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

[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?

[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.

[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

[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.

[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.

[JesseK]

I wouldn't suggest 4 Omnis + 2 traction. Even an omni bot or mecanum drive bot can play excellent defense against you. Actually, they see you and salivate at the defense they can play against you since they can also outmaneuver you.

Last year 1885 did a 6WD rocker with 4" wheels. It is my opinion that we turned "too well", and the center of rotation kept changing from one side of the bot to the other. This proved to be detrimental with sudden shifts in speed and direction, as we could very easily tip if we did the incorrect sequence of movements. We tried to limit max turn speed via the gyro but that went south pretty quickly as it was a last-minute "feature".

For this coming year, we're toying with 4 traction wheels + 2 omnis. This is so the center of gravity and center of rotation are more stable. It also allows us to adjust the distance between the rear and middle traction wheels so we can adjust our traction wheel base, whereas with a rocker the wheel base is always set in stone. This will play a key role in weight placement because the students have already derived the advantages of trying to keep the center of mass just behind the middle traction wheel axles.

So, rocker pros:
It turns very well.
More than likely you have 4 traction wheels on the ground at all times for good enough pushing.
It does not inhibit other design decisions, such as transmission types, overall forward speed/acceleration, etc.

Rocker Cons:
Others can still turn you very well when you're up on only 2 wheels.
Your center of gravity and center of rotation change, constantly.
It's hard to control how well it turns (well, it's not something that can be programmed spur-of-the-moment)
You need more advanced drilling techniques for the axle holes, otherwise if you weld the frame you will definitely see some warping, leading to wierd rocking or cambered wheels.

On March 9th, 2008 I will tell you what I really think about 4 traction wheels + 2 Omnis.

[EricH]

Originally Posted by JesseK

Last year 1885 did a 6WD rocker with 4" wheels. It is my opinion that we turned "too well", and the center of rotation kept changing from one side of the bot to the other. This proved to be detrimental with sudden shifts in speed and direction, as we could very easily tip if we did the incorrect sequence of movements. We tried to limit max turn speed via the gyro but that went south pretty quickly as it was a last-minute "feature".

Sounds like a weight distribution issue to me. Was your CG right over the center wheels? If so, you would have had a two-wheel drive robot (effectively) with a chance that any loose weight (tubes and the like) could easily shift during a match and slow one wheel down more than the other.

330 has only tipped twice using their 6-wheel rocker (6" wheels), and one of those was due to tangling with another robot high up (and we still pulled them across half the field. The other was due to being on a ramp in 2006 and going down the "wrong" way by mistake. No tips at all in 2007, other than being on a partner's ramp and sliding off for whatever reason. (And who didn't do that?)

[AdamHeard]

Let's try to keep the anecdotal evidence out of this thread in terms of pro/cons. (not talking about you Eric).

It seems some of you are thinking of some of the pros/cons of a 6wd in general and randomly putting them among the two groups.

In my opinion (assuming the traction wheels have a mu of 1.3, the omni's are AndyMark's with 1.0, the rocker has a reasonable center wheel drop, and the non rocker is 4 traction wheels with two omnis with no center wheel drop)

These are just rocker vs. non rocker w/ omnis. The general pros/cons of all 6WDs are left out.

Rocker Pros;
Well, your overall traction will be higher as all load bearing wheels will have a higher coefficient of friction.
Much more difficult for opposing robots to disrupt you.

Rocker Cons;
Well, if the center wheel drop is wrong you can have a robot that rocks really bad. However, this is more of a specific problem than the overall design. Our 2007 base had 6 high traction wheels with about .15 drop and it not only turned very well, but didn't have any noticable rock.
Will probably turn slower than with omnis, but I don't think turning speed is an issue.

Non rocker Pros;
Well, it it will turn faster if that is what you want....

Non rocker Cons;
It will be easier for other robots to turn/disupt you.

[Madison]

Will everyone that's made comments about omniwheels on each corner of a 6WD system that has first-hand experience with that configuration please raise a hand?

Our robots will be disappointed to learn that, among other things, they didn't handle inclines well.

http://www.chiefdelphi.com/media/photos/28462

http://www.chiefdelphi.com/media/photos/23716

Our prototype chassis had its endmost wheel raised 1/8", but the next iteration of the design eliminated that and replaced it with a single set of omniwheels on one end. My team was urging me to go back to omniwheels on all corners, but I've been resisting. This thread is making me reconsider the decision -- we've had good success with that setup for the past two years and there's no use in changing a good thing. "Mr. T," as the prototype has been affectionately called since we "pity the fool," will very likely have omniwheels on all corners in 2008.