I recently started doing research to build one of these (popular?) two-wheeled balancing things. (A segway clone) I don't know how to calculate the proper size motors (a pair of motors) for this. I know you have to consider the torque and rotational speed required to size the motor. Desired rotational speed is easy, I just take a desired speed and a wheel size and figure out how fast it needs to rotate to reach that speed. I'm not sure how to calculate torque requirements.