Need help in deciding the configuration for an off season chassis prototype

[ErvinI]

Originally Posted by EricH

How possible is doing both?

Generally speaking, WCDs are run long configuration. Sometimes someone will try a wide configuration. Not too many try a square.

It's the offseason. If you can't decide based on some other parameters, play a couple games of rock-paper-scissors-lizard-Spock to pick one, and put the other one on the list for next offseason.

Team 254 was 27.75"x28". Technically wide, although it was pretty much square.

Is there a specific reason why you believe that WCD's are run in a long configuration? Is it just what you've seen, or is there an underlying design advantage for a WCD that is best seen in a long configuration?

[cadandcookies]

Square chassis have a major advantage with the advent of the "perimeter" rule, as they maximize volume. With a WCD/generic 6wd they also let you avoid a drop center (if you space things correctly).

That being said, go with whatever the students are motivated to do. Don't lose valuable time and energy debating the chassis dimensions when the point is giving the team practice.

Speaking of the point, when your team is making the decision, make sure to take into account the intended purposes-- to practice machining? To prove that you can do it? To have an excuse to hang out in the shop over the summer? The core purpose of this project should guide it from beginning to end.

[Madison]

Originally Posted by cadandcookies

Square chassis have a major advantage with the advent of the "perimeter" rule, as they maximize volume.

For a given perimeter, a circle's your best bet. A square is a pretty inefficient shape, actually.

[cadandcookies]

Originally Posted by Madison

For a given perimeter, a circle's your best bet. A square is a pretty inefficient shape, actually.

I apologize. What I meant was that squares maximize your volume for a rectangle, which is the easiest chassis shape to build. You are of course correct-- saying a square maximizes the volume of a general shape is patently false. If I wasn't past the statute of limitations on post edits I would edit mine to reflect that. Apologies if I misled anyone.

[Torrance]

Originally Posted by ErvinI

Team 254 was 27.75"x28". Technically wide, although it was pretty much square.

Is there a specific reason why you believe that WCD's are run in a long configuration? Is it just what you've seen, or is there an underlying design advantage for a WCD that is best seen in a long configuration?

At least for the past 3 years I've been on the team, usually the primary driving (pun not intended) force behind the shape of the Drivebase frame is due to what we predict the superstructure will require. In 2012 we needed a long bot to fit a large hopper in the back, in 2013 having it be square made us more compact and thus easier to climb and drive under/through the pyramid. In 2014 we had to be a little wider to fit the ball inbetween the superstructure tubes on the sides.
So I guess my point is this: While your driver may prefer a specific shape of chassis, at some point you'll have to differ from previous years to fulfill design requirement and rule changes so I recommend prototyping lots of shapes of frames and getting your drivers used to all of them over time!

While I am not a driver, I have heard that our drivers prefer driving square 6WCD over long 8WCD because it easier to get out of T-bones.

[who716]

for the past two seasons where the perimeter defined the size, we have made a square robot 28X28 it has been absolutely beautiful, handling wise as it will turn slightly better then other frame configuration with wider wheel bases, we had hi-grip wheels both seasons two, i am an advocate for 28X28

[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.

[EricH]

Originally Posted by ErvinI

Is there a specific reason why you believe that WCD's are run in a long configuration? Is it just what you've seen, or is there an underlying design advantage for a WCD that is best seen in a long configuration?

It's what I've seen since 2003/2004, when the WCD was first unleashed. In general, most WCDs I've seen have been long. (For that matter, most robots have been long, too.)

There is one potential advantage, and it could be a key reason for the long robot. 6WD WCDs (OK, any 6WD drop-center drive) tend to be a little "tippy" when accelerating front-to-back or vice versa--how tippy depends on the drop height. A wide robot that tips front-to-back will often have its CG move dangerously close to outside the contact patch, which condition--if met--generally results in a tipped robot. But a long robot doing the same tip has a lot more room for the CG to not be outside the contact patch. Proper engineering will mitigate that little problem; improper engineering will at best leave it as is; at worst, the robot just tips when suddenly switching directions.

[lamk]

Agree. A 28 x 28 6wheel WCD drop centre wheel is basically two wide configuration 28 x 14 (width x length) mate together. From my understanding any 6 wheel drive wider than a square should not have a drop centre. The AM14U(kit chassis) does not drop the centre wheel when configured to wide. If the square prototype is successful, I'll likely build at least one more version, most likely long, for the drive team to try.

[Mark Sheridan]

Originally Posted by EricH

It's what I've seen since 2003/2004, when the WCD was first unleashed. In general, most WCDs I've seen have been long. (For that matter, most robots have been long, too.)

Just for the sake of history. the fall 2003 60/254 collaboration was a WCD in a wide configuration. This was the robot the two teams built to prepare for their 2004 collaboration , which was of course long.