Robot Carrying Weight up a Ramp

I am entering a competition where part of it is carrying as much weight as possible, both on a flat surface and up an 18° ramp. Currently I’m thinking of a robot shaped like an upside down T, with two motors (These monsters?) at the back pushing it forward and a caster at the front. I was thinking of using the standard 70 or 80mm Pololu Wheels with the gummy tires. Assuming my frame is strong enough, what do you guys think will be the limiting factor, torque or traction? Larger wheels give more traction and go faster (which is also good), but smaller wheels give more torque. Also, gummy tires only fit on 70 and 80mm wheels. If traction is not a problem, I would probably use 90mm for the flat and 60 for the ramp, but I have no idea how to calculate how much traction I will have or need like I can for torque. Should I just try out different wheel sizes before the competion? I would rather not buy all of the different sizes of wheels.


I expect that the required torque will be less than the available friction, so you can probably do the calculations with just gravity and torque. I don’t think a larger-radius wheel gives you more traction.

- Ryan