6WD Pros and Cons
 

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.

Re: 6WD Pros and Cons
 

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

Re: 6WD Pros and Cons
 

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)

Re: 6WD Pros and Cons
 

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?

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

Re: 6WD Pros and Cons
 

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

Re: 6WD Pros and Cons
 

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.

Re: 6WD Pros and Cons
 

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.

Re: 6WD Pros and Cons
 

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).

Re: 6WD Pros and Cons
 

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.

Re: 6WD Pros and Cons
 

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.

Re: 6WD Pros and Cons
 

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.

Re: 6WD Pros and Cons
 

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

Re: 6WD Pros and Cons
 

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

Re: 6WD Pros and Cons
 

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

Re: 6WD Pros and Cons
 

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.

Re: 6WD Pros and Cons
 

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.

Re: 6WD Pros and Cons
 

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?)

Re: 6WD Pros and Cons
 

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.

Re: 6WD Pros and Cons
 

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.

Re: 6WD Pros and Cons
 

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.

Re: 6WD Pros and Cons
 

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.

Re: 6WD Pros and Cons
 

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.

Re: 6WD Pros and Cons
 

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.

Re: 6WD Pros and Cons
 

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.

Re: 6WD Pros and Cons
 

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).

Re: 6WD Pros and Cons
 

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.

Re: 6WD Pros and Cons
 

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.

Re: 6WD Pros and Cons
 

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.

Re: 6WD Pros and Cons
 

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.

Re: 6WD Pros and Cons
 

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.

Re: 6WD Pros and Cons
 

Wheelbase, while taking about turning, acts only as a short hand description of considerations of friction, coefficient of friction and torque. The behaviors described by Kevin's comments are absolutely a function of coefficient of friction and resulting traction. There are a handful of ways to make your robot turn more easily -- one is shortening the distance between the center of mass and a wheel's contact point, reducing the torque required to rotate the robot as a result of reducing the distance variable in your torque equation. Conversely, you might reduce the force variable in the equation by using wheels with a lower coefficient of friction; it is by this principle that omniwheels work. You could, with more difficulty, also increase the available torque.

I'm sure you understand this, but some of the folks coming down the line later may read this without some of the inherent assumptions I think you're making -- namely, considerations for all other things being equal. I just want to be sure that we avoid having someone think that they can use wheels with a higher coefficient of friction and simultaneously shorten the wheel base and have everything come up roses.

Re: 6WD Pros and Cons
 

You are right, I was making a few assumptions (gotta stop doing that :p ).

corpralchees' first post only talked about a 6WD vs 4WD high traction setup
To me, this sounded like if he just added 2 more powered wheels would it be better. The answer to this is just adding 2 more of the same type of wheels, at the same level, without changing wheelbase will not assist in turning nor will it (in and of itself) give you more traction.

So now that we are knee deep into the discussion, let me ask you what you mean by 'better'. Are you looking for more pushing force? better manuverability? Easier turning?

Edit: Also corpralchee, Wider tires do not give more traction, in and of themselves. So Widening the center tires, alone, wont directly help. There's a whole science around tire width, contact patch, durometer and sidewall height ... etc.

Re: 6WD Pros and Cons
 

Daniel,

You're correct that adding 2 more wheels at the same level won't help traction, but it will help with turning. It won't help as much as a pair of lowered wheels to shorten the wheelbase, but assuming equal weight distribution on all six wheels, then each wheel has 33% less weight on it than a 4wd configuration and will be 33% easier to scrub sideways. Assuming the robot turns about the midpoint of your two center wheels, you still have only 4 wheels scrubbing but now with 33% less weight. So the robot is theoretically at least 33% easier to turn. That's not counting gains in the torque the middle wheels put out since they're operating under static friction instead of dynamic friction. So adding 2 more wheels certainly does help turning a little.

I'll admit the above all happens in the land of point masses and spherical chickens, but I don't think reality is THAT far off in this case.

Re: 6WD Pros and Cons
 

Your point is more valid than you think. Doing some reseach previously I found that adding wheels in the center leads to them carrying almost 2/3 of the total weight. See the supporting Machinery Handbook snippet posted here http://www.chiefdelphi.com/forums/sh...d.php?p=605839

I'd also like to point out for the silent masses that a white paper is available that explains, in detail, the physics of skid steering.
http://www.chiefdelphi.com/media/papers/1443
There is also a spreadsheet based upon this paper for 4x4 chassis.
http://www.chiefdelphi.com/media/papers/1917

Re: 6WD Pros and Cons
 

Hmmm ... maybe I'm being a dolt here and not understanding.

Each axle on a 6WD robot has 33% of it weight over it (assuming level loading).
Each axle of a 4WD robot has 50% of it's weight over it (again assuming level loading).

A 6WD robot has to overcome the traction of 2 of it's axles in order to turn, or in otherwords overcome (66% of it's weight over wheels x CoF).
A 4WD robot only needs to overcome the traction of 1 of it's axles (or 50% of it's weight over wheels x CoF).

