6WD Pros and Cons

[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.

[Taylor]

Has anybody toyed with putting omnis in the center of the robot, and treads at the corners? I would be curious to find out how that setup handles, both coplanar and rocker.
Please only reply if you've used this setup, even if it's prototyping. Thoughts or inferences won't help - I'm looking for anectdotal evidence.

[Andrew Bates]

Sorry if this is the wrong thread for this question but seems like it would fit. My team is wondering if going with a 6wd traction wheels all on the same level would be better than a 4wd traction wheels all on the same level.

[Kevin Sevcik]

Originally Posted by corpralchee

Sorry if this is the wrong thread for this question but seems like it would fit. My team is wondering if going with a 6wd traction wheels all on the same level would be better than a 4wd traction wheels all on the same level.

It would be, but not by a whole lot. Assuming a dead center weight distribution on both bots, you'd still be trying to skid the outside wheels. The only advantage you have is that your center wheels don't need to side-slip (approximately), so they don't add any resistance to your robot turning. Since they support one-third the weight of your robot, your outer wheels now support two-thirds the weight and effectively resist turning a third less than they used to. It's sort of like magically reducing your weight by a third while keeping the turning torque approximately the same. It's probably a lot more like reducing the side-to-side friction of your wheels by a third while keeping the forwards-backwards friction the same.

Point being, those wheels and your robot will still resist turning, they'll just resist it somewhat less.

[AdamHeard]

Originally Posted by corpralchee

Sorry if this is the wrong thread for this question but seems like it would fit. My team is wondering if going with a 6wd traction wheels all on the same level would be better than a 4wd traction wheels all on the same level.

Kevin is right.

It'd be better to go with 6WD, or make two of the four wheels on a 4WD omniwheels. 70/494 had a cool way of doing it in 2006 where the opposite corners were omni's so they weren't as easily rotated by opponents.

[Daniel_LaFleur]

Originally Posted by AdamHeard

Kevin is right.

It'd be better to go with 6WD, or make two of the four wheels on a 4WD omniwheels. 70/494 had a cool way of doing it in 2006 where the opposite corners were omni's so they weren't as easily rotated by opponents.

Here, I completely disagree.

A well designed 4WD robot can have as much traction as a 6 or 8 or 14 wheel drive robot.

Coefficient of friction is Coefficient of friction. Contact patch size or number of wheels or whether the starts are aligned has nothing to do with traction. The only 2 things that do are Cof (Mu) and maintaining contact with the surface in question.

The only advantage that a 6WD robot can produce is a shorter wheelbase ... and that can be mimic'd on a 4wd robot by driving wide instead of long.

[AdamHeard]

Originally Posted by Daniel_LaFleur

Here, I completely disagree.

A well designed 4WD robot can have as much traction as a 6 or 8 or 14 wheel drive robot.

Coefficient of friction is Coefficient of friction. Contact patch size or number of wheels or whether the starts are aligned has nothing to do with traction. The only 2 things that do are Cof (Mu) and maintaining contact with the surface in question.

The only advantage that a 6WD robot can produce is a shorter wheelbase ... and that can be mimic'd on a 4wd robot by driving wide instead of long.

I didn't say a single thing about traction. I was addressing turning ability.

Assuming all the wheels are the same, and all are in the same plane, a 6WD will turn better than a 4WD. both systems will have the same traction.

Also, if you take out the two opposite corners of the 4WD for omnis, it will turn easier, but it will have slightly less traction (based on currently available omniwheels).

[Daniel_LaFleur]

Originally Posted by AdamHeard

I didn't say a single thing about traction. I was addressing turning ability.

Assuming all the wheels are the same, and all are in the same plane, a 6WD will turn better than a 4WD. both systems will have the same traction.

Also, if you take out the two opposite corners of the 4WD for omnis, it will turn easier, but it will have slightly less traction (based on currently available omniwheels).

The original question from corpralche was about traction, thus I responded about traction.

And turning problems have as much to do with wheelbase as CoF. If you drive with the 36" wide, then you will have shortened your wheelbase enough to compete (and even win against) 6WD "rocker" robots. Omni's on the corners are not required and reduce the traction advantage of well designed 4WD robot.

If you look at my signature you will see 1824's robot from last year. It is 4WD, Skid steer, with High CoF (~1.3) wheels and we had no problems turning and we were able to push most 6WD robots with little problems.

[lukevanoort]

I'm going to go out on a limb and say that with a coplanar 6WD, assuming that you use the same wheels, you could quite likely end up with more traction than a 4WD. Now, here's my reasoning and it is completely unrelated to any idea that "more wheelz equalz more trakshun dudez" or any similar thinking. With a coplanar 6WD, you end up with significantly less load on each wheel, and this results in less wear. With some tread materials, traction will reduce as the tread wears down (think about what happens to old sneakers). So, since the 6WD reduces wear, you might end up keeping your wheel's high initial traction for longer. Probably not a big effect (especially if you can replace treads like with IFI wheels or have smooth wheels anyway), but it is something to consider.

[Andrew Bates]

Thanks for the responses. From what I gather turning will most likely be easier with 6wd instead of 4wd. However it sounds like there won't be much or any increase or decrease in traction. If we went with the 6wd would making the center wheels wider than the outer wheels help/hinder or no change at all.

[Alan Anderson]

Originally Posted by Daniel_LaFleur

The original question from corpralche was about traction, thus I responded about traction.

I believe you misread the question. It was about 6WD vs 4WD with traction wheels as opposed to omniwheels.

Kevin's answer is a good one. As long as the corner wheels are in the same place in both systems, a coplanar six-wheel drivebase should indeed turn more easily.

Luke's comment about tread wear brings up an interesting detail. The corner wheels of a six-wheel system will see more wear than the center wheels as they scrub sideways during a turn. That will eventually result in the equivalent of lowering the center wheel a teeny bit, since its tread will be fractionally thicker, and it will turn even more easily -- at least until the corner treads are replaced.

[Daniel_LaFleur]

Originally Posted by Alan Anderson

I believe you misread the question. It was about 6WD vs 4WD with traction wheels as opposed to omniwheels.

Kevin's answer is a good one. As long as the corner wheels are in the same place in both systems, a coplanar six-wheel drivebase should indeed turn more easily.

Luke's comment about tread wear brings up an interesting detail. The corner wheels of a six-wheel system will see more wear than the center wheels as they scrub sideways during a turn. That will eventually result in the equivalent of lowering the center wheel a teeny bit, since its tread will be fractionally thicker, and it will turn even more easily -- at least until the corner treads are replaced.

I guess I'm just not explaining myself well here .

My initial response to AdamHeard was about his agreeing with Kevin that you would get more traction from a 6WD with traction tire setup over a 4WD with traction tire setup. Assuming that both setups would use the same tires and were the same weight, then both would (Theoretically) have the same CoF.

Kevins comments about being able to turn better are a function of wheelbase and not traction or CoF. Because of this I commented on shortening the wheelbase of a 4WD system would (by driving 36" wide rather than 36" long), in effect, create an equivelent wheelbase on a 4WD robot, and thus the same turning advantage of the 6WD system.

Lukes comment about wear is interesting ... but it does not create more traction. It will allow for easier turning as the corner wheels wear because the robot will "rock" on the shorter wheelbase (center wheels to 1 set of end wheels). Traction will stay the same, as will pushing force.