6WD Pros and Cons

[ChuckDickerson]

Could anyone with experiance with both of the following 6WD systems please describe some of the pros and cons of each?

1) Center lowered "rocker style" with all 6 traction type wheels

2) All 6 wheels level but with omni wheels on one or both ends.

I am not looking for a cut and dry answer as to which is "better". I am looking more for pros and cons of each to make a more informed decision when the time comes (January 5th). Assuming we could build either just as easily I am trying to figure out why we would choose one or the other. The sort of knowledge that only can come from teams that have tried both options.

[Simon Strauss]

For your First option of six traction wheels with a rocker setup

Pro- Really nice traction and suitable turning abilities (we pushed every one around last year with it)

Con- Its difficult to do, especially if you do not have access to the proper milling equipment

Omnis in the front and back

Pro-better turning abilities

Con- Not only can you turn your self more easily, every one else can turn you against your wishes

[David]

1. "Rocker Style"
Pros:
Decent Turning (Acts Like 4wd)
Great Traction
Designed Into Kitbot Frame
Good for climbing
Cons:
As the name suggests, if you do not balance it properly, it will rock back and forth constantly.
(I believe Beach Bots 2007 bot are a good example of a well balanced one)

2. Omnis
Pros:
Great Turning (when you want to)
Good traction
Stable (doesn't rock)
Cons:
Easily turned (when opponents want to)
Not as good as "Rocker Style" at climbing

A third type is having 6 narrow traction wheels all touching. This has about the same pros and cons as "rocker style", but it does not rock (Children of the Swamp Designed a nice a year or so ago, and I think Cybersonics use it)

[ChuckDickerson]

Originally Posted by Simon Strauss

For your First option of six traction wheels with a rocker setup

Pro- Really nice traction and suitable turning abilities (we pushed every one around last year with it)

We have used the lowered center wheel "rocker" style in the past and I agree that it gave us very good pushing power, however, why would a "rocker" style 6WD inherently have more traction than a "level" wheel 6WD?

Originally Posted by Simon Strauss

Omnis in the front and back

Pro-better turning abilities

Con- Not only can you turn your self more easily, every one else can turn you against your wishes

I am wondering if a "level" 6WD with omnis on only one end would be a good compromise?

[EricH]

Originally Posted by David

1. "Rocker Style"
Pros:
Decent Turning (Acts Like 4wd)
Great Traction
Designed Into Kitbot Frame
Good for climbing
Cons:
As the name suggests, if you do not balance it properly, it will rock back and forth constantly.
(I believe Beach Bots 2007 bot are a good example of a well balanced one)

2005-2007 all used a similar drive system. Two pneumatic tires as the center wheels; a pair of kit wheels as the front, and either two more kit wheels (2005 and 2007) or another set of pneumatics (2006) in the back.

Actually, it's all about weight placement with the rockers, not the balance. For 2005, we had a lot of the weight even with or behind the center wheels and didn't have a lot of drop. 2006 had a pretty balanced load and you could tell as it rocked a little bit more. 2007 had almost all of the weight in the back. This reduced the rock. Also, because we had pneumatic tires in the middle (I'm not sure where we got them from, sorry), we were able to adjust the rock as desired. Less rock? No problem, just let air out. More rock? Pump them up. We did go through the tires, though. I think I counted three or four sets of center tires that were dead from two robots this year.

As clear a view as you can get of our 2007 drive, aboard the practice robot: http://www.chiefdelphi.com/media/photos/27675
And the 2006, pre-front wheel change: http://www.chiefdelphi.com/media/photos/23376
And both, for comparison (post-wheel change for 2006): http://www.chiefdelphi.com/media/photos/27581

[lukevanoort]

1) Lowered centre
Pros:
Traction - All wheels that are on the ground are powered (unlike 2WD), and you can pretty much use as high traction wheels as you want without turning issues. Also, high traction normal wheels can get a mu of 1.2+, whereas omnis are around 1.0 (assuming AM omnis... I guess with custom one you could get better traction, but your options still are probably not going to be as broad as with normal wheels).

