Best Form Of Six Wheel Drive?

I have been doing a bit of research on drive trains for next year I am currently leaning toward some form of six wheel drive. I have noticed numerous variations on the conventional six wheel drive. I have narrowed it down to:

A six wheel drive with two traction wheels in the center and 2 omnis in the front and back

A six wheel drive with a wider track at the center and an octagon shaped base(Haven’t chosen wheels yet)

A six wheel drive with six traction wheels and a lot of power (to brute force the turns)

And A six wheel drive with the center wheel slightly lower than the others ( I personally dislike this design)

Any advice or examples would be greatly appreciated.

                                                                        -Dustin

Drive with lower centered wheel and all six are traction wheels

Our team uses a six-wheel design, with the center wheels lowered slightly (1/8 to 1/4 inch). A bunch of teams in the Pacific Northwest use this basic design. We learned it from team 492.

The past two years, we’ve used high traction IFI wheels in the center and Skyway wheels at the corners. If you can keep your robot reasonably balanced over the center wheels, you get a powerful base that can turn in place, pushes well no matter how the weight is distributed and climbs ramps very well.

What about this design do you dislike? We’ve been using it at 418 for three years counting '07 and it’s worked great. Some people like to lower the wheels a lot, effectively creating two 4WD bases that you rock between, but you don’t necessarily need to do that. If you lower the center wheel only very very slightly, then what happens is that you just sink into the carpet a little bit and more of your normal force ends up on the center wheels. The robot doesn’t really rock back and forth, but it does turn well.

Our team used a 6-wheel drive with pneumatic skyways this season. We used 2 speed AM shifters and lowered our center wheels 1/8"

Video
Another Video…

You can see how agile and speedy it was on pavement.

Well it didn’t work that well on carpet…:o Although it still turned in low gear, it was completely unmenuverable in high. It also started to vibrate horribly after spinning in place for a few seconds. After we lowered the center wheels some more it started to turn better.

Brute force turning is successful at low speeds - but it won’t work at all if you start going faster. Omnis are the way to go when it comes to making fast turns.

We at 665 use a somewhat different system. We use two pneumatic tires in the back for each side. Then we use an omniwheel on each side in the front. The back two wheels are closer together to give more traction from getting pushed from the side and the omniwheels are really close to the front, giving us a good turning radius. The robot turns like a dream, there is no jumpy problems and it is fairly accurate and we can do some good pushing. Even with those omnwheels.

As seen here: http://www.flickr.com/photos/60426630@N00/399274777/

The omniwheels are kind of a black blur, you’ll have to take my word for it that they are omniwheels.

-Greg

With 6 traction wheels, a lot of robots have trouble turning without lowering their center wheels. However, certain teams (team 25 comes to mind) have such a powerful drivetrain that they can skid wheels when turning without a problem. I think they even went 8 wheels this year without anything lowered. :yikes:

Yeah, we prefer to “brute force” our turns. We use 9x2 skyways. We custom cut them so that the outer wheels slip a little bit more than the inner ones, which helps a lot with turning. We’ve been going with a similar design since 2003 and have never had a problem with turning at all.

If you want to check out our drive train, we will be bringing a display one from our practice base to Atlanta (you might have seen it at NJ or UTC). Feel free to stop by and look at it. We’d be happy to answer any questions to have about it!

Out of all the ones I have worked with, I like this one that was Inspired by Team 1114.

This year team 1726 went with 6 wheel drive with all traction wheels and a lowered center wheel (seen here). After it’s remarkable performance, I doubt that the team will be changing it any time soon.

I’m personally in favor of the kind that drives, and preferably drives only when under operator (or autonomous) control…

But anyway, we’ve gone with a all traction wheel, all driven system the past two years, and it’s treated us great. This year we added on a 2 speed gearbox, and buffed up the whole setup, and we’ve been running great the whole time.

in my opinion, the six-wheel base that we are using this year is the best idea that we have ever had. this base has a lowered center wheel, so the robot can rock ever soooo slightly.
http://www.virginiafirst.org/FRC/images/thurs/target94.html

Ive designed our 6wd the last 2 years for 1015, both were a success.

2006: center wheels were lowered slightly and had tread on it. Outer wheels did not have tread on it, but i cut groves into them using a soldering iron.

2007: Center wheels were treaded, and ever so slightly larger then the other 2 wheels, and outer wheels were just the kit 6" wheels.

Both worked real well for turning, because the outer wheels act sort of like omni wheels because they can slide along the carpet, however not so much that we are easily pushed arround. I think we have the perfect balance.

I’ve done two kitbot-derived 6WDs in my days, both with the little bit of center-wheel drop inherent with the kitbot.

In 2006 with 1293, we used an 8x2 IFI wheel in the center and Skyways on the corners. The result held up fine through Palmetto, and some off-season thrashing the team did following my departure. (Suffice to say, I noticed some grass around the kit gearbox output shaft when I paid them a visit.)

This season with 1618, we had a bit of funding to use on the drive system, so we switched to AndyMark Gen2 Shifters with a big and small CIM on each side. (We were using one of the small CIMs elsewhere, so we couldn’t just go four smalls.) This year, we’re using six AndyMark kit wheels, each skinned of its gray rubber surface and replaced with roughtop or wedgetop with rivets. We didn’t get the chance to test it on carpet, but tests in the hallway were impressive. (Ask me in a week for better results.)

I haven’t experimented with other forms of 6WD yet, but the one I have used hasn’t let me down yet.

First of all, this is only our second year, so take this for what it’s worth, we’re still new at this; that said:

1732 uses grippy (roughtop) 1.5"W x 6" dia, six wheel drive with the center wheels dropped. We use a kitbot chassis setup for a 13" wheelbase. With 2 CIMs per side we can turn from a standstill easily in Low gear (geared for 5 ft/s max), and reluctantly in High (geared for 11 ft/s max). (We use 36tooth wheel sprockets and 2 speed AndyMark shifters to achieve these ‘gearings’).

We ‘totter’ very slightly and our turning axis shifts between mid-front and mid -back depending on instantaneous force distribution between the front and rear wheels (longhand for ‘which way we’re tottering’).

Ease of turning is a double edged sword. By managing the ratio of wheelbase to track (width between wheels)and lateral friction coefficient of the wheel, you can achieve the degree that works best for you.

A robot that is too difficult to turn wastes power in turns and handles ‘clumsily’ (or doesn’t turn at all). However, that same robot drives straight much more easily, which is is often helpful for driving up ramps (especially when combined with the traction bonus of grippy wheels).

A robot that turns too readily can generally be easily ‘directionally reoriented’ by opposing robots, and often tends to oversteer (keeps turning even when you want to stop turning).

My thinking is the optimal lies somewhere in between these two extremes; at least for standard four and six wheel ‘tank style’ drive types.

But then, there are several teams that can change lateral mobility on the fly. Some have used casters that are only deployed for turns (341 and others?), or a perpendicular set of wheels to roll sideways (175), or use swerve 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

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).

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.

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.

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