Lowered Center Wheel for 6 wheel drive
 

Hi,

We are testing a six wheel drive train and I was wondering how much some teams lower their center wheel. We are thinking about lowering it 1/8 inch....
Also, it would be nice if you also posted how soft your wheels are.

Thanks a lot

Re: Lowered Center Wheel for 6 wheel drive
 

lol, I was just looking into this same question.

229's 2005 robot had the wheels lowered by 3/16th of an inch, and it used the same rought top that IFI sells with its wheels. It didn't hop very much, but it rocked nastilly. Most robots at 1/8th of an inch don't rock unbareably, but if their CG is high, then they hop in an annoying way.

5/32nds of an inch is what makes sense right now, unless the CG is low, in which case 1/8". 3/16", I guess, is too annoying of a rocking motion for me.

There are threads on this, BTW, and I'm not a MechE, I just happened to have looked into this earlier.

Re: Lowered Center Wheel for 6 wheel drive
 

1/8" is, imo, the ideal lowering of the center wheel. When you say soft, you mean how much grip it gets? We used the McMasterCarr Roughtop conveyor belt material, which has a cof of 1.3 to the carpet. Worked pretty well, we were able to push just about anyone with our AM shifters.

Re: Lowered Center Wheel for 6 wheel drive
 

1/8" with Wedgetop.

You have to be careful if you're welding your frame though. It's very easy for the frame to warp and nearly negate your dropped middle wheels. This happened on our practice robot this year.

Re: Lowered Center Wheel for 6 wheel drive
 

swampthing in 2005 was 3/8"

Re: Lowered Center Wheel for 6 wheel drive
 

We did that for our 2005 bot and for our 2006 bot(even though it was a 8 wheel drive). We lowered it about 3/8-1/4 inch, It was variable. We did this not by making the center holes lower, but by making bearing blocks for the back wheels ,which had slots. Therefore, the assembly was adjustable. This allowed us to adjust how quick or how unstable the chassis was when turning. For tread, we used SBR rubber (its the stuff that makes the soles of your tennis shoes). We also got it at McMaster. Our wheels were made out of milled 6061 Aluminum. To hold the rubber in place, we used Loctite 404. Loctite 404 was designed to glue drag racer tires to the hubs. However, I recommend against using the glue because it can be temperamental and result in the rubber peeling off. A better solution is to use pop rivets to hold them in.

Re: Lowered Center Wheel for 6 wheel drive
 

In 2005 we had the center wheel 3/16" lower which was fine. 1/8" probably would have been fine.

Re: Lowered Center Wheel for 6 wheel drive
 

Our first year of lowering our center wheel we dropped it down 3/16, too much rocking for us. Last year we dropped it to 3/32 i believe and it worked alot better. Keep your CG low as possibly for the best handling when you do that. Also the rocking could be productive if you can drive it but its difficult.
-Guy

Re: Lowered Center Wheel for 6 wheel drive
 

We did .200" this year because we weren't sure if warpage would be an issue when we welded the frame. I would be comfortable decreasing it a bit but there are other factors beyond just the amount of rock to account for when we do our final drive train layout.

Re: Lowered Center Wheel for 6 wheel drive
 

This question has been answered before, specifically in the following thread.

CD Poll

It seems that your wheel's COF, placement, and size all play a part in determining how much to drop the middle wheel. The best answer is to play around with different drop heights and figure out what is best for your drivers. YMMV.

Best of luck,

BEN

Re: Lowered Center Wheel for 6 wheel drive
 

I have used 1/4" over the years, but I now think 1/8" is enough. Actually, the more rigid your frame and wheels, the less offset you need.

Also, if you use pneumatic wheels, you can play with inflation levels of the center wheels to get more or less rock. In general, lower inflated wheels grip better so there are limits to what you want to do here, but it does give you some freedom.

Joe J.

Re: Lowered Center Wheel for 6 wheel drive
 

For the 2006 HOTBOT, we had the four corner wheels with rough top tread and the center two with wedge top tread. This made the center slightly higher than the outer wheels just with the tread difference. On top of that, we offset the outer wheels 5/16" or 3/8" (I can't remember which anymore). This worked extremely well for us.

Re: Lowered Center Wheel for 6 wheel drive
 

in 2004 and 2006, 1369 dropped the center wheels by .100". in 04, we used beadlok wheels, and in 06, we used roughtop tread. both years, we had excellent maneuverability.

Re: Lowered Center Wheel for 6 wheel drive
 

Last year I believe our center wheel was lowered 3/16" and for wheels we used six of the KOP 8" wheels ground flat with SBR Roughtop attached using small wood screws. We were very happy with this setup, it turned easily, and pushed hard, although it was rather heavy. (Mostly due to twelve 54 tooth sprockets)

Re: Lowered Center Wheel for 6 wheel drive
 

2005 - 3 mm (just under 1/8")

2006 - 2 mm (just under 3/32")

In 2005 we had a bit more rock than we wanted. In 2006 we used 30% more current to turn than in 2005. We found that the closer your CG is to the center, the less the center wheel has to be lowered. We had a very stiff sheet metal chassis.