Ramp climbing - It is much harder to bottom out with that wheel in the center than without one, such as in a 4WD. It is also less likely to rotate sideways as it goes up a ramp than a design with omni wheels.

Easy to drive - Since it drives forward on four wheels, but often (depends on how central your weight distribution is) pivots on two, it is not exceedingly difficult to drive in a straight line (if you think this can't be a serious problem, try driving a fast 2WD) yet still turns very easily (In fact, in some of our drivers' early opinions of our 2006 robot, too easily). I have never driven one, but I imagine driving a 6WD with corner omnis takes a light touch (or PID) to avoid driving in unintentional arcs and spinning too fast.

Cons:
It rocks - While this may seem obvious, it can become somewhat problematic at times. For example, when we extend up to score on the top spider our rocking 6WD, combined with our somewhat loose turret, causes our arm to sway a good bit. We can correct pretty easily (the arm has 5 degrees of freedom though, so your mileage may vary), but this can get annoying (especially if this was 2005 with 15+ feet tall arms carring 9 lb tetras instead of 9ish feet tall arms carrying 1lb tubes)

Heavy - Well, this is a characteristic of all 6wds, they will invariably be heavier than if you had made the same system with only four or two wheels.



Something I've always wanted to try is a coplanar 6WD using four traction wheels and two omnis, with the omnis at opposite corners on opposite ends. Kinda like the 2006 494/70 drive system, but with an extra pair of wheels in the middle. I dunno if such a system is necessarily a worthwhile design, but is seems like a good idea to me.

[EricH]

Originally Posted by lukevanoort

1) Lowered centre
Pros:
Traction - All wheels are on the ground, and you can pretty much use as high traction wheels as you want without turning issues. Also, high traction normal wheels can get a mu of 1.2+, whereas omnis are around 1.0 (assuming AM omnis... I guess with custom one you could get better traction, but your options still are probably not going to be as broad as with normal wheels).

Actually, that's not exactly what happens. Really, only four wheels are fully on the ground at any one time. The "extras" just kind of skim along. How much they help is determined by the weight distribution and the drop of the center wheel. In a turn, you might only have two wheels down.

[Lil' Lavery]

While I haven't used a 6WD system with omnis, I have used both a 6WD "rocker" and a 6WD co-planar (with all 6 traction wheels).
In 2007, 116 used a 6WD with 6 AndyMark performance wheels with blue nitrile roughtop tread, 2 CIMs per side, and AndyMark 2 speed shifters. For VCU and Championship the wheels were coplanar. In low gear the turning was still far better than the "average 4WD kitbot" as it much higher torque. In high gear the turning struggled though.
For IRI we lowered the center wheel 1/8" to improve out turning in high gear (enabling us to use it much for frequently). The turning was actually almost too good, especially in low gear, as the robot would rotate very rapidly when we tried to make fine adjustments to pick up the tubes (even when barely touching the sticks).

[=Martin=Taylor=]

Originally Posted by DeepWater

I am wondering if a "level" 6WD with omnis on only one end would be a good compromise?

Yes. This is a great compromise. We did this last year and it worked great.

There is only one thing to be aware of - your robot will turn about its 'end' rather than about its center. In other words, the omni wheels will swing around a lot. This isn't bad, just different.

Our whole robot's movement was substantially changed when we added two omnis to the front. It also didn't seem to allow other robots to push us around.

[Simon Strauss]

Originally Posted by DeepWater

... why would a "rocker" style 6WD inherently have more traction than a "level" wheel 6WD?

It is not that a rocker 6WD has more traction, it is that it has the right amount. If you try to use traction wheels with level system you can barely turn, and if you go level and use omni wheels you have less traction than if you had used traction wheels.

[ChuckDickerson]

Originally Posted by Hachiban VIII

Yes. This is a great compromise. We did this last year and it worked great.

