[NotInControl]

When dealing with a skid-steer chassis, as in 6 wheel or 8 wheel drive (traction wheels) It is very important to note that performance of skid steers have an optimal trackwidth to wheel base ratio for better turning performance.

Trackwidth = the width of the chassis, and wheelbase = the length of the chassis.

Without diving into the math, to acheive better performance, (assuming the cg is at the center of rotation of the chassis, the trackwidth must be larger than the wheelbase. (wider bots). This ensures the torque moment is placed outside enough the center of rotation to have the base spin. Determining if you have enough torgue to move/turn effieciently, is another problem and I won't talk about it here. If the C.G depending on where the weight of the robot is distributed, the ratio moves.

When you have a 6 wheel drop center, the robot turns on either the forward 4 wheels, or the rear 4 wheels. Depending on the c.g it can be biased to one side more than the other.

The reason square chassis with drop center works is because essentially, when you have a 28W x 28L chassis. When you turn, it is as if you have two 28W x 14L chassis. This means trackwidth is greater than wheel base, and the turning performance is maximized.

The reason I bring up the above is a pre-cursor to talk about wide bot and long bot configurations.

When dealing with a wide bot, you can guarantee the ratio where track width is greater than wheelbase.

However, depending on a longbot configuration, this may not be the case. For example if I design a 12W x 28L Long bot chassis, with 6 wheels and a dropped center, then when I am turning I will be turning on a 12W x 14L chassis. The performance of this will be very inefficient. (This example is for illustration only, not suggesting that a 12 x 28 chassis is someones intent.)

I just wanted to throw this out there, because the real key to choosing the best dimensions for a chassis is to choose the dimensions of the wheels YOU TURN ON and keeping that dimension within the butterzone ratio. The overall chassis dimension is a second constraint to live within FRC rules.

Hope this helps,
Kevin

P.S. This makes the assumption that your center of gravity is near your center of rotation, and that the wheels your using have similar friction coeff in all directions. As any of those assumption changes, the ratio changes.

P.P.S This only looks at the question at hand (dimension of the chassis), it does not consider the c.g of the robot, or the torque (gear ratios) required for effieint driving. All of these need to be considered simultaneously when considering chassis performance, and the engineering decisions that go along with them.