Re: Best Form Of Six Wheel Drive?
 

We have used 6 wheel drive 2 out of our 3 years so far. Last year we used the kitbot to build a 6 wheel drivetrain and skyway wheels retreaded with roughtop. Overall it worked well with the center wheel offset 1/8". It turned well and still had plenty of strength. Our main problem was with the frame itself as after one regional of very tough play, the frame was very bent causing many chain related problems. This year we solved all of the problems by going with a custom welded frame with 2" wide colson wheels and andymark shifters. We also switched to live axles which made running the chains much easier. The robot turns very well and after 2 regionals we have yet to be pushed. I think this will be our drivetrain for a long while with some improvements, mainly reducing the weight.

http://www.chiefdelphi.com/media/photos/26794

Re: Best Form Of Six Wheel Drive?
 

116 went to a 6WD for a variety of reasons this year, namely the turning issues with our 4WD traction-wheeled bot in 2006, as well as the shorter length between wheels increasing our ability to climb ramps.
Our 6WD incorporates 6 6" AndyMark Performance Wheels, with rough-top nitrile tread (same as seen on 1345's bot). We have a Andymark 2-speed shifting transmission, with 2 small CIM motors on each side. This is further reduced by a 12:30 sprocket reduction to the 6" wheels, giving us theoretical speeds of roughly 4 fps (low) and 10 fps (high) including inefficiencies. We designed the drive-train in such a manner that center wheel could easily be lowered if needed, and we can lower it at any point we wish by removing 8 bolts and adding spacers between the frame and center bearings. Currently, we are running the wheels co-planar, and it handles outstandingly in low gear, but not as well in high. If we deem that we need to use our high gear (and turn in our high gear) more in Atlanta (we very rarely needed to use it in Richmond), we may lower the center wheel.
So far the drive-train is our best in recent history (just ask our driver), is durable, and easy to maintain. We have only thrown one chain so far (in our first match), and even throwing it didn't really hinder our ability to drive (due to separate runs between wheels). The system is lightweight (under 50 lb.s for our entire drive-train and frame assemblies), easy to construct, and quite effective. It has given us more than adequate pushing power in direct pushing matches, but (like virtually every robot FIRST) can still be rotated by an opposing bot. It has also been able to climb every ramp we've encountered (even some very narrow and precarious ones, although some credit for that must be given to our driver).
http://www.invisiblerobot.com/roboti...h/p1280253.jpg

Re: Best Form Of Six Wheel Drive?
 

We have have had a drive train with the center wheels lowered by 1/8" in the past and it worked pretty well. This year we had 4 traction wheels, two in the middle and two in the back, and then two unpowered omni-wheels in the front. We were told by several teams at our regional that this was a bad design, and that everyone would push us around by spinning us. But actually, no one did.

We had a 4 CIM drive train, geared more for torque than speed, and we were able to push most of the robots we went up against, except that beastly 48 machine. The key for us this year was the welded frame. Because we didn't flex, we pretty much kept 4 wheels on the ground all the time. So we were not easily spun at all. But since the traction wheel axes were only 13" apart, it was very easy to turn. We also implemented a "reverse" button, so we could change the direction of front and back, because we found that for some ramps (those that didn't flex), it was easier to go up leading with the rear traction wheels while for flexible ramps it was better to lead with the unpowerer omni wheels. With last year's (unintentionally) flexible chassis this design might not have worked so well.

Re: Best Form Of Six Wheel Drive?
 

We went with a kitbot chassis (cut to 37x27), but didn't drop the middle wheels. Our wheelbase is 30 inch, split 16/14 so the middle wheels are offset one inch from the true center, toward the claw. AM aluminum six inch omnis on the corners and AM kit wheels in the middle, driven by two CIMS through 12:1 BB 56mm kit gearboxes (unmodified, with no bowtie failures luckily) and 15/28 sprocket ratio. With the winch and compressor in the front and the battery in the back our weight distribution made for very easy maneuvering. And we had no trouble climbing ramps. See our chassis here.

Of course we don't push anyone around but that has not been an issue: we have seeded better than in any of our previous years (6th at St. Louis and 11th at Boilermaker) and were drafted fifth at both events. Didn't make it past the semifinals but we had a lots of fun and put up quite a few ringers. Even managed to hang a couple of keepers. If I had it to do over, I'd probably only use omnis on one end, and would definitely go with two CIMs per side; didn't want to risk that this year due to concerns about the BB carrier plates. If the budget allowed I'd buy AM Shifters, and go with higher traction center wheels.

Re: Best Form Of Six Wheel Drive?
 

6WD. Drop the center wheel 3-4 mm, nothing but traction. The crush of the carpet makes the rock almost imperceptable. This was the drivetrain that 1189 used this year. I think the folks at Boilermaker will attest both it's pushing power and agility (for a 2 degree of freedom drive system).

http://www.gpgearheads.org/gallery/album08/IMGP1542

Don't have a good picture of the finished drivetrain, but the finished version had 6 8" AM Performance wheels with roughtop treading.