There is only one thing to be aware of - your robot will turn about its 'end' rather than about its center. In other words, the omni wheels will swing around a lot. This isn't bad, just different.

Our whole robot's movement was substantially changed when we added two omnis to the front. It also didn't seem to allow other robots to push us around.

This is pretty much what I was thinking. It would seem to me to give the advantages of both sytems but I am just trying to figure out what hidden tradeoff it might have not having tried it. I am wondering what difference it would have on traction and pushing ability given everything else being equal.

[David]

We used a chassis that had 4 traction wheels in the back and two omnis in the front last year. We had great traction because we used custom 3 inch wide wheels with neoprene rough top treads. We decided not to power the front 2 omni wheels because the chain would have been heavy and extremely long (we didn't want to need tensioners). Like others have said, it turns easily, but not at the center. The wheels were powered directly off a AndyMark Gen2 Shifter, so our low gear was around 11:1, so we were very fast, but not that powerful.

Picture of our chassis

[dtengineering]

Originally Posted by DeepWater

...why would a "rocker" style 6WD inherently have more traction than a "level" wheel 6WD?

I am wondering if a "level" 6WD with omnis on only one end would be a good compromise?

There is no inherent traction advantage for a rocker robot over a level robot, until you consider the wheels.

If the level 6wd robot had omnis front and back (which tend to have a lower co-efficient of friction than traction wheels) then 2/3 of your weight (assuming a centre of gravity over top of the centre axle) would be transmitted to the ground through the low-friction omnis.

In a "rocker" set up, if all the wheels are traction wheels, then 100% of the robot weight is transmitted to the ground through high-friction wheels. Therefore you have more traction.

Two years ago we built a 6wd robot that was pretty good at pushing, and climbing using the kitbot frame (with lowered centre axle), 6"x2" IFI traction wheels, and two CIMs per side driving the 12:1 KOP gearbox, further geared down by the sprockets driving the wheels to give a max speed in the 6-8 fps range as I recall.

We noticed that even with the lowered centre wheel that we had difficulty turning on carpet with six traction wheels. We replaced the rear (heavy side) traction wheels with omnis. Now the front traction wheels were slightly in the air, the majority of the weight (approching 2/3) was supported by the centre traction wheels, and we could turn with no problems. When it came to pushing or climbing, the front traction wheels would often come into contact with the ground, preventing some of the problems of the "flat with four omnis" problems in climbing.

Jason

[caffel]

We have used 4 WD and 6 WD.
Ironically, six turns better. The reason is that the middle wheels do most of the turning with very little skidding.
This year we used the kit chassis with a little rock as designed in and six traction wheels, 1.5 " wide When the drive was running well, turning was fine and traction was excellent, climbing all ramps and pushing well. When we had trouble with the digital filter and voltage to the motors was down to 10.5 v, then turning wasn't good enough.
Next year if turning becomes a problem or if pushing is de-emphasized we'll use narrower traction wheels on the corners.
The year we used omnis at the corners went well, but they broke easily and climbing was only adequate.

C Affel, mentor 423

[TheOtherGuy]

I was told one thing about the "rocker" setup and did notice it on the field; when you're pushing someone, the robot gets on the back four wheels. This provides a greater amount of 'traction' or 'torque' (maybe 'power' is the right word? not sure...) because the weight is applied where the robot needs it. With traction wheels and the center wheel lowered about 1/8" (and if you can get your CG close to the middle) you should be able to make a drive train that has lots of pushing power and can turn on a dime.

Also, I don't believe it's too difficult to fabricate a pultruded fiberglass frame for a "rocker" setup. We didn't use any 'fancy' machining on it, just a reciprocating saw and drill (and some very accurate measurements!) Here are two examples of our drive train from this year (I don't have any build pictures uploaded... sorry!)

EDIT: that 1/8" is incredibly hard (if not impossible) to see in those pictures, but the center wheel doesn't need to be much lower for the drive train to work properly.