Best Form Of Six Wheel Drive?
 

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

Re: Best Form Of Six Wheel Drive?
 

Drive with lower centered wheel and all six are traction wheels

Re: Best Form Of Six Wheel Drive?
 

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.

Re: Best Form Of Six Wheel Drive?
 

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.

Re: Best Form Of Six Wheel Drive?
 

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...:o 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.

Re: Best Form Of Six Wheel Drive?
 

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

Re: Best Form Of Six Wheel Drive?
 

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. :yikes:

Re: Best Form Of Six Wheel Drive?
 

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!

Re: Best Form Of Six Wheel Drive?
 

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

Re: Best Form Of Six Wheel Drive?
 

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.

Re: Best Form Of Six Wheel Drive?
 

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.

Re: Best Form Of Six Wheel Drive?
 

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

Re: Best Form Of Six Wheel Drive?
 

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.

Re: Best Form Of Six Wheel Drive?
 

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.

Re: Best Form Of Six Wheel Drive?
 

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.

Re: Best Form Of Six Wheel Drive?
 

We have used 6 wheel drive 2 out of our 3 years so far. Last year we used the kitbot to build a 6 wheel drivetrain and skyway wheels retreaded with roughtop. Overall it worked well with the center wheel offset 1/8". It turned well and still had plenty of strength. Our main problem was with the frame itself as after one regional of very tough play, the frame was very bent causing many chain related problems. This year we solved all of the problems by going with a custom welded frame with 2" wide colson wheels and andymark shifters. We also switched to live axles which made running the chains much easier. The robot turns very well and after 2 regionals we have yet to be pushed. I think this will be our drivetrain for a long while with some improvements, mainly reducing the weight.

http://www.chiefdelphi.com/media/photos/26794

Re: Best Form Of Six Wheel Drive?
 

116 went to a 6WD for a variety of reasons this year, namely the turning issues with our 4WD traction-wheeled bot in 2006, as well as the shorter length between wheels increasing our ability to climb ramps.
Our 6WD incorporates 6 6" AndyMark Performance Wheels, with rough-top nitrile tread (same as seen on 1345's bot). We have a Andymark 2-speed shifting transmission, with 2 small CIM motors on each side. This is further reduced by a 12:30 sprocket reduction to the 6" wheels, giving us theoretical speeds of roughly 4 fps (low) and 10 fps (high) including inefficiencies. We designed the drive-train in such a manner that center wheel could easily be lowered if needed, and we can lower it at any point we wish by removing 8 bolts and adding spacers between the frame and center bearings. Currently, we are running the wheels co-planar, and it handles outstandingly in low gear, but not as well in high. If we deem that we need to use our high gear (and turn in our high gear) more in Atlanta (we very rarely needed to use it in Richmond), we may lower the center wheel.
So far the drive-train is our best in recent history (just ask our driver), is durable, and easy to maintain. We have only thrown one chain so far (in our first match), and even throwing it didn't really hinder our ability to drive (due to separate runs between wheels). The system is lightweight (under 50 lb.s for our entire drive-train and frame assemblies), easy to construct, and quite effective. It has given us more than adequate pushing power in direct pushing matches, but (like virtually every robot FIRST) can still be rotated by an opposing bot. It has also been able to climb every ramp we've encountered (even some very narrow and precarious ones, although some credit for that must be given to our driver).
http://www.invisiblerobot.com/roboti...h/p1280253.jpg

Re: Best Form Of Six Wheel Drive?
 

We have have had a drive train with the center wheels lowered by 1/8" in the past and it worked pretty well. This year we had 4 traction wheels, two in the middle and two in the back, and then two unpowered omni-wheels in the front. We were told by several teams at our regional that this was a bad design, and that everyone would push us around by spinning us. But actually, no one did.

We had a 4 CIM drive train, geared more for torque than speed, and we were able to push most of the robots we went up against, except that beastly 48 machine. The key for us this year was the welded frame. Because we didn't flex, we pretty much kept 4 wheels on the ground all the time. So we were not easily spun at all. But since the traction wheel axes were only 13" apart, it was very easy to turn. We also implemented a "reverse" button, so we could change the direction of front and back, because we found that for some ramps (those that didn't flex), it was easier to go up leading with the rear traction wheels while for flexible ramps it was better to lead with the unpowerer omni wheels. With last year's (unintentionally) flexible chassis this design might not have worked so well.

Re: Best Form Of Six Wheel Drive?
 

