6WD Pros and Cons

[Kevin Sevcik]

Originally Posted by Daniel_LaFleur

Hmmm ... maybe I'm being a dolt here and not understanding.

Each axle on a 6WD robot has 33% of it weight over it (assuming level loading).
Each axle of a 4WD robot has 50% of it's weight over it (again assuming level loading).

A 6WD robot has to overcome the traction of 2 of it's axles in order to turn, or in otherwords overcome (66% of it's weight over wheels x CoF).
A 4WD robot only needs to overcome the traction of 1 of it's axles (or 50% of it's weight over wheels x CoF).

Again, maybe I'm being a dolt here and not seeing the obvious, but why would it be easier to turn against more weight on wheels?

** as a note: this is all theoretical. It assumes many things which in nature just don't happen such as weigh being perfectly distributed over each wheel, the plane that the wheels sit on is perfectly flat, all wheels touch the plane in 1 (and only 1) point, the effective suspension for all wheels are identical, etc **

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The scenario you posit may be the case, but it still works out in favor of the 6WD. The difference is that you're interested in the torque necessary to turn the robot. The torque would be coming from the wheels applying force around the center of rotation. So if the 6WD is turning about the center of the robot, you have 33% weight x 2 axles x 1/2 wheelbase = 33% weight. (I'm ignoring the CoF). For your 4WD scenario, you'd have to turn around the midpoint of an axle. So, 50% weight x 1 axle x 1 wheelbase = 50% weight.

This is, in fact, just a very rough approximation, as the width of your wheelbase affects how much skidding happens and what the actual torque arm for the friction is and what the torque arms of the wheel turning forces are, etc. etc. etc. I'm assuming all that is covered in much more detail in those whitepapers kramarczyk linked.

Also, for the record, I have not convinced myself that a traction wheeled robot necessarily turns about its center of mass. My intuition tells me it's a bit more complicated and has more to do with what wheels are slipping and how much.