We will probably go with 3m this year.

Re: Lowered Center Wheel for 6 wheel drive
 

we have used 6 wheel drive on 2 of our last 3 and I know that this year it was lowered 1/8 and in 04' I am pretty sure it was the same. :)

Re: Lowered Center Wheel for 6 wheel drive
 

3 meters seems a bit extreme :)

Re: Lowered Center Wheel for 6 wheel drive
 

I am sure that he ment 3mm if you read the post. :) .

Re: Lowered Center Wheel for 6 wheel drive
 

I take it you have the inside scoop that the height restriction is going to be quite tall in 2007? ;)

Re: Lowered Center Wheel for 6 wheel drive
 

No, he's talking about new tread material... I guess FIRST is lifting the "No Adhesives" ban... ;-)

Re: Lowered Center Wheel for 6 wheel drive
 

you don't lower the middle wheel-

you use progressively larger tires from front to back

:cool:

Re: Lowered Center Wheel for 6 wheel drive
 

<offtopic>Can someone explain in businessmajorese how this would work effectively? It sounds interesting.</offtopic>

Re: Lowered Center Wheel for 6 wheel drive
 

I believe 1/8" is the best option but if you're not sure I would definitely stick with getting pneumatic wheels for your center wheels so you can adjust the wobble with air pressure. Check out skyway for some nice wheels.

Re: Lowered Center Wheel for 6 wheel drive
 

We've been debating using a Nascar technique to robot design. After having realized that the most efficient setup for a 6 wheel robot is to lower the center wheel, we statistically analyzed all of our matches and noted that we turn left much more than we turn right, and as such we are considering making the wheels on the right hand side a tad bit thicker (perhaps a few mm), and having the left hand train on a slight angle < 3 degrees. This compensates for the drag of the wheels when turning and allows for much faster game play.

Re: Lowered Center Wheel for 6 wheel drive
 

Wow, that's pretty in depth analysis. You guys are actually considering making the robot turn one way better than the other just based on driver preferancec?

Re: Lowered Center Wheel for 6 wheel drive
 

Nice job with the stats, I figure you will be using the same driver that you collected those stats on. Let us know how that works out, I like it... unless your driver/ game strategy changes...just a thought...

As for the drivetrain q, Tank ('06) was a six-wheel drivebase, with the middle being down 1/4''. We used 12''d, 4''w wet-track go-cart tires on the middle and back, and the fronts were narrow (to accomodate our pickup) 8'' pneumatics on our own beadlock-style rims. Our frame was welded, but we used bearing blocks to mount all our wheels, so warpage wasn't an issue. We located our CG slightly behind the middle wheel (partly by accident, I must admit), and this meant that we were generally running on our wide, tacky back wheels. We could push or at least hold everyone we came up against, but with the center wheel so much lower, it turned amazingly well. We didn't have any problems with wheel hop, for our CG wasn't above the top of our big wheels.
A lot has been said about experimenting with pneumatic tire pressure, but we didn't so much because we wanted as much tread as possible to be contacting the floor (running on a dusty floor is a GREAT way to check for this, by the way), but if you were using narrower tires it probably wouldn't be as big a deal.
I wouldn't recommend using four go-cart tires in a drivetrain unless you plan to be a "defensive specialist", as we were, or you can build a very light yet effective offensive apparatus (like our pick/unload roller).
I would recommend a six-wheel drive base if possible because you can get away from all the wheel hop problems of a four-wheel.
Good Luck, y'all. Let us know how it pans out. -JH
P.S. sorry, I tend to ramble. :o

Re: Lowered Center Wheel for 6 wheel drive
 

In 2005 237 dropped their center wheel in a 6 wheel drive setup. They lowered the wheel 1/4 and it worked out perfectly. In 06 we also decided to go with a 6 wheel drive setup with the center wheel dropped. I can't remember 100% but I believe it was either 1/4 inch or 1/8 inch. Either way it worked really nice. The drive system was always beastly.

Re: Lowered Center Wheel for 6 wheel drive
 

I've thought about this for a bit and, operating under the assumption that all the wheels remain on the ground all the time, I can't understand either how such an arrangement is helpful.

Re: Lowered Center Wheel for 6 wheel drive
 

I don't know if it is a joke or not. I have heard that back when he was a competitive bodybuilder, the current Gov of CA used to psych out his opponents by leaking crazy stories to the press about the "secret" to his success (e.g. eating huge amounts of some strange food or sleeping 20 hours each day rather than the hard work of actually bodybuilding).

Between this "larger wheels from front to back" posting and the NASCAR posting from Neil Parikh, it is quite possible that Team #25 is pulling our collective chains. But...

...this is how it may work and work quite well.

Put all axles in a line. Have the front/middle/rear wheels with radii X/X+a/X+a+b respectively where a and b are small positive numbers (both with respect to X and in absolute terms) and b<a.

This does provide a similar effect as lowering the middle wheel; on a flat carpet, only the front and middle wheels OR the middle and the rear wheels can touch the ground at any one time (I leave this as an exercise for the reader -- but trust me it is true). This changes the aspect ratio of the width/wheelbase and makes it easier to turn.

