Lowered Center Wheel for 6 wheel drive

[987HighRoller]

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

[Joel J]

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.

[Jeff K.]

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.

[Cory]

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.

[Tytus Gerrish]

swampthing in 2005 was 3/8"

[edthegeek]

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.

[sanddrag]

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

[Guy Chriqui]

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

[Peter Matteson]

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.

[Joe Johnson]

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.

[Pat McCarthy]

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.

[RogerR]

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.

[Ben Piecuch]

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