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

[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.

[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.

[DampRobot]

One thing I recommend experimenting with is how easily you can get out of T Bone pins with your drive base. If you're planning on putting up a significant number of points next year, you will get T boned more than you like, and you likely will wish you designed your drive base to make it hard to T bone pin you. Think about what frame dimensions will give you the most turning moment against a pin, and what "add ins" like drop down casters, omnis, etc, can help you get out of pins. Oh yeah, and it's a ton of fun practicing pinning with another robot, so I encourage you to test it after you built the base. You'll get ideas for improvement too.

Test out a bunch of different wheels (VersaWheels, AM stuff, different treads omnis, etc) to see what you like best in terms of traction and turning. With a WCD, it's super easy to change out wheels, and you'll be glad you bought the wheels (even if you don't like them all that much) because they'll be a part of your inventory for next year.

Make your bumper mounts rock solid. So many people (including me) mess this up the first time around with WCDs. Design the side bumper mounts as if the robot will be slammed on the side with sledgehammers during an earthquake. Trust me, this is one of the few things on your robot you will not regret overdesigning. And then test the heck out of them by going to offseasons or by having another robot slam the bejesus out of the bumpers for days in end.

If you can, put some kind of manipulator on top of the drive base you're building and go to an offseason or three. Planning on going to a competition will give you a big motivation to finish, and will be a ton of fun once you get there. Sometimes, grand summer plans can fall by the wayside because there wasn't enough motivation to get them done. Deadlines (and looking forward to offseasons) helps with that.

Oh yeah, actual dimensions. I'd go 28x28 and see how you like it, but it really doesn't matter too much either way.

[Brandon_L]

Originally Posted by EricH

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

WCD generally has nothing to do with wide, long, square, or even the amount of wheels or type of wheels. It comes in all shapes and sizes, just like any other drive style. This style seems to be defined by the cantilevered wheels, which makes it easy to service.

Originally Posted by lamk

There is a few reason why I am leaning towards a square 6 wheel WCD. Our driver has experience driving a long configuration for a whole season so I want them to drive something different to see how this configuration handle.

Our drivetrain (WCD) this season would of been square, if not for the balls being so wide. Our base ended up being 28x27, wide. (We need just that extra inch for our intake to work ) With a six wheel drop center, the thing handled like a dream.

For an example of a square 4 wheel WCD, see 2590's 2013 robot Athena

Its all about the effective wheelbase compared to the track. When building a skid style drivetrain where unlike cars or bikes the front wheels wont pivot to turn, too small of a wheelbase will result in uncontrollable turning (turning to well). Too big of a wheelbase, and you'll have trouble turning. Its simply a ratio.

[Tim Lehmann4967]

4967 Decided to cut the KitBot chassis into a 27.5inx27.5in square. This gave us a lot of frame rigidity. I can post pictures if interested.

[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.

[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.

[lamk]

Originally Posted by Tim Lehmann4967

4967 Decided to cut the KitBot chassis into a 27.5inx27.5in square. This gave us a lot of frame rigidity. I can post pictures if interested.

Please do!! My original plan was to exactly do that for the off season chassis but decided to try WCD because I feel WCD is likely more easier to configure and maintain.

[Bryan1625]

Team 1625 was a 27x28 bot (wide). I absolutely loved the drive this year too. It ran very smooth with a quick turn radius

[Brandon_L]

Originally Posted by Bryan1625

Team 1625 was a 27x28 bot (wide). I absolutely loved the drive this year too. It ran very smooth with a quick turn radius

4wd? 6wd? 8?

Frame dimensions generally don't decide how smooth your bot turns or doesn't turn, it's all about the wheelbase/track width. A 4 wheel drive of X by Y size will have more trouble turning then a base of the same size with a six wheel drop center.

[Oblarg]

We ran the kitbot wheel setup in a 29x26.5 configuration this year, and it turned like a champ. Probably worth noting that it was also very well-balanced; CM was maybe an inch or two forward of the geometric center of the robot.

[Bryan1625]

Originally Posted by Brandon_L

4wd? 6wd? 8?

Frame dimensions generally don't decide how smooth your bot turns or doesn't turn, it's all about the wheelbase/track width. A 4 wheel drive of X by Y size will have more trouble turning then a base of the same size with a six wheel drop center.

Its a 4 Wheel drive with 6 CIMs

[Abhishek R]

Originally Posted by Bryan1625

Its a 4 Wheel drive with 6 CIMs

Though I don't think it applies to this thread as much because your turning radius will be largely improved by having 2 omnis and 2 traction wheels.