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Originally Posted by Andy A.
Why use 6 wheels when you could get away with 2?
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Pushing Force!
If you are using 2WD with 4 skids on the corners, you have at most 6 points of contact with the ground, and for most operation at least 4 points of contact. (Sometimes it balances on just 2 wheels, but not in the cases I'll be describing).
This means, there is weight resting on non-powered wheels (casters, skids, whatever). Why is this bad?
Simply:
Pushing Force = (Normal Force of Robot resting on Drive Wheels) * (Wheel coefficient of Friction)
By putting weight on "Dead wheels" you are greatly limiting your pushing force. If you have 4 points of contact, but only 2 of them are powered (i.e. 2 wheels, 2 casters) you are only resting 1/2 your robot weight on the drivewheels, and only utilizing HALF of your potential pushing force. (Yes, this is a simplification assuming weight is evenly distributed over all wheels, but it's still a valid point).
So, here is the blunt (however opinionated

) truth:
If you use 2WD, you better not plan on winning any pushing contests.
Physics is against you, and it's not just a polite suggestion -- It's the law!
John