The Center Wheel

[Akash Rastogi]

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).

.

[Cory]

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

Originally Posted by Cory

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

Same here.

[Eugene Fang]

Originally Posted by Cory

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

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.

[NickE]

Originally Posted by EugeneF

You don't need a lot of "drop" to get the effect you want.

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.

[EricH]

Ditto on the drop not being a problem for 330.

We also used sticky pneumatic tires in the center and AM 6" wheels on the corners; one year we tried 6 stickies and quickly swapped at least two of them after having problems turning.

Bear in mind that 3 of the 4 years I know of us doing a 6WD drop, we had arms to place objects. We never had an issue placing or acquiring in a big hurry, or on the go.

[Triple B]

233 has dropped the center wheels since 03.
This year we dropped the center 4 wheels.
In my experience the hopping is caused by a flexing chassis.
We always try and build the frame as stiff as possible.
mike d

[Al Skierkiewicz]

The reason behind dropping the center wheels is due to the current drawn by the drive train with all the wheels on the carpet. If all wheels are high friction types (in all directions) then the current on the drive motors skyrockets to near stall current. With a CIM motor drive that is 129 amps per motor. With four motors pulling over 400 amps, the power supplies feeding the Crio and radio start to fail or at least disable Crio output. In some cases the hopping robots you see are due to the wheels actually breaking friction with the floor and in others it is due to the Crio shutting down the outputs. In worse case, the Crio reboots and you all know how long that takes for your individual robot design and software. The decision to keep all wheels on the same level is a risky one that each team must make based on their own robot and wheel design. You can minimize the current by altering the turning radius in software, using omni wheels, dropping the center trucks, or dropping a helper wheel to lift the front or back wheels during a turn.

[R1ffSurf3r]

Originally Posted by NickE

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.

we have less of a drop, and when our robot straddles the plywood transition, it rotates on axis painfully slow. but on the flat it is fine

[RoboTIP]

Originally Posted by squirrel

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.

excellent idea! Anyone willing to share the relevant equations?

[Jon Stratis]

Whenever we've done 6-wheel designs, we've dropped the center wheel. In Overdrive, we had a forklift-like elevator that got up probably 10 feet high with dropped center wheels... and we never tipped over. We also had a giant steel plate on the bottom of the robot to lower the CG.

Dropping the center wheel a fraction of an inch does make the robot look a little unstable as it bounces between front and back wheels, this is true. But in reality, if your CG is fairly low, it introduces no risk of tipping while on flat ground (and even climbing ramps in Rack and Roll it didn't seem to matter).

You may want to create a small test bed to see the results and prove it to yourself. Create a standard robot chassis with 6 wheels, but make the center wheel's height adjustable for example, you could securely mount a screw vertically that, when tightened, pushed the axle downwards, giving you a dropped center wheel, assuming you're using dead axle's). Drive it around with both dropped center and non-dropped center, and see how it handles with each. Design some tests for stability to determine if that would be a problem. In short, get some data before you decide which way to go.

[Joe Ross]

Originally Posted by RoboTIP

excellent idea! Anyone willing to share the relevant equations?

http://www.chiefdelphi.com/media/papers/1443

[ajlapp]

How about making your drop adjustable?

Also remember that you can drop the center wheel or lift the outer wheel.

When making an adjustable setup it may be easier to lift the outer wheels that probably are not interfaced with your transmission directly.......depending on the rest of your design.

[Rich Kressly]

Please note I'm talking about using "standard" type (kit or similar) wheels for purposes of this discussion. The use of omni wheels, mecanum drive, holonomic drive, etc. all provide very different characteristics.

In general the "hopping" / excessive current draw / etc problem isn't limited to a six (or more) wheel drivetrain. I've seen it in four wheel drives as well - in fact our team made a design mistake in 2007 with our four wheel drive configuration which led to excessive current draw and poor turning at times.

Turning torque (also called turning scrub) is the culprit and is largely affected by the wheel base width vs. length. Yes wheel types, etc can have an effect too, but as a general rule in standard tank style drives (4,6,8, no matter how many wheels) you'll want your drive base width (distance between wheels left to right) to be equal to or wider than the drive base length (distance from center of front wheel to center of back wheel).

The VEX Classroom Competition teacher's handbook, the Inventor's Guide, and the Autodesk curriculum all carry more detailed information on the topic - diagrams, etc.

In an FRC robot, if you build your chassis in the classic long rectangular configuration (ala 254 most years - ~38" long x ~28" wide), a six wheel drop center configuration provides huge maneuverability advantages and cuts out excessive current draw because only 4 wheels are ever on the ground at one time - cutting the effective front to back wheel base in half, making your effective wheel base on a flat surface wider than it is long.

If you build your FRC robot in "wide body" configuration (~28" long x ~38" wide), there's most likely (depending on other variables) no need to drop a center set of wheels as the wide configuration and shorter front to back wheel base/footprint provides for easier turning all by itself. Team 25 has built some great 6-wheel wide body robots and some long ones as well. In those machines (long), as stated before, clearly they are using means other than a drop center wheel to overcome the turning scrub/current draw issue and they'd be best suited to answer the question as to how they get it done.

If your drop distance (my team has always used .125") is in a decent range, rocking should never be a problem. We did encounter an issue this year like one other post here stated. We started out long configuration, drop center and because our CG was so low and our mass was so evenly distributed around the center, we had a difficult time driving the robot because we were effectively turning on two wheels most of the time. So we took our drop shims out in Washington, DC which made us easier to control and almost impossible to turn (dropping to like 7 volts on a fresh battery). After that we put a pair of omni wheel on one end making turning much easier, but maintaining our control while driving/turning.

Whatever you decide, just be sure you understand the way turning torque/scrub changes with the configuration of your robot chassis and wheel position. Those simple factors should enable you to make sound decisions without things getting too very complicated.

[Jeff 801]

Originally Posted by Don Rotolo

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.

Team 1323 did this and you can see the rails that they used here
http://www.chiefdelphi.com/media/photos/32378