I am trying to figure out how to design a stair-climbing device similar to a hand-cart that climbs stairs. I know that the loading will be ~500 lb. It uses a 3-wheel climbing design design like http://www.northerntool.com/webapp/wcs/stores/servlet/product_6970_200381541_200381541" except motorized. I am trying to figure out how much torque I will need at the output of the gear box. I am assuming that this is simply a function of the distance from the climbing-wheel to the loading. I also would like to figure out what size motor would work in that it should be relatively small and run off of a reasonably small portable power source. I have no real design experience, so I was hoping for some input/advice. What kind of small portable motors are out there and where can I see their specs? What kind of power sources are out there? If my questions are not that clear, let me know and I will try to elaborate. Thanks, Casey EDIT to help put this into better perspective, the motor is to power a 3-wheeled design like the one shown above, but it is to be used on an Ambulance Gurney (stretcher) to assist in moving 'larger' people up and down stairs.