We went with a kitbot chassis (cut to 37x27), but didn't drop the middle wheels. Our wheelbase is 30 inch, split 16/14 so the middle wheels are offset one inch from the true center, toward the claw. AM aluminum six inch omnis on the corners and AM kit wheels in the middle, driven by two CIMS through 12:1 BB 56mm kit gearboxes (unmodified, with no bowtie failures luckily) and 15/28 sprocket ratio. With the winch and compressor in the front and the battery in the back our weight distribution made for very easy maneuvering. And we had no trouble climbing ramps. See our chassis here.

Of course we don't push anyone around but that has not been an issue: we have seeded better than in any of our previous years (6th at St. Louis and 11th at Boilermaker) and were drafted fifth at both events. Didn't make it past the semifinals but we had a lots of fun and put up quite a few ringers. Even managed to hang a couple of keepers. If I had it to do over, I'd probably only use omnis on one end, and would definitely go with two CIMs per side; didn't want to risk that this year due to concerns about the BB carrier plates. If the budget allowed I'd buy AM Shifters, and go with higher traction center wheels.

Re: Best Form Of Six Wheel Drive?
 

6WD. Drop the center wheel 3-4 mm, nothing but traction. The crush of the carpet makes the rock almost imperceptable. This was the drivetrain that 1189 used this year. I think the folks at Boilermaker will attest both it's pushing power and agility (for a 2 degree of freedom drive system).

http://www.gpgearheads.org/gallery/album08/IMGP1542

Don't have a good picture of the finished drivetrain, but the finished version had 6 8" AM Performance wheels with roughtop treading.

I recommend that all teams (especially those having difficulty turning) reference the following white paper:
Drive Train Basics (How to Be Sure Your Robot Will Turn)
http://www.chiefdelphi.com/media/papers/1443
And the spreadsheet derived from it.
http://www.chiefdelphi.com/media/papers/1917

Re: Best Form Of Six Wheel Drive?
 

One of the reasons that I do not personally like about 6 wheel drives with a lowered center wheel is that I am afraid of the rocking back and forth. I assume that if you make the amout that the wheels are lowered minimal (1/8") then it would rock very little. I was just hoping to achieve the ability to spin in a dime.

Re: Best Form Of Six Wheel Drive?
 

I think the most important part of having a successful drive is putting enough power into it. As you look at most of these posts, you will not that a vast majority have had 4 motors in their drive-train, typically the 4 small CIMs. This provides enough torque to turn well, whether you a brute forcing your turns, using omnis, or lowering your wheels. Don't overkill your drive though. Be wise about motor selection and motor quantity, and you will do well.

Re: Best Form Of Six Wheel Drive?
 

I'd be interested in learning how many other teams have run 6WD with omniwheels at the corners before we started to do so last season and what their experiences were. I can't recall anyone doing it prior, but my knowledge of robots is hardly encyclopaedic.

I don't like rock, so I moved away from 6WD with a lowered wheel beginning last season to eliminate the rock. It's marginally more maneuverable than designs without omniwheels, as well, and that's worked well for us for the last several years. We haven't run into many problems with being turned in place, but we have also purposefully and successfully avoided roles that draw attention to ourselves and bring defense.

I've been thinking a lot lately about why some teams use a high traction wheel as the center pair in a 6WD. There may be some benefit -- again with respect to maneuverability -- but such an arrangement does not offer increased pushing force over 6WD with three pairs of identical wheels, so I'm not convinced that accomodating the differing wheels is worth the trouble.

Re: Best Form Of Six Wheel Drive?
 

It depends on your design. The chassis that I designed didn't rock back and forth at all. The center wheels were lowered 1/8th of an inch and the wheels were modified andymark kit wheel. I used roughtop for my design. Roughtop has a coefficient of friction of 1.3 (I know many of you know this, but there are others who doesn't).

Also, I saw a similar 6wd when I went over to team 1251's home. They are using 4 inch wheels and middle wheels were dropped .25". I didn't see any rocking on their bot, and even if there was any it was very minimal. They used wedgetop. The coefficient of friction for wedgetop is 1.2. The designer of the robot made a statement as follows, "We lowered the middle wheels .25" but the wedgetop compresses about 1/8" so in reality, it is only 1/8" lowered."

I have got a calculus exam Monday morning that I need to study for. I know there are few posts here that asks how to design a 6wd, or how it works (similar to those questions). If you got questions and would like to have a discussion, you have got my aim screen name or email in my profile. Feel free message me.

