[RyanCahoon]

Originally Posted by GeeTwo

In this case, there is no reaction force on the left end of the beam as shown in my diagram

The beam I was considering was the half-module from the drive axle to the wheel that was touching the floor. I was ignoring the other wheel.

Originally Posted by GeeTwo

If the distance between the drive axle and the wheel axle is increased (and the wheel and wheel gear geometry remains constant), this means that the force D is being applied by the drive gear at the end of a longer moment arm from the drive axle, so of course it will require a greater torque from the drive shaft, though it will turn at a lower angular speed. For a given robot acceleration, the torque load required by the motor system will increase, though D remains the same.

Agreed, but in that case, since D and N are being applied at different points on the lever arm, equation 3 needs to incorporate that.

Either:
- analyze it from a force perspective, where you have to consider all 3 forces (D, reactive force from wheel axle bearings, reactive force from drive axle bearings), or
- analyze it from a torque perspective, which allows you to choose your origin so that r*F is 0 for one of the forces, but you have to include the radius for the other two