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

We used the long configuration AM14u for aerial assist. It got us to St. Louis. As an off season activity I would like to prototype a 3 cim 2 speed 6 wheel west coast drive. I am leaning towards a 28 x 28 square configuration. My student wants to stick with the 23x32 long configuration. Any suggestions.

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

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.

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

Honestly, I don't feel it really matters what the dimensions are for your offseason drive. However, I would make them so they fit all of the previous years rules to give you practice designing a drive that not only works and looks cool, but also follows all of the perimeter (and maybe even bumper rules if you think that far ahead).

Offseason drives are also great bases for offseason manipulators. Think of what you might put on top of it. Yes, this is hard thinking about future years offseason projects, but next offseason you might decide to prototype a elevator or whatnot. Building a complete robot over the course of many seasons is a extremely rewarding task.

You might also want to use the same dimensions you used for your 2014 robot, because you could possibly (if time permits) take the manipulators of your 2014 robot, and place it on top of your fancy new drive base.

So in short, plan the dimensions of your drive base with what you might possibly build or place on top of it.

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

I hope you mean 6 cims in total and not just 3. :ahh:

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

Just a notion: Traditional West Coast Drive is just (relatively cheap) 1x2 tubing, creatively machined and assembled. Do you have the resources to machine the tubing, then transplant the gearboxes/wheels/axles to try it both ways?

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

Make sure you have somebody who can do the machining. Also, make sure your chain tensioning is good. We had a lot of problems because we did not use screw tensioning or cam tensioning. We did a thing with slots, pulling, and locknuts. Bad plan.

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

Have you seen these?

We used them this past year, and were nothing but happy with their performance. Not only do they provide easy tensioning by just sliding the blocks in and out, but they only require a drill press for machining.

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

Yes 6 cim total 3 cim per gear box

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

That's exactly what I am planning to use!!

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

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. I don't know it it matters: all the Einstein drivetrain this year were square drive including the poofs.
Once we have some experience building this chassis I don't think it'll be difficult for us to build a long configuration.
I'll be using the vex versachassis and 3 cim two speed ball shifter gear box.

I do like the suggestion of building a chassis that has the same dimension as our competition bot and put this year superstructure on it.

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

I feel that has less to do with a square drive being "better" then a long drive and more of due to perimeter constraints, it was hard to acquire a ball being long. Thus many teams were forced to be much more square.

That said, there are a few things that I forgot to mention in my first post, that being long allows you to be taller without having to worry about tipping over as much as being square (or wide and having a shorter wheel base). But being square (or wide and having a shorter wheel base) usually enables you to turn easier with less scrubbing of the wheels.

I am not sure how much different it is to drive a square vs. long robot, but I doubt it would be significantly different. Maybe someone can chime in about that?

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

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?

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

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.

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

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

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

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.

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

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.

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

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.

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

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.

Re: Need help in deciding the configuration for an off season chassis prototypes
 

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.

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

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.


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 :rolleyes: ) 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.

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

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.

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

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.

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

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

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

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.

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

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.

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

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

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

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.

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

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.

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

Its a 4 Wheel drive with 6 CIMs

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

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.

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

Just a reminder, off the subject of dimensions, make sure you test the base under full weight. If your team generally build way under the weight limit and is 30lbs underweight, make sure you account for that. If your team pushes the limit to 119.9 lbs like we usually do, then your base when being tested should be around 140 lbs total (robot, battery, bumper). If you don;t test with weight you will see dramatically different traction results

If you want to get fancy you should even raise the weight a bit to account for how high your Center of Mass will be. Accurate testing in the offseason yields predictable results in the onseason.

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

While that is true, there is also the major downside of being spun around in every collision that teams usually try to avoid.

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

I'm having some issues uploading the photographs from my phone due to the weird format it saves them in. I will have them converted/uploaded later this evening. Sorry about the wait.

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

Right, I was referring to 1625's setup this year, that it's not a standard WCD because of the two omni wheels they had.

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

Personally, I'd much rather spin out of collisions than get locked up in a t bone pin by them. We had two omnis on front and 4 VersaWheels on back of our 6WD this year, and it was still much harder to get out of T bones than I would have liked.

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

Okay, I put the best two I have in Google Drive and pasted the links here and here.
I also asked my teammates for more, as these two do not show of our chassis perfectly. If interested I can try to get a SolidWorks render of just the chassis also.

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

Looks excellent. I like how you guys make the cut out in the front for the ball collector and still comply with the bumper rule. Very clever.

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

If you look closely you will see that it is not actually a cut out. There are two aluminum boxes in each front corner. These boxes sit in front of a aluminum frame rail, the same as seen in the rear of the robot, the only difference being the front rail is flipped backwards. The space in between is where our pickup forks lower. The boxes give us quite a bit of frame rigidity in those corners. This is a bit hard to explain so I would like to show you in photographs.

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

I can sort of glimpse that in the two image you posted. Love to see more picture

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

Team 1318 is working on a few summer projects, including an 8 Colson wheel, 4 CIM, West Coast Drive using dual speed WCP DOG shifters, and a modified 6 Colson wheel AM14U kit bot chassis outfitted with shifter upgrade kits. I reccomend moving out of the box and trying something new when it comes to summer projects (more below).

1318 (The IRS) has been known through out the PNW for their relatively simplistic, and well performing, designs. Last year we manufactured our own chassis, dimensions were based off of the 2013 kit bot chassis. This year, we decided to use the provided kit bot chassis in a 28x28 configuration, which took us up to Galileo division finals (although the front cross brace extrusion is bent in about an inch after division semifinals). By branching out this summer we hope to educate our team about a higher performing level of drive train, and comparing and contrasting different designs (6 wheel vs. 8 wheel, kit bot vs. WCP drive base, etc.)

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

As another design constraint to consider....

Can it get through your room door with bumpers on?

Being slightly under this dimension when you are driving out the door, saves a lot of time when you want to go show off your hard work in the gym, cafeteria, etc...

(AM14U, two speed upgrade, 28x28 configuration, number 4 alliance captain on Archimedes, 1st year not designing WCD base)

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

IIRC, the rules are designed to make sure any rectangular robot can fit through a door with bumpers on in at least one dimension.

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

Depends on your door... but it is better to be a long bot to drive through the door, as to be wide, and have to pick it up and carry it sideways.

Just saying, it's a demo, make it easy on you....

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

Y'all need a cart with caster wheels then.

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

Agree! That's why I will go with the WCD 28 x 27 to get away from the am14u.
Will also buy different vex wheels and colson wheels to try out.

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

Update:
We have prototyped a square configuration 6 wheel 3 CIM ball shifter VersaChassis WCD. Haven't done AB comparison driving with our AM14U since it's the summer holiday. Will start AB testing in a few weeks once all the students returns to school.
Our team has made a Step by Step guide to building the VersaChassis. Hopefully it will help anybody who wants to try something different than the KOP AM14U.
http://www.chiefdelphi.com/media/papers/3050

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

That is a really awesome paper. It would have really helped me when I was making a parts list for building a WCD with VersaChassis earlier this summer!

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

Fantastic white paper. We are looking at purchasing the VersaChassis system to test out here in the Fall and this is a great paper to read for anyone who hasn't used the system before.