What goes into to determining what amount of center drop to have on a 6WD. What factors should you consider?
Some of the things to consider are:
-Rubber material and how much it will compress.
-Type of wheel. Might not need drop if using omnis at one of the ends.
-Distance between centers of the wheels. The further out the more difference there is in the “rocking” of the robot. (As pointed out, less rocky as you spread out the wheel base)
-Frame deflections, if any. Not much of a problem if your have a nice and rigid frame.
From my experience I suggest you go with 1/8" drop on the center wheel. I have also used 3/16" in the past. I don’t think 1/16" is enough in my opinion, and depending on your specific set up, you might encounter difficulty turning.
Keep in mind that the center drop mainly affects how much your robot rocks back and forth. While driving, depending on your mass distribution, you will always be riding on the front or back 4 wheels.
Correction: You’ll rock more when the edge wheels are closer to the center; just do some geometry to see why.
In a prototype of ours, we tried 1/16 drop with roughtop tread on a 6WD with 4" wheels and there was too much resistance when turning. 1/8 works pretty well in our experience.
1/16" is not going to work at all unless we have a game like 09 again.
What type of wheels did you use?
We have done 1/8" drop with 6 plaction wheels. We had no difficulty turning, but the rock was certainly noticeable. Depending on what else the robot is doing this may or may not be a problem.
I should have been more specific, 
, I meant any drop really, and what considerations should be taken in determining the magnitude of the drop
In which case there’s no reason to drop the wheels at all… Sounds like 1/16" is just too little. I assume this is with the rubber tread you guys usually use.
Why not drop for an 09 style game?
I guess it’s something I never even really thought about, it seemed obvious. My thinking was that because the wheels that year slid sideways with no issues there was no reason to restrict your contact to four wheels. Upon actually thinking about it makes sense that you might want to still put all of your limited normal force on those wheels to make turning more effective…
And upon looking at some of the 6 wheeled robots that year particularly for that detail I see that indeed teams (including you guys… 
) used a drop (or a raised outside wheel).
We used AM 4" performance (with rough top). There’s noticeable tread wear on the outer wheels as compared to the center wheels as well from the excessive dragging.
Side note: The Andy Mark CAD for the wheels shows less material around the center than the actual wheel has. Not a huge problem, but it did cause some surprises when we got our hands on the wheels.
Our wheels wore down in 2009 until there wasn’t a drop any more.
A better question would be, why drop? Dropping’s downside is the naturally rocking, and there wouldn’t be much of an upside due to the naturally low friction. If the rocking potentially messes with the ball sucker mechanism and costs you quite a few points, would it be worth the… whatever the gain is?
The improved turning of course (and the rock caused zero detriment to our intake).
Low friction doesn’t change the dynamics of how a drivetrain turns much (as the friction is still the same in the x-y axis more or less). A wider base still turns better than a narrow base.