[lukevanoort]

1) Lowered centre
Pros:
Traction - All wheels that are on the ground are powered (unlike 2WD), and you can pretty much use as high traction wheels as you want without turning issues. Also, high traction normal wheels can get a mu of 1.2+, whereas omnis are around 1.0 (assuming AM omnis... I guess with custom one you could get better traction, but your options still are probably not going to be as broad as with normal wheels).

Ramp climbing - It is much harder to bottom out with that wheel in the center than without one, such as in a 4WD. It is also less likely to rotate sideways as it goes up a ramp than a design with omni wheels.

Easy to drive - Since it drives forward on four wheels, but often (depends on how central your weight distribution is) pivots on two, it is not exceedingly difficult to drive in a straight line (if you think this can't be a serious problem, try driving a fast 2WD) yet still turns very easily (In fact, in some of our drivers' early opinions of our 2006 robot, too easily). I have never driven one, but I imagine driving a 6WD with corner omnis takes a light touch (or PID) to avoid driving in unintentional arcs and spinning too fast.

Cons:
It rocks - While this may seem obvious, it can become somewhat problematic at times. For example, when we extend up to score on the top spider our rocking 6WD, combined with our somewhat loose turret, causes our arm to sway a good bit. We can correct pretty easily (the arm has 5 degrees of freedom though, so your mileage may vary), but this can get annoying (especially if this was 2005 with 15+ feet tall arms carring 9 lb tetras instead of 9ish feet tall arms carrying 1lb tubes)

Heavy - Well, this is a characteristic of all 6wds, they will invariably be heavier than if you had made the same system with only four or two wheels.



Something I've always wanted to try is a coplanar 6WD using four traction wheels and two omnis, with the omnis at opposite corners on opposite ends. Kinda like the 2006 494/70 drive system, but with an extra pair of wheels in the middle. I dunno if such a system is necessarily a worthwhile design, but is seems like a good idea to me.