[AdamHeard]

Like mentioned before, it's based on the stall torque provided by the motors and the maximum traction force, whichever is lower.

For the force the motors can provide, calculate the torque at the wheel, and divide it by the radius of the wheel (T = F x D.... so F = T/D.... D is distance).

For the maximum traction force, you must lookup/experimentally determine/calculate the Coefficient of friction of the wheels and multiply that by the weight of the robot. Hopefully this number is the smaller of the two.

Now, that's just theory; I'd expect it to be off by a little bit when measured 842 style.