Re: Best Form Of Six Wheel Drive?
 

AndyMark wheels in the corners, dropped center wheels are McMaster pneumatics. Tread wears off the pneumatic wheels, but they're good for at least two events. Pneumatic wheels also allow us to adjust the rock. (We don't really have one--we put most of the weight in the back, so the back likes to stay down.)

Re: Best Form Of Six Wheel Drive?
 

Just curious, why don't you want the robot to rock? considering most 6wd bots have about 1/8" drop, rocking is minimal anyways....but what are the negatives?

Re: Best Form Of Six Wheel Drive?
 

You can see it here: http://www.chiefdelphi.com/media/photos/27927?

It barely has any rocking, it turns on a dime even at 13 ft/sec and so far we have pushed any robot we have tried to.

Re: Best Form Of Six Wheel Drive?
 

A rocking robot base means the end of a horizontal extension (such as a tube-holding arm) moves up and down. Lots of rock means lots of up and down. That's an inconsistency that can't easily be countered by sensors on the robot. If your scoring relies on accurate placement of the arm, rocking can be a significant problem.

Re: Best Form Of Six Wheel Drive?
 

1270 went with an eight wheel drive base this year and Im pretty sure we are going to keep it due to how well it has performed. We took the Andy Mark 2 speed transmission and integrated it into our 4 inch box tube frame and we also put the wheels in there. We used 4"x1.5" IFI wheels for the four center wheels and 4"x1" IFI wheels for the four outer wheels. The base has a lot of pushing power, turns on a dime, and is fast. The four center wheels are lowered 1/8".

I posted a video of the base if you havent seen it yet just click the link.
http://www.chiefdelphi.com/media/photos/26804

Re: Best Form Of Six Wheel Drive?
 

1731 has traction wheels in the center (lowered about 1/2 inch) & omni wheels in the corners. since we are slightly heavier in the back we don't really rock that much -- we basically only use our front wheels to climb ramps.

Even though we are an offensive robot with an arm & ramp were were chosen by the top seed at VCU for our defense & our ability to climb.

Re: Best Form Of Six Wheel Drive?
 

Should have posted this earlier...

The center axle in a 6WD system carries a disproportionate amount of weight on it. In a uniformly distributed frame the center axle carries about 62.5% of the weight vs. 18.75% for the outer axles. Since traction is proportional to weight, increasing the friction coefficient at the center axles has the biggest bang for the buck in terms of pushing force while minimizing the reduction in turning due to high skidding losses.

Assuming a uniformly distributed 120lb bot with 6 skyways (0.7 CoF) then the pushing force is 84 lbs. Swap the center wheels to a ridgetop material (1.3 CoF I think) and the pushing force jumps to 129 lbs (120*.625*1.3+120*.1875*.7*2), an improvement of over 50%. In reality most bots are not uniformly distributed, but instead biased to the center which makes this change even more significant.

Re: Best Form Of Six Wheel Drive?
 

Also wouldn't changing the center wheels to omnis allow other robots to translate you sideways in addition to being able to spin you around the center axis? Then they wouldn't even have to hit you on the corner, any part of either side of your robot perpendicular to the drive direction would do.

Re: Best Form Of Six Wheel Drive?
 

This makes sense, I think. I had neglected to consider that the weight would be distributed unevenly on 'typical' 6WD arrangements since they don't use all six wheels at a time and I presume that's where the 62.5% figure comes from.

Perhaps I'm conceptualizing something incorrectly or misunderstanding your math, but wouldn't it seem that the most torque the outer wheels could transmit before slipping in the scenario above is 120*.1875*.7 (for the pair), or ~15.75 lbs? Once the output torque of your gearbox exceeds that value, those wheels no longer transmit torque to the carpet. In such a case, the torque transmitted to the carpet then becomes only that of the center wheel and its higher coefficient of friction -- 120*.625*1.3 or ~97.5 lbs. This is still an improvement over a .7 coefficient of friction at all points of contact, but it's not as significant as 50%.

Re: Best Form Of Six Wheel Drive?
 

just because they're slipping, does not mean that they are not transmitting force. The coefficient of sliding friction is usally less than that of static friction, but is still much higher than zero.

Re: Best Form Of Six Wheel Drive?
 

One other thing to consider is how your weight distribution characteristics change when you are pushing hard against something like another robot. When happens to a lot of well balanced robot is a tendency for the front to raise slightly, causing the center of mass to shift backward. This can be a quite pronounced shift if your robot rocks, even a small amount.

