BIG Tires

[Joe Johnson]

The worse you are with the ratio:

Side to side distance between wheels touching the ground
-----------------------------------------------------------------
Fore/aft distance between the wheels touching the ground

the worse you be able to turn.

It's painful to get this wrong.

I give you,

Dr. Joe's Seven Stages of Dealing with a 4WD Robot with Grippy Tires

• First, you'll blame the drivers:

  "Just TURN already will you! How hard can it be?"

• Second, you'll blame your batteries:

  "Why do we keep browning out during a match? Are you SURE that battery was a full charge?"

• Third, you'll try more motors:

  "How about if I have 3 CIMs per side? That's got to work. No? What if I add a mini-CIM? Still can't turn without moving! I've heard good things about those unlimited 775Pro's. 5 motors per side! What can possibly go wrong?"

• Fourth, you'll go for a higher ratio:

  "What do you mean we can't fit a 6 tooth sprocket on that gearbox?!?! I've got to have more POWER*!!!"

• Fifth, you'll wonder why you can't even play defense:

  "We finally got enough motors and ratio to be able to actually turn and NOW the robots we are supposed to be playing defense against keep driving past us by like we're standing still "

• Sixth, you'll try a set of slippery wheels:

  "Now that we've wrapped the front wheels in duct tape we can actually turn with our original ratio... kinda stinks playing defense and crossing defenses with half our wheels slipping and sliding but it's better than what we had."

• Finally, you'll vow to do better next season:

  "There are a lot of people with a lot of experience in FIRST who seem to think that 4WD without some method to address the turning problem is... ...a problem. Maybe I should find out what they know before the next kickoff?"

Dr. Joe J.

*But of course you mean TORQUE. You already tried more POWER back in stage three!

[Jim Schaddelee]

Looking though old pictures and I think these are the largest wheel in first history.

[Joe Johnson]

Originally Posted by Jim Schaddelee

Looking though old pictures and I think these are the largest wheel in first history.

Jim,

Those are in fact very large wheels for a FIRST robot but you know this isn't going to end well for you...

I know 107 is indeed a low numbered team but with tens of thousands of robots in FIRST over the quarter century, you KNOW that someone is going to say, "no so fast there, Mr. S. There was one regional back when dirt was new that had wheels the size of ..."

Hold Strong,
Dr. Joe J.

[techhelpbb]

Anyone thinking about the old time bicycle design with a big front tire and smaller rear tires on a swivel caster for balance?
If you keep the weight closer to the large tires but between them and the follower, and power train on the big front tires, the rear tires just have to follow along like a train.

Get one of the exercise balls from a few years ago and use it as a huge ball caster in the back .

[Jim Schaddelee]

Joe,

Just to Clarify, I strongly wound not suggest wheels like this for Stronghold. But on another note, This is my 20th year do this and it nice to still see some "old" faces like yours around. You and Your Chief Delphi team was a major inspiration for me in the early days of FIRST.

jim

[Alex Chamberlin]

Originally Posted by Joe Johnson

The worse you are with the ratio:

Side to side distance between wheels touching the ground
-----------------------------------------------------------------
Fore/aft distance between the wheels touching the ground

the worse you be able to turn.

It's painful to get this wrong.

Do you want a higher or lower ratio?

[techhelpbb]

Big tires you say?
DIY
Smaller of the same idea but no rubber

Seriously what is stopping someone from making a wooden, or combination aluminum/3D printed rim, for any size tire they like?

Same basic ideas could be applied to this:
12" spoked tire from Northern

[Joe Johnson]

Originally Posted by Alex Chamberlin

Do you want a higher or lower ratio?

Good question. The way I showed the ratio, bigger is better.

WideBots can turn. NarrowBots have troubles.

The reason lowering the center wheel works so well on a 6WD robot is that it allows NarrowBots to pretend that they are WideBots (in a sense) because only the 4 front wheels or the 4 back wheels can be in contact with the ground at once SO the ratio is approximately doubled (the top stays the same, the bottom is halved).

Careful readers will notice something and be worried (possibly). In order for this to work you have to actually get a set of wheels off the ground (or at least unloaded). With pneumatic wheels and uneven floors the assumption that lowering the center axles 1/8" is going to be enough may not be such a great assumption. It also means you are going to have to pay attention to the pressure in your tires -- you can easily make a great turning robot into a dog by allowing the pressure in the center tires to go down a bit.

Think about it.

YMMV.

Dr. Joe J.

[techhelpbb]

Another way to reduce the turning radius of an otherwise 4 wheel robot could be the old Batmobile stunt:

Lift the robot up on smaller pivot underneath breaking the traction between your large outer 4 tires and the ground then turn on that pivot. Course you probably have to stop to do that.

[Joe Johnson]

Originally Posted by techhelpbb

<snip> Course you probably have to stop to do that.

Not true. I think many teams have done something like this where they dropped a pneumatic turn assist foot whenever they needed help turning.

I will let other who know better give examples but i am 99% that a number of robots designed by Raul the Magnificent formerly of Wildstang (#111) now (like so many great FIRST engineers) working at Bob & Tony's Excellent Adventure in TX.

Let those with knowledge share it.

Hold Strong,
Dr. Joe J.

[dmorewood]

Well you first problem is that it's a wheelbarrow wheel. The hub on that is designed to spin freely. So your going to have to make and design all new hubs for every wheel and these new hubs are going to have to be able to take all the torque your putting on them.

[MrForbes]

I was thinking about how to drive the "wheelbarrow" wheels, which have a tube with bearings in the ends. My idea is to make a steel hub, with an ID the same as the OD of the tube, and a 6 hole bolt pattern to match common robot sprockets. Tack weld the hub to the tube.

But I haven't actually done this yet! and I have some equipment that other teams may not have access to, although I think you could do it with nothing more than a hole saw and a drill press and hack saw, if you needed to. Oh, and a MIG welder, although there are plenty of shops that could weld something like that pretty easily.

[techhelpbb]

Depending on the forces involved:

In the past I bought some tires from Tractor Supply made a jig to drill out a bolt pattern that I also put into some G&G Manufacturing cast sprockets. I put a piece of PVC around the bolt pattern as a big spacer. A good shot might break that alone but I packed it with Shapelock. In place of PVC one might use EMT as well.

It simply worked and I've seen a few go-karts setup like that as well.
Just keep in mind you need to be careful matching your part selections to make this as easy as possible.
BTW it's heavy so if weight is your concern think it over some more.

[GeeTwo]

Originally Posted by MrForbes

I was thinking about how to drive the "wheelbarrow" wheels, which have a tube with bearings in the ends. My idea is to make a steel hub, with an ID the same as the OD of the tube, and a 6 hole bolt pattern to match common robot sprockets. Tack weld the hub to the tube.

But I haven't actually done this yet! and I have some equipment that other teams may not have access to, although I think you could do it with nothing more than a hole saw and a drill press and hack saw, if you needed to. Oh, and a MIG welder, although there are plenty of shops that could weld something like that pretty easily.

If you're firing up a welder, why not just use the COTS built-in bearings on dead axles and weld the sprockets to the wheel, possibly with a short length of tubing to move the sprocket away from the bearing? (You will need steel sprockets, but no hub, so it shouldn't weigh any more.)

[DaveL]

The manufacturer claims that they are not for highway use.