BUT... what else does it do?

It also means that the "ground speed" of each tire is slightly different (assuming that the wheels are driven by a single chain and the sprockets are the same size).

What difference does that make?

The first thing it will do is increase rolling resistance. When you are driving in a straight line, with two wheels on the carpet, it means that one wheel must be slipping on the carpet. This, in turn, means that the straight line rolling resistance will be higher than it would with the same size wheels. This sounds bad, but, when you have 3 motors per side like I understand Team #25 typically has, you have power to burn, especially when you are driving is a straight line.

BUT... ...what happens when this same robot turns? Now one wheel is already slipping on the carpet.

I remember a prof. in college beating one idea into us in his class, "Friction is a VECTOR and the MAGNITUDE of the vector is at MOST muXN." This sounds like a stupid and obvious idea, but it is surprisingly important. He would give us examples of where this is important like pulling a cork out of a wine bottle (by twisting the cork as you pull it out, you point some of the muXN Vector in a direction that is 90 deg. from the direction you are pulling... it is easy to pull out) and a rear wheel drive car on snow (if the driver guns the gas, slipping the rear wheels, very small forces toward the back of the car can easily move the car from side to side).

How does this apply to this case?

Well, it may mean that by having one set of wheels already slipping on the carpet, turning ability may be improved because there is a smaller component of friction available to resist the scrubbing of the tires on the carpet.

I am not sure that I have convinced myself that it is a good idea, it would take some noodling to figure out the right values of a & b for example, but I am not sure that it is as far fetched as I had initially thought.

Joe J.

Re: Lowered Center Wheel for 6 wheel drive
 

Gee Joe- Us Joke?

would we do that?

actually- everybody asks us how much we lower the center wheel and we always tell them that we dont lower it at all. But then we are told we NEED to so I guess our bots just dont work.

We don't lower the wheel. The robot overpowers lateral friction to make the turns. The wider the wheel base the better it works.

No matter- the Evil Machine base design is being revamped for 07 to be "a bit more powerful". There were a few bots this year that were able to move us when we had the brakes on so we want to have a little more "umph". We gave a bunch of teams the schematics through last season so we know there will be others like us out there this year. It is still a great basic drive design- just not strong enough for some applications (like pushing free of a corner goal).

WC :cool:

(PS- I have some kidnapped Moe Sticks sitting here in with a pile of FIRST mailboxes if you want them)

Re: Lowered Center Wheel for 6 wheel drive
 

I can't wait to see it! It's amazing how you guys always find a way to improve.

Re: Lowered Center Wheel for 6 wheel drive
 

look out ... they are all liars ;) In 2007:
25 will go with five omni wheels
217 will become a proponent of tank treads
357 will get sick of Jester Drive and revert to the kit wheelchair wheels

It'll be chaos I tell you ...
but Dr. Joe's thoughts on progressively large has me thinking...

Re: Lowered Center Wheel for 6 wheel drive
 

sssshhhh! darn- you said you wouldnt tell! :cool:

Re: Lowered Center Wheel for 6 wheel drive
 

We have a six-wheel drive setup with fairly soft six inch wheels, and we just used the standard kit-bot deviation... It worked like a charm. I believe that it is 1/8 of an inch, but if I'm wrong please correct me.

Re: Lowered Center Wheel for 6 wheel drive
 

Ah, I see the problem with my thinking! The wheels are not linearly larger as you go back! Interesting, and Dr. Joe, I think you are completely right in the uN component arguement. Unfortunately, I would think you would wear tread quite quickly. You also sacrifice a bit a of anti-push resistance when you're moving, because the wheels that would normally be providing static frictional force to oppose movement are now only providing sliding frictional force. Still, food for thought...

You could also use 4 wheels of the same size, and two in the front (or back) that were smaller, and achieve the same effect. We did the same thing, now that I think about it, in 2005 to reduce a hopping problem.

Re: Lowered Center Wheel for 6 wheel drive
 

Note very closely the placement of the middle track sprocket on our '06 shooter bot without the drop you cant really turn very well. We use a machined aluminum box frame so warpage was no issue.

-Q

Re: Lowered Center Wheel for 6 wheel drive
 

I was in a rush to get dinner when I posted the above. Since eating, the blood has returned to my brain. My analysis in Case 2 is just wrong. The case is a classic example of a "statically indeterminant" system. In such cases, you need resort to the stiffnesses of the members to determine the loading on each wheel. But... ...even so, in this case, to the extent that the middle wheels carry any load at all, it is better than the 4WD case with wheels in the corners. And with reasonable assumptions about the chassis it is not too hard to get the same answer as the one in the above message, even though I arrived at it in error.

Joe J.

Re: Lowered Center Wheel for 6 wheel drive
 

Do teams ever adjust their middle wheel height automatically to accommodate turning. I swear I saw the triplets lowering theirs when they turned last year but I am baffled how.
Can someone point me to the right forum/ upload some pictures or CADs. I've searched around a bit and it seems to be nonexistent on the forums....probably the better question I should be asking myself is why arent I using a swerve drive.