# gear ratio

## How to estimate max speed of electric bike

By Winston at Tue, 07/20/2010 - 15:37For those of you who want to build an electric bike/minibike this Excel spreadsheet (w/graphs) I made might be useful in estimating your max speed. I did all of my research using "Build Your Own Electric Vehicle." It is only for a single gear though and you'll probably have to do some editing in the 'MOTOR SPECS' area if you want to use it. The biggest problem I had in putting this chart together is that none of the smaller sized scooter motors I've found online include torque curves, so I went off of these equations I found about calculating RPM decrease due to rising current. Who knows if they're entirely accurate, but the relationships make sense to me. The only extra thing you have to measure is the resistance of the motor's coils - Rm. With the spreadsheet you just enter in all the bolded values and it will change the graphs accordingly. Where the orange line intersects each of the required torque curves depicts the max speed at a certain degree of inclination. Then you can take a look at the RPM graph to see if the RPM of the motor is too high for that speed or you can look at the current graph to see if the current draw is too high in general.

Also, I put in a small section on the spreadsheet that will give you the discharge time (until empty) of lead acid batteries rated at a certain Ah (20hr rate) when used at a certain current. (Peukert's Law rearranged). The 'POWER NEED TO ACCELERATE VEHICLE' section should give you a general idea of the power you need to accelerate your vehicle to a certain speed in a certain distance.

Hope this can help someone! Any comments would be greatly appreciated.

Haha, well I can't figure out how to attach a file, and I don't know how to delete this post... oh well!

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