Again, maybe I'm being a dolt here and not seeing the obvious, but why would it be easier to turn against more weight on wheels?

** as a note: this is all theoretical. It assumes many things which in nature just don't happen such as weigh being perfectly distributed over each wheel, the plane that the wheels sit on is perfectly flat, all wheels touch the plane in 1 (and only 1) point, the effective suspension for all wheels are identical, etc **

I

Re: 6WD Pros and Cons
 

You would be right, but 4WDs turn around their center, and not one pair of wheels. (unless there is something funny with the wheels)

Re: 6WD Pros and Cons
 

In the instances above, you're assuming a different center of mass in each case. If the weight is distributed evenly across the robots, the center of mass will be in the center of the robot and it will, thus, turn about that point.

Re: 6WD Pros and Cons
 

The scenario you posit may be the case, but it still works out in favor of the 6WD. The difference is that you're interested in the torque necessary to turn the robot. The torque would be coming from the wheels applying force around the center of rotation. So if the 6WD is turning about the center of the robot, you have 33% weight x 2 axles x 1/2 wheelbase = 33% weight. (I'm ignoring the CoF). For your 4WD scenario, you'd have to turn around the midpoint of an axle. So, 50% weight x 1 axle x 1 wheelbase = 50% weight.

This is, in fact, just a very rough approximation, as the width of your wheelbase affects how much skidding happens and what the actual torque arm for the friction is and what the torque arms of the wheel turning forces are, etc. etc. etc. I'm assuming all that is covered in much more detail in those whitepapers kramarczyk linked.

Also, for the record, I have not convinced myself that a traction wheeled robot necessarily turns about its center of mass. My intuition tells me it's a bit more complicated and has more to do with what wheels are slipping and how much.

Re: 6WD Pros and Cons
 

Yup, adds up the contribution that each wheel adds to the turning moment (M=f*d) less the resistance of each wheel to the turning moment.

Summary

Re: 6WD Pros and Cons
 

If you gear it right, 6 wheels are better for most first games. But not always.


P.S. do not lower the middle wheel on a 6 wheel drive robot.

Re: 6WD Pros and Cons
 

C'mon Shaun. You have to know this statement is totally misleading.

The vast majority of teams who do 6wd need to drop the center wheels (even if only slightly) to allow their robots to turn. In the past, we have had turning issues, even on a robot with dropped center wheels.

As stated before, by dropping the wheels you effectively have shortened the wheelbase to 50% it's previous length, as your weight is being supported by 4 wheels at any given time. By shortening the wheelbase you make it much easier to turn.

Every year you guys claim your robot turns perfectly without a dropped center wheel. You are known for your high traction wheels, and you have a wheelbase length that is twice that of most other teams. While many teams have low gears for turning, you have only a single speed which is geared to 9+ fps.

Until you can show everyone the physics or math that validate that system, stating that no team should run a 6wd system with dropped center wheels just means that anyone who takes your advice will find themselves wondering what good it did to add 2 more wheels, when their robot still won't turn. Clearly your exact setup works well for you, but to claim categorically that you should not drop the center wheel is just plain wrong and totally misleading.

Re: 6WD Pros and Cons
 

So why? This is exactly the sort of blanket statement I was trying to avoid when I started this thread. As Cory indicates in the previous reply, Team 25 is known for their awsome 6WD setup. I was hoping that teams with experiance in 6WD setups (like Team 25) would post the pros and cons of the 6WD systems they have developed over the years rather than making a blanket statement that this one or that one is "better" with no supporting evidence as to why. Come on nuggetsyl! You can do better than that. ;)

Re: 6WD Pros and Cons
 

I know there was a discussion on this the previous year, but the topic of 25 not lowering center wheels reminded me of one of the more interesting ideas, like in this post by Joe Johnson.

It's a possible explanation of 25's drive system, and even if it's not exactly right, is an alternative for dropping the wheel on a 6 wheel drive system. The use of kinetic friction to turn is a novel idea, as you get the turning advantages when you want to turn, but your opponent still has to fight the full force of static friction when your robot is stationary.

Re: 6WD Pros and Cons
 

I am trying to keep the answers short and to the point. K.I.S.S. The point of having 6 wheels is for traction. By lowering the middle wheel you have just defeated the purpose of using 6 wheels. And make it much easier to be pushed from the side corner of your robot.

here is you pros and cons

pro- much harder to push, drive stright lines better, easier to control in tank drive.

cons- turns choppy, heavy if weight is a concern, need to put alot of time into gearing it right.

Re: 6WD Pros and Cons
 

As I see it the point of lowering the center wheel is to make it easy to turn your robot but still maintain the ability to push people when you want to. Turning your robot works both ways. If it's easy for you to turn, then it will be easy for others to turn you. If you build your robot so it can't be turned by other bots, it will be very hard for your robot to turn by itself.

