Best Form Of Six Wheel Drive?

[thefro526]

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

[Robo_Coyote]

Drive with lower centered wheel and all six are traction wheels

[Tom Saxton]

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.

[Gerry Salinas]

Originally Posted by thefro526

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

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.

[=Martin=Taylor=]

Originally Posted by thefro526

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

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... 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.

[Greg Peshek]

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

[Lifelessimp]

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.

[Alexa Stott]

Originally Posted by Lifelessimp

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.

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!

[Arefin Bari]

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

[s_forbes]

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.

[cziggy343]

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/ima.../target94.html

[cire]

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.

[Billfred]

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.

[John Wanninger]

Originally Posted by thefro526

Any advice or examples would be greatly appreciated. -Dustin

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.