Such changes can make your back wheels (relative to the object being pushed) as or more important than center front wheels. We found this out the hard way last year. (Fortunately we were able to correct at the Championships.)

Re: Best Form Of Six Wheel Drive?
 

Yes, this is also true. Laziness always gets me into trouble. :)

Does anyone know the coefficient of friction for the KoP AndyMark wheels whilst they're sliding? I'm interested now in seeing how large of a benefit there is to varying your wheel types, though almost nothing will get me to abandon "rock-less" 6WD. :p

For 2008, I'm trying to stress efficiency of design and making the best use of resources in our drive rather than trying to win by overwhelming force. We've ended up running this season with a single CIM per side at 17.5:1 with the AndyMark KoP wheels and AndyMark omniwheels and have had very little trouble making our way around the field. It's been an eye-opener and a refreshing change from the prevailing mindset that seems to indicate that you won't survive a match without the biggest, meanest tread you can find and nineteen motors powering your wheels.

Re: Best Form Of Six Wheel Drive?
 

That's an interesting way of looking at it.

My oldest son Steve (a freshman in engineering school now) went with us the AZ regional, and noticed that 60 was doing very well with a very light robot. It does have a powerful 6wd (rocking) drivetrain, but didn't have all the weight that we normally associate with strong pushing bots. He's trying to convince 1726 to try a few matches without ramps or bumpers, to see if the added agility of a lightweight bot will be more advantageous than the pushing power of the full weight version. The difference would be about 25 lbs.

I was thinking about the rocking thing some more...in this years game, it looks to me like rocking is not much of a concern at all, as long as you are hanging tubes by dropping or hooking them onto the spider foot. And with the rack moving around sideways as much as it does, if you are scoring some other way, vertical rocking would be a very small concern.

Re: Best Form Of Six Wheel Drive?
 

Andy his whole line of wheels in one place http://www.chiefdelphi.com/forums/sh...ad.php?t=50284 'course the KOP wheels were still a surprise to come, but based on the numbers he arrived at I think that using a sliding friction value thats ~90% of the should be reasonable. Sooooo...... If they are 1.0 static then ~.9 for the KOP wheels

Updating my math for the sliding friction...

with skyways...
120*.7*.9 = 75.6 lbs
swap center skyways to traction...
120*.625*1.3*.9+120*.1875*.7*2*.9 = 116.1 lbs

As far as rock from a dropped center wheel goes, with our 1/8 drop across a ~30" wheel base the max rock is ~0.5 degrees. We can see it on concrete, but not carpet... it basically is rockless. C'mon, drop that wheel I know you want too. Everybody's doing it. :cool:

BTW... does anybody have a good metrics for describing a drivetrain?
Pushes around 170 lbs (static)
15.62 fps High (analytic)
6.10 fps Low (analytic)
turning moment of 2400+ lbf*in (Is this a good metric for turning? Is current draw better?)

Re: Best Form Of Six Wheel Drive?
 

For traction, I like to use the "V-Neun" unit of traction. One V-Neun is equivalent to the pushing force that year's kitbot's could produce, assuming it has enough torque to do so, and also assuming a weight of 133 lbs (robot + battery). So, in this years case, 1 V-Neun = 133lbs * .5 (for the amount of weight over the drive wheels) * 1.05 (I averaged AM's values for the kit wheel) = ~69.8 lbs of force. The reason I like this unit is that it gives some basis of comparison since, at least at competition, what matters more is how much traction you have relative to other robots, not the absolute numbers. So, instead of saying our robot has 139.7 lbs of pushing force, which leaves scouts trying to figure out if thats a lot/little/medium, I can say we have 2 V-Neuns, or twice the kitbot's pushing force, they can visualize that amount easier. Also, it takes into account yearly changes in the easiest to make drive system. For example our robot's 139.7 lbs of force would push a good number of teams in 2005, for example, but not so many number this year, since the base kitbot has more traction. (Amoung other factors, like the proliferation of IFI and AM traction wheels) This is reflected in the V-Neun number, this same robot has 3 V-Neuns of traction using the 2005 kitbot as the standard.

Re: Best Form Of Six Wheel Drive?
 

there might be an error in your formula...the factor of 0.5 for weight distribution over the rear wheels ignores weight transfer when pushing. If you put a scale up against a wall, and have the robot push against that, you might discover the real number to be higher than you thought.