Re: 6WD Pros and Cons
 

You are correct (as usual ;) ), if the weight is evenly distributed the robot will "hop" and rotate around the CoM. I've generally set our weight distribution to 2/3 weight over the rear axle and only 1/3 weight over the front axle. That allows the robot to provide enough force to have the front wheels break free (sidewards) allowing the robot to rotate around the rear axle.

And by shortening the wheelbase (by driving 36" wide and 28" long, giving us a wheelbase of ~19"), having a high CoF (~1.3, adjustable because they are pneumatic tires) and providing plenty of torque (4 small Cim ... 1 per wheel ... with 16:1 reduction) we've been able to turn without difficulty, been able to push almost anyone, and have been very difficult to be pushed or turned by others (including 6 and 8WD opponents).

** Note: I use the term axle, but we usually power each tire individually and there is no contiguous (sp?) axle across the rear (or front) 2 tires.**

Re: 6WD Pros and Cons
 

I would recommend 6 wheel with the center wheel lowered just a bit. I have found that the amount at which you should lower the center wheel depends on the type of wheels that you are running and the type of tread that you are using. When you lower the center wheel you want to be careful not to lower it too much because the robot may bounce back and fourth when turning hard. I have found that the optimal height to lower the center wheel is between 1/16 to 1/8 of an inch. This Has worked for me, but if you use thicker tread, or tread that has a lot of grip you might want to change it a bit. My suggestion would be, if you have time, make the center wheel adjustable and let your drivers test out variable heights. Make sure to allow for the extra weight if you are only running a test chassis, as the extra 80+ pounds that you put on top of the chassis will affect the way the robot reacts.

--hope this will come in handy--

Re: 6WD Pros and Cons
 

I think most people would disagree with you here. My guess is the majority of teams who move to 6wd do so to improve the turning performance of their robots.

If that's why you guys in specific use 6 wheels, there's nothing wrong with that. But assuming everyone else is using the same reasoning, and making a blanket statement like "dont drop the center wheel" is misleading to people who do not fully understand the physics behind drivetrains.

Re: 6WD Pros and Cons
 

I see 25's setup working towards the strategy they depicted, such as 2006 they human loaded then ran in a straight line to the goal and fired minimal turning recquired especially since they had a turret.
Last year however there was quite a bit of turning involved depending on how you played the game. So ultimately to figure out whether you want the center wheel lowered or not you have to anazlyze your game strategy, and what type of driving you'll be doing

Re: 6WD Pros and Cons
 

Team 766 has used 6wd with rocker for the last three years and that has worked out very well.

Pros:
- Heavy, which resisted being pushed, and this year we didn't need to go all that far considering the rack in the middle of the field as opposed to 2006, with the goals, defense and ramps (Biased a bit here)
- Low cg for pushing others
- Turning is definitely better than 6wd no rocker

Cons:

- Center wheels wear down much faster than the outer four (This is one problem related to our choice of tread this year; we used this thick, heavy, rough stuff that we screwed to our inch-wide Andymark wheels that would actually wear down to show the screw heads. The center wheels wore down much faster than the outer four and lasted perhaps one regional. Also, not good for PR demos on concrete, asphalt, etc.)
- Heavy, but this is a con only if you need speed (Cheesy Poofs are known for speed, and a very light robot)

We have toyed with the idea of omniwheels, but haven't gotten around to using them.

Re: 6WD Pros and Cons
 

Our robot was actually exactly at the weight limit with the ramps. It's very possible to make a light 6wd drivetrain, without needing to make all your gears out of aluminum like we did.

Re: 6WD Pros and Cons
 

About steering, why don't you link the left and right sets of wheels on a 6WD robot togeather, then you can use one joystick for the left, and one for the right. I have an RC car that operates on this exact principle, turns easily and on a dime.

Re: 6WD Pros and Cons
 

If you mean all the left wheels tied together (with chain or such) and the same with the right wheels, most teams do this and it's called Skid (or tank style) steering.

If you are talking about like a car turns, it's called Ackermann (or car) steering. Most teams don't do this because it does not allow them to turn in place (it has a turning radius above 0), but it will have less (almost) no scrubbing of the wheels when turning .

Re: 6WD Pros and Cons
 

One thing to watch out for, don't try and save weight by using too light of a material on the center axle. Last year, we drove the robot around all day during Space Day at the National Air and Space Museum and the axle and wheel ended up shearing off completely at the end. We replaced them with steel instead of aluminum ones after that.

Re: 6WD Pros and Cons
 

What alloy Aluminum were you using?

Re: 6WD Pros and Cons
 

And what diameter?

Re: 6WD Pros and Cons
 

Sorry for the late reply. I do not remember, as I was a measly programmer/drive team member.

Re: 6WD Pros and Cons
 

Go with a 8 wheel drive system :D

Re: 6WD Pros and Cons
 

So...now that the challenge has been announced, is anyone going with either of these? Why?

Re: 6WD Pros and Cons
 

We are going with the same base from last year with very minor changes, but we are keeping the 8 wheel drive system we had last year.