I recommend that all teams (especially those having difficulty turning) reference the following white paper:
Drive Train Basics (How to Be Sure Your Robot Will Turn)
http://www.chiefdelphi.com/media/papers/1443
And the spreadsheet derived from it.
http://www.chiefdelphi.com/media/papers/1917

Re: Best Form Of Six Wheel Drive?
 

One of the reasons that I do not personally like about 6 wheel drives with a lowered center wheel is that I am afraid of the rocking back and forth. I assume that if you make the amout that the wheels are lowered minimal (1/8") then it would rock very little. I was just hoping to achieve the ability to spin in a dime.

Re: Best Form Of Six Wheel Drive?
 

I think the most important part of having a successful drive is putting enough power into it. As you look at most of these posts, you will not that a vast majority have had 4 motors in their drive-train, typically the 4 small CIMs. This provides enough torque to turn well, whether you a brute forcing your turns, using omnis, or lowering your wheels. Don't overkill your drive though. Be wise about motor selection and motor quantity, and you will do well.

Re: Best Form Of Six Wheel Drive?
 

I'd be interested in learning how many other teams have run 6WD with omniwheels at the corners before we started to do so last season and what their experiences were. I can't recall anyone doing it prior, but my knowledge of robots is hardly encyclopaedic.

I don't like rock, so I moved away from 6WD with a lowered wheel beginning last season to eliminate the rock. It's marginally more maneuverable than designs without omniwheels, as well, and that's worked well for us for the last several years. We haven't run into many problems with being turned in place, but we have also purposefully and successfully avoided roles that draw attention to ourselves and bring defense.

I've been thinking a lot lately about why some teams use a high traction wheel as the center pair in a 6WD. There may be some benefit -- again with respect to maneuverability -- but such an arrangement does not offer increased pushing force over 6WD with three pairs of identical wheels, so I'm not convinced that accomodating the differing wheels is worth the trouble.

Re: Best Form Of Six Wheel Drive?
 

It depends on your design. The chassis that I designed didn't rock back and forth at all. The center wheels were lowered 1/8th of an inch and the wheels were modified andymark kit wheel. I used roughtop for my design. Roughtop has a coefficient of friction of 1.3 (I know many of you know this, but there are others who doesn't).

Also, I saw a similar 6wd when I went over to team 1251's home. They are using 4 inch wheels and middle wheels were dropped .25". I didn't see any rocking on their bot, and even if there was any it was very minimal. They used wedgetop. The coefficient of friction for wedgetop is 1.2. The designer of the robot made a statement as follows, "We lowered the middle wheels .25" but the wedgetop compresses about 1/8" so in reality, it is only 1/8" lowered."

I have got a calculus exam Monday morning that I need to study for. I know there are few posts here that asks how to design a 6wd, or how it works (similar to those questions). If you got questions and would like to have a discussion, you have got my aim screen name or email in my profile. Feel free message me.

Re: Best Form Of Six Wheel Drive?
 

AndyMark wheels in the corners, dropped center wheels are McMaster pneumatics. Tread wears off the pneumatic wheels, but they're good for at least two events. Pneumatic wheels also allow us to adjust the rock. (We don't really have one--we put most of the weight in the back, so the back likes to stay down.)

Re: Best Form Of Six Wheel Drive?
 

Just curious, why don't you want the robot to rock? considering most 6wd bots have about 1/8" drop, rocking is minimal anyways....but what are the negatives?

Re: Best Form Of Six Wheel Drive?
 

You can see it here: http://www.chiefdelphi.com/media/photos/27927?

It barely has any rocking, it turns on a dime even at 13 ft/sec and so far we have pushed any robot we have tried to.

Re: Best Form Of Six Wheel Drive?
 

A rocking robot base means the end of a horizontal extension (such as a tube-holding arm) moves up and down. Lots of rock means lots of up and down. That's an inconsistency that can't easily be countered by sensors on the robot. If your scoring relies on accurate placement of the arm, rocking can be a significant problem.

Re: Best Form Of Six Wheel Drive?
 

1270 went with an eight wheel drive base this year and Im pretty sure we are going to keep it due to how well it has performed. We took the Andy Mark 2 speed transmission and integrated it into our 4 inch box tube frame and we also put the wheels in there. We used 4"x1.5" IFI wheels for the four center wheels and 4"x1" IFI wheels for the four outer wheels. The base has a lot of pushing power, turns on a dime, and is fast. The four center wheels are lowered 1/8".

I posted a video of the base if you havent seen it yet just click the link.
http://www.chiefdelphi.com/media/photos/26804

Re: Best Form Of Six Wheel Drive?
 

1731 has traction wheels in the center (lowered about 1/2 inch) & omni wheels in the corners. since we are slightly heavier in the back we don't really rock that much -- we basically only use our front wheels to climb ramps.

Even though we are an offensive robot with an arm & ramp were were chosen by the top seed at VCU for our defense & our ability to climb.