This weight transfer thing plays an important role in drag racing, cars that are set up right have the front wheels just skimming the track at launch, all the vehicle weight is on the rear wheels.

In robot pushing matches, a similar thing happens, but for a different reason. Draw a free body diagram of the robot, with the pushing point centered on the bumper zone, see what happens.

Re: Best Form Of Six Wheel Drive?
 

How do you know a given team drives with the driven wheels in the rear? For example, our rookie year we used 2WD with the front wheels driven, then in '02 we drove the rear wheels. I just use a 50% weight distribution for two primary reasons 1) Its easier to calculate 2) The above issues with teams driving differently.

Re: Best Form Of Six Wheel Drive?
 

I'm going to have to agree with you on this statement. Dropping the center wheel of a drivetrain is the perfect fix for adding maneuverability to a six wheel drive robot, and the rocking that results is so small that it will have no effect on the performance of the bot. (I can't even see it on carpet)

As far as calculations go for drivetrains, I don't think we are able to make even close estimates to the amount of pushing force you can get out of a certain drivetrain. It depends on more than just the tread material; the geometry that you have your wheels set up in has an enormous effect on how much pushing power you get. Take team 842's drivetrain for example. They have four wheel drive, but because they have traction wheels set up the way they do they can get much more pushing force than a typical 6 wheel drive bot (240 lbs, actually). There are tradeoffs, though. Carl Hayden's bot can only push hard in one direction, and it can easily be spun by opponents.

After pondering this for a while, I've been thinking that an ideal set up may be what 703 has done with their drivetrain this year, with a large amount of wheels set up in a large arc. Does anyone know how much their robot can push?

Re: Best Form Of Six Wheel Drive?
 

If you are just looking for a quick calculation that has little to do with how hard a robot can actually push, fine....but as you should be well aware, the estimate will be way off compared to real life. We know all about how powerful FWD robots are (not), having done that rookie thing also. :)

Re: Best Form Of Six Wheel Drive?
 

After 3 years of playing with the idea we finally got what team 501 found to be the best possible drivetrain for us. 4 close grouped 8in by 2in wide high traction wheels at the back and 2 8in powered omnis at the front. It makes the robots center of turning at the back but this helps us in this years game. The layout is powered with 4 small cims and 2 2speed transmissions. This design has already helped us win a regional as well as part of the GM Motors Industrial Motors award this year.

Re: Best Form Of Six Wheel Drive?
 

Hmm. I've not studied these sort of things (I want to though), but that seems to make sense, except I would imagine that traction is much more than the weight pushing down on top of the wheel, although that would have a noticeable effect. You also have to keep in mind the material used as tread (Say, a plastic/perspex "tread" would be much less efficient than normal traction tread) and the driving surface matters too (such as carpet, hardwood floor, concrete, cobblestone, sand, etc.).

How then, would you calculate the traction of a robot whose weight is mostly on their right side, and the rest is evenly distributed front to back? It's a 4-wheel bot, though. (I'm asking because 675 is using 4 CIM's/4 56mm Gearboxes @ 12:1 and we can push another bot AND the rack with normal traction tread on carpet, without losing traction)

Assuming that the chassis is not a rocker chassis and also that the other 4 wheels on the bot are normal traction (which would be quite unintelligent, omnis in the center but tractions on the corners... wtf?), then maybe. If the chassis was a rocker, then yes, and probably with ease.

The optimal omniwheel/traction wheel combination is omnis at the corners, tractions in the center. Rocker chassis or not, it offers the turning radius of omnis, with the traction of, well, tractions.

I say if you're going to try to make an interesting drive system, go with mecanum. I think that a bot that moves in all directions is pretty cool.

Hmm, good point.

Anywho, if you break traction in a pushing fight and you're sliding around, it doesn't matter the torque you're transmitting to the carpet, it just matters that it's below the number you want. ;)

Re: Best Form Of Six Wheel Drive?
 

It depends on what you want the robot to do. In typical game competitions, the robots play offense and defense, and a robot that resists being turned (and also has to work a bit to be able to turn itself) seems to do pretty well at being able to score while being "pestered". Omnis make your robot easier to be turned around by other robots. There's always a trade-off!


I agree...my sons made some mecanum wheels for VEX, it's fun to drive that one around!

Re: Best Form Of Six Wheel Drive?
 

On 234 we went with the swampthing-esk drive where you have all traction wheels, still lowered the center wheel 1/8" with the octagon style outside. http://www.chiefdelphi.com/forums/sh...&highlight=234 We really like the "deflector shield" effect. A lot of robots like to hit other robots on the corners, but when you hit the cut-off corners the hits are kind of deflected to sides more than spinning the robot.

