The Center Wheel

FIRST General Forum
1

I’m designing a 6-wheel drive train and I’m debating on whether or not to drop the center wheel. I know of several teams that drop the center wheel, but I also know of several teams that don’t.

Dropping the center wheel can improve the maneuverability of a robot, and can also prevent “hopping” when the robot turns. However, in my opinion, it reduces the robot’s stability footprint. Unless I’m convinced otherwise by this thread, I’d like to keep all 6 wheels on the ground.

I’d really like to hear from teams that don’t do dropped wheel drive trains, particularly regarding how they prevent their robots from “hopping”. Teams that immediately come to mind: 254, 233, 968, 25

2

I would recommend dropping the center wheel. We have done so since 2005 excluding 2009 with varying amounts of success. Over the years, we have dialed in the drop to around .8" to .125". The only time we had trouble with dropping the center wheels was in 2008. I believe we had like 3/16 of drop that year and a very centrally located CG. We were to maneuverable to the point where the robot was tough to control.

On the Issue of stability, we have never had an issue. Maybe because we always try to keep our CG low.

Hope this helps.

-Ryan

3

Assuming you are using 6 traction wheels, you need to drop the center wheel. The robot wont turn if you don’t.

If you dont want to drop the center wheel, you can use either 2 or 4 omni wheels on the corners so that they roll instead of skidding.

4

Every one of those teams bar 25 does dropped wheels.

I actually don’t know of a single other team that consistently does 6WD non dropped wheels, to give you an idea of its popularity.

5

The decision to drop or not depends upon the traction of your outer 4 wheels. Less traction means less need for drop, more traction means more. Thus mikelowry’s comment “Assuming you are using 6 traction wheels, you need to drop the center wheel. The robot wont turn if you don’t.”

Make a set of side plates that have an adjustment for center drop, get a bunch of different wheels and experiment. Best way to learn.

6

Which wheels do 254 and 968 drop on their 8-wheel drivetrains?

7

The 4 center ones.

Drop your center wheels, unless you don’t want to turn. 25 may not drop their wheels, but you’ll find 25 teams who don’t drop their centers and can’t turn for every 1 team that can

8

Don made a great suggestion (as usual), try some different things and see how it works.

And you can play engineer first, figure out what will happen using math and physics equations, then see how close your predictions are to your experimental results.

9

i am 90% sure 233 drops their center wheel, just very slightly.

we have always dropped the center wheel. Depending on how far u drop it and how your CG is situated your bot will rock. I find this to be the best way to do tank drive. i personally hate the omni wheels in the corners as they make the bot far to easy to spin (from a defenders point of view). You can make a robot just a maneuverable and many times stronger by dropping the center wheel and putting real wheels on the corners.

my $0.02

10

We get this question all the time, with people curious about our turning. We find there to be no real use in dropping our center wheels, because if we do, at any given time, our 6 wheel drive train could just as well be a 6 wheel drive train with two wheels in the air.

I’ve noticed the “hopping” robots on the field, and wondered why we’ve never had the problem. The skyway wheels that we tread allow us to drive with 6 as well as turn great with 6. They also have a wider width then most wheels used by FRC teams.

11

yeah I was thinking if you did an 8-wheel with the middle wheels dropped you could form the equivalent of a square chassis. I may be wrong but I’m pretty sure the skidding come from the drive being rectangular, causing conflicting vectors.

I know its a bit off topic, but has anyone tried a square mecanum drive? If so did you see a difference?

12

Coefficient of friction of the wheels on the carpet is one of part of the equation…normal force is another. If you have less friction, you can have more normal force, and still be able to steer as easily.

13

It seems like that your problem with dropping the wheel is just because you think it will decrease the robot’s stability, which in my opinion it will although I don’t have an answer for you on how much, but if this is your problem I say go with your instincts and keep them at the same level. Although in order to gain some of the manueverabilty that dropping allows, make your outer wheels omni wheels and have your center wheel be traction.

On a different note and this is something I think we’ll be trying in the offseason is, and we stole this from SparX (1126), do a 6 wheel with a 1/8" center drop, with the center wheel be traction wheels along with one of the outer sets. While having the other end be omnis, this will change the bot in a way that if you go in one direction, so all traction wheels are touching, you have more traction to the ground, or travel in the opposite direction, with one set of traction and one set of omni wheels touching the ground, which will give you more maneuverability

14

The relatively wide wheels Team 25 uses helps avoid hopping. Their siping program is a science and an art.

15

If you are very adamant about not dropping your center wheel, you can use outside wheels with a lesser coefficient of friction than your center wheel.

However, this will decrease your tractive force.

16

If you are also concerned about the amount of teetering you’re getting from a dropped center, make sure your outer wheels are as far apart as you can go. Its an obvious solution to a very minor problem once CG is maintained as close to the center of your robot as possible (barring any intentional CG changes given certain manipulators).

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17

From the experience we’ve had with a dropped center 6WD the rock has never been a problem. Not once.

18

Same here.

19

Same here, too. You don’t need a lot of “drop” to get the effect you want. And that little amount of drop results in a barely noticeable rock.

20

On a flat field, at least.
With the thick plywood under the carpet near the bumps this year, our robot needed a larger drop to turn well near the bumps.