How to calculate potential speed and what gears to use?

[RayB]

Hi all,

Well, when it comes to mathematics I am a bit of a dunce.

Anyone know a good online calculator or can tell me how to figure out the best sprockets to buy to attain a specific speed.

An example is I am looking at having a sprocket on a disc hub, perhaps 42T, and drive that with a smaller sprocket on a motor. The smaller sprocket perhaps being a 12 or 14T. If this is going on a 700c wheel and the motor does 180rpm... is there an easy way to calculate this? I am looking for sprocket sizes I would need to hit 30/35km per hour.

Thanks for any advice.

 

[liveforphysics]

You need to know the KV number for the motor. This is the number of RPM's per volt. Next you need to multiply this by your pack voltage to know the peak unloaded speed of the motor.

Calculate the distance you move for each revolution of the wheel (3.14 * (diameter of the tire in feet)). Decide on a top speed requirement. If the speed you choose is in MPH, divide this by 60(to get miles per minute), then multiply this number by 5280 to get feet/minute.

Divide the number of feet/minute by the circumference of the wheel, and you now know the RPM you need the wheel to spin to reach your intended top speed. Take the unloaded speed of the motor, and multiply this by something like 0.8 to give a rough guess of loaded peak motor RPM. Divide the motor top speed RPM number by the top speed wheel RPM number, and you will know the ratio between motor speed to wheel speed.

From there it's a simple matter of looking at sprockets available, and finding a combo that divides to give you something close to that calculated ratio. For example, if it works out to be something like 8, and the smallest front sprocket you want to fit is a 12 tooth, then you know your rear sprocket needs to be a 96 tooth (because 8 *12=96).