[EricH]

Originally Posted by JVN

The fun one:
Ever wonder why some teams can hang more of their robot off the bridge than others? CG is only one half of that story... Support Polygon is the other.

Can you think of a situation in which an (evenly spaced) 6WD would be better for balancing than an (evenly spaced) 8WD?

Situation: The 8WD's CG is in the middle of the middle 4 wheels, and the 6WD's CG is in between the middle wheels and one end's wheels. I'm assuming that the end of the 6WD with the CG is on the bridge, and the CG is fairly close to the middle but not exactly there. (There are other reasons to not have the CG exactly in the middle of the 6WD, but that's beyond the scope of this discussion for now.)

I've included my reasoning. Before looking, see if you can figure it out (unless you're JVN, in which case you've probably already figured it out). Use a pencil and paper if you need to.

Spoiler for Why:

We'll assume that the bridge can't leave level configuration first, just to make life a little bit easier.

When the robot is just under halfway off, the 8WD's CG is already causing the robot to think about falling off the end of the bridge, due to the support polygon suddenly shortening (something about one side running out of support, then the frame reforming the support polygon by landing on the bridge). The 6WD's primary support polygon is still fully on the bridge.

Now move the robot to exactly halfway. The 8WD is pretty much toast now; its CG is about to go off the bridge completely (if it hasn't already due to height of the CG). The 6WD is also very close to going off, but still has its full support polygon, if barely.

Now, go just over halfway. The 8WD's CG goes off the bridge (groans from the crowd and a crashing noise), while the 6WD's support polygon just got shorter due to the center wheels going off of the bridge--and the 6WD's CG is getting very close to the edge of the bridge if it didn't just go off of it.

BUT! Now it gets even better. We repeat the experiment with a pivoting bridge, with a 150# robot-sized counterweight fully on the bridge. At just under halfway off, the bridge is starting to tip--throwing the 8WD's CG just about fully off of the shortened support polygon. But the 6WD, while also at risk due to CG placement, probably still has a little bit of support polygon left. At halfway... goodbye 8WD, 6WD is starting to go over but might have just that little bit of support polygon that it needs to stay aboard.