6WD Pros and Cons

[Daniel_LaFleur]

Originally Posted by Kevin Sevcik

Daniel,

You're correct that adding 2 more wheels at the same level won't help traction, but it will help with turning. It won't help as much as a pair of lowered wheels to shorten the wheelbase, but assuming equal weight distribution on all six wheels, then each wheel has 33% less weight on it than a 4wd configuration and will be 33% easier to scrub sideways. Assuming the robot turns about the midpoint of your two center wheels, you still have only 4 wheels scrubbing but now with 33% less weight. So the robot is theoretically at least 33% easier to turn. That's not counting gains in the torque the middle wheels put out since they're operating under static friction instead of dynamic friction. So adding 2 more wheels certainly does help turning a little.

I'll admit the above all happens in the land of point masses and spherical chickens, but I don't think reality is THAT far off in this case.

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 **

I