Unstable AM14U2 Chassis

[GabrielKB]

As the title implies, our 2015 KOP drivetrain is somewhat unstable. This is due to the fact that the center wheels are about 0.6 cm below the other wheels, causing one end to be 1cm off the ground when the other is touching. During assembly this was noticed and attributed to the different wheels we chose for middle and sides, but once we changed them all to uniform 6" wheels we realized that the axles are at different heights.
Is this an intentional part of the design or did we just horribly mess up? If it's intentional what is its purpose? If not, has anyone else encountered this or has an idea of our mistake?

When flipped over the top of the gearbox and the two outer wheels form a stable plane and the difference in height on the middle wheel becomes obvious:
http://i.imgur.com/z3tI1Sy.jpg?1
The CAD model provided by Andymark seems to have the middle wheels 0.35 cm below the others, but that still isn't close to this height.

[JorgeReyes]

Center wheels are dropped so that the robot could turn easier

[Peyton Yeung]

What you are describing is a drop center and it is intentional. This allows for a smoother turn.

[EricH]

To expand further, what you're seeing is known as a 6WD drop center drivetrain. The center wheel is dropped for easier turning; effectively the robot's wheelbase is cut in half. Incidentally, this is THE most common general drivetrain setup in FRC, at least the last 5-10 years or so. (8WD drop center is close.)

Here's an exercise if you're interested: Drive the robot without the middle wheels being installed at all. Then drive it with the middle wheels. You should see a difference.

Now, how to cope with the drop/rock: You'll probably want to put all your heavy items on one end. This would be the battery, the high-pressure side of the pneumatic system, and some if not all of the control system. That should hold that end down, giving the robot more stability.

[GabrielKB]

Thanks for the replies! Last year's didn't have this so I was unfamiliar, and wasn't sure what exactly to search up. The whole reason we were trying to have different center wheels was to allow for better turning, but I guess that's already been made easier. Learn new things every day!

[GeeTwo]

As I described in my drivetrain class at our "robocamp" [mostly] for the incoming freshmen this summer -- the drop-center tank drive train gives the advantages of a short wheelbase for turning, and the advantages of a long wheelbase for stability (which I defined as not falling over in a hard acceleration or brake).