The calculator doesn't say it isn't possible to drive a man around, only that it cannot go at the specs you entered. For example, did you measure the acceleration or incline angle or efficiency for that 20mph run?
Please reread my post, i'm not aiming at 20mph, but 1.1 mt/sec -> 2.4 mph, 3.9 km/h which is about a walking human.
Also, where did you get the rpm/torque data for the motor? To use it properly, you need to get it from a rpm to torque curve on the motor datasheet . . .
I've got them from the motor's datasheet
For example, you wrote the motor goes at 71rpm with a wheel diameter of .3 meters. If you do the math, that only comes out to 2.5 mph.
Waaaaay lower than your 20mph
2.4 mph is just what i was expecting, Waaaaay similar to what you calculated
Take a look at this scooter http://www.electrikmotion.com/gt450main.htm
, it is equipped with a 450w motor and with a man on it (80kgs ?) does 15 mph, lets assume it is optimstic, lets say 10 mph; why i cannot do that with four 450 watt motors and 30 kgs?
I just want to understand, i'm not saying your applet is wrong, i feel it useful and i just have to size motors for my project