Downside: they are more complex to make and they weigh more.

That's just what I've seen. Like every other design there are pros and cons.

Re: Best Form Of Six Wheel Drive?
 

Our drivetrain has six wheels and the key point we have in it is that our center wheels are 4mm lower than the outer wheels. This allows us to turn cleanly and to keep our robot low to the ground.

Re: Best Form Of Six Wheel Drive?
 

You are on the right track

How to calculate traction... well, traction is friction ; where mu is the coefficient of friction (CoF) and Fn is the normal force acting against the friction surface.

You are correct in stating that the traction is dependant on the surfaces involved and that is what the CoF charaterizes. This information must be arrived at through testing, fortunately some wheel vendors have done the testing for us and provide those numbers along with the rest of thier product data (Thanks guys!) Keep in mind that a material may be directional and have more friction in one direction than another. Think omni-wheels.

The normal force is something that can be calculated, and for a simple case of a robot sitting on level ground the Fn is equal to the weight of the bot. If the bot attempts to climb a ramp (not sure why they would want to do that :ahh: ) then the (more detail and pics at http://www.ac.wwu.edu/~vawter/Physic...PlaneMain.html )

OK, so now what about if you have different styles of wheels on the bot?
In this case address the situation on a wheel by wheel basis. First figure out how much of the bot's weight is being carried by each wheel (possibly CAD, or several matched scales) and the appropriate Fn for the wheel. Multiply the wheels Fn by the wheels CoF to get the contribution for that wheel. After this is done for all of the wheels then add up thier contributions to determine the traction limit for the bot.

So has anyone noticed that area didn't come up? Friction, and therefore traction (for our purposes) is independant of area. For those that are getting ready to bring up dragster tires and such I suggest that you read this first. http://www.newton.dep.anl.gov/newton...ysics/PHY2.HTM
cliff notes version... dragsters use wide tires to limit wear, deal with thermal expansion, and take advantage of the adhesive like properties of the rubber compounds.

wheew...

Re: Best Form Of Six Wheel Drive?
 

This is from the Machinery Handbook 26th ed. Keep in mind that the total load is 2W (each span has a load of W) so the reactions at the bottom need to be halved relative to the total bot weight.

I believe this attachment falls within fair use for educational purposes.

Re: Best Form Of Six Wheel Drive?
 

Working with both The Visoneers (73) and G.R.R. (340) this year in the same house allowed me to see two styles of six wheel drive. The Visoneers went with traction wheels in the middle with omnis on all fours corners. They had awesome maneuverability. G.R.R. went with six traction wheels with a lowered centered wheel. That gave them awesome traction. To answer the question which style is better depends on what you want to achieve.

Re: Best Form Of Six Wheel Drive?
 

Our team also uses 6 wheel drive with all traction wheels. The center wheel is dropped slightly to increase turning ability. If you do not drop the center wheel you will need to run the robot with a lower gear ratio to enable turning. Unless of course you run with a ton of motors in your drive train. The other option that a few teams use is to use pneumatic cylinders with delrin blocks or small rollers on the end at each corner of the robot. When they turn, certain cylinders fire allowing the robot to skid during a turn. When the turn is done, the cylinder retracts and you are at full pushing power again. I think Wildstang has done this in the past. Good luck with the drive design.:cool:

Re: Best Form Of Six Wheel Drive?
 

Our team uses the drivetrain with the lowered center wheel.
(hands down most popular it seems)

Last year we used just 4 wheels, one at each corner like a car. It was a terrible drivetrain. We were the slowest robot at the PNW Regional. By the end of qualifications, we were first place. Some of the judges even came up to us and said that our robot should have been losing, it was that bad of a drive train. The only reason we never lost in the qualification matches was because our robot could score all 10 points in autonomous and get onto the ramp in the end.

In the finals, the bad drive train finally got to us and we lost right away. Luckily we have a better drive train this year.:D

Re: Best Form Of Six Wheel Drive?
 

Me and Team 501's drivetrain

Re: Best Form Of Six Wheel Drive?
 

Well, if somebody's going to be running Omniwheels, though, then having them at the corners as opposed to the pivot point would make more sense to me, but yes, there is a trade-off, sadly.


Thanks for that, cleared some of it up. I'll probably get around to reading that again when it's not 10:20PM and I have to pack for a Regional. ;)