistance, incline force)? A torque vs rpm curve for the motor and a chart of resistive force (in terms of torque needed at the wheel at a certain degree of incline) vs speed are needed to more accurately estimate max speed. Torque curves don't exist for smaller scooter motors so you have to calculate the torque from the rated power and rpm. But that's only one torque value to work with. How do you calculate rpm when the motor is drawing less/more amps? I used the formulas on this website to figure out rpm loss due to rising current. The only extra thing I needed to do was measure the resistance of my motor's coils. Who knows if it's entirely right, but it made sense to me.

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Re: How to estimate max speed of EV

This is what is holding me up on my Trike project...I have a 36 volt 1000 watt motor and I am planning on 3, 12AH SLA's so I think I will have plenty of power, (torque).

I can calculate max speed using these calculators...but just how limiting are these other variables based on the fact that I should have a good amount of power?

Therefore, what should my sprocket sizes/ratio be?

I am very confused...

Viva Electrica!

Winston
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Re: How to estimate max speed of EV

I would say 12Ah is a bit low. Using Peukert's rearranged equation, I get about 17.5 mins till empty. This number underestimates the actual range because you assume the motor is drawing around 1000W/36V amps continuously, which it obviously won't be. But it gives you a rough idea. Also, it's bad to drain general purpose SLA batteries beyond about 15% remaining capacity.
In my experience it really isn't possible to calculate the perfect gear ratio because you need data about the motor that simply doesn't exist. I built a minibike that weighed (with ev thing on it but me) about 117 lbs. I used a 36V 1000W motor, and 3 26Ah 12V batts. It has 11" wheels. My experimental gear ratio was 4.8, i.e. output teeth/motor teeth = 4.8. The speed was around 20 mph during the peak of my batteries' lives. I was glad I chose a higher gear ratio because it had plenty of umph going up hills. So yeah, just think of a bike - high gear (low gear ratio) means good speed on straightaway, but serious trouble when going uphill - low gear (high gear ratio) means not great speed, but easy to get up hills.
More recently I built an electric mountain bike - basically replace pedals with motor and add battery rack. It weighs about 65 lbs. I used a 24V system to make it lighter. Even though it's much lighter, I again chose a high gear ratio, 6.54, because the tires are 26". It's max speed is 30 mph and it can easily zip up hills. The bigger the tire the motor is trying to turn, the greater the torque supplied by motor must be and therefore it must work harder. By using a higher gear ratio you, in a sense, amplify the torque generated at the motor.
So to summarize, I would look most closely at the weight of your vehicle, the size of your tire, and where you'll be riding it to come up with a best guess for the gear ratio.

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