What am I not considering about using huge tires (like these)?
The robot being designed is not going under the low bar. The plan would be to use the long kitbot chassis but instead of 6 wheels just have 4. For ease of use I would assume we would still mount the gearbox in the center and drive two center sprockets that are then connected to the sprocket on each wheel. Assume that we could space everything appropriately. Would we have turning issues? Doesn’t seem like it.
I realize that the distance between the inner and outer plate would have to be widened to accommodate the wider tire. What other challenges would we face?
Any idea how to attach a hub/sprocket to a wheel like that?
Any thoughts on why this wouldn’t be a good idea from a game sense? Seems like it would make many of the static obstacles much easier while giving more ground clearance. I know this is rather out of the ordinary and nobody is set on it, but I am trying to come up with good reasons not to do it.
The biggest problem you are going to have is turning with only 4 wheels. The reason for a 6WD is that the center wheel is dropped slightly making it easier for your robot to turn.
Interfacing aside, you need a larger gearbox; the reduction needed for significant force is higher. (Holding torque constant, force decreases with wheel size)
By going to a larger wheel, you:
Probably lift your CG up (gearboxes are higher up as well as framing), decreasing stability
Shorten the contact patch of the bot (your area of contact is now 14.5" less than the length of your bot instead of only 8"), decreasing stability. Now your contact area is around only 15" (assuming squarebot). I made an offseason bot with this C-C distance once with 10.75 reduction from a single CIM onto 6" wheels, and had enormous issues with being spurred into a wheelie.
Force yourself to get rid of a center wheel, so you can’t run a drop and decrease turning scrub in this fashion.
All of this would deter me from running these wheels
One thing to consider, if you are putting 3 wheels per side at 14.5" per wheel, plus clearance, makes each side of your robot 46in. This leaves you 14" of width for your robot, at least 8" of which will be used by wheels and sprockets/pulleys.
Leaving you six inches of width to build the rest of your robot.
You don’t need different gearboxes, but you might want them. We run the old kit drive train in our off-season and t-shirt toy that looks like Dozer with 10 wheelchair tires. (Interfaceand finished product)
You don’t have to worry about CG because those things weigh a ton and will keep you planted just fine.
The end result is a machine that, with enough runway, can go really fast. Scary fast indoors, on a football field it’s not so bad. We also run ours off a deep cycle marine battery.
My biggest concern would be weight, those things are heavy, and the drive to hold them together is heavy, and the chain to move them is heavy. We build a 6wd with 10 rubber tires and had to ditch it. The drive base weighed in over 70 lbs without a single manipulator on it.
Turning scrub would also be an issue to worry about.
if you kept whatever is going inside the chassis short enough, stability wouldn’t matter. if you flipped over, you’d still be able to drive. the bot might even be able to right itself against a wall
Turning is a big concern. You’ll want to make sure the track width is greater than the wheel base using 4CIMs and a reasonable gear ratio (10-12 ft/s). The greater that ratio is the less current your motors will draw when you turn, meaning you’ll have better batteries for the end of the match (important for SCALING).
Track width is distance between the two sides of the robot as measured from the center of the wheel contact patches. Wheel base is measured from the rear-most to the front-most wheel, and usually you can easily do it by the center of the holes in the axle.
For example, say you want 15" tires, 4WD. Assume a 1" gap between the tires just for math’s sake. We have then figure out how wide the drive base is allowed to be based upon the FRAME PERIMETER rules. If there is 1" from the front of the FRAME PERIMETER and the tire, and also 1" on the back, your total robot length is 33". Thus, the maximum allowed width of the FRAME PERIMETER is 27".
A cursory search for 15" tires shows the skinniest coming in around 4", but let’s say you do bike tires that are about 2" wide at the tire. Using the KOP drive base, there will be about 1" between the edge of the FRAME PERIMETER and the edge of the wheel, then another inch from the edge of the wheel to the center of its contact patch (so the contact patch is 2" from either side).
The distance between the axles, or wheel base, is a wheel radius + 1" gap + wheel radius = 16".
The side-to-side distances between the contact patches is 27" - 22, or 23".
Generally speaking, you’ll probably be fine so long as you aren’t geared to go super fast and the tires aren’t super knobby. It’s how Team 25 got away with 6 traction wheels on the floor for so long
My team has done 4 wheels every year since 2012. This may be the first year in a while we’ve done 6 wheels, but we haven’t given up on the idea of 4 just yet…
The other issue with very big tires is simply the torque necessary to turn them (and the loss of torque you’ll have when they interact with objects.) They’ll be ~3.5x harder to turn, and the same factor faster, than 4" wheels off of the same gear boxes…so plan on gearing that down heavily compared to most drive train gear boxes.
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!
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 …”
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 ;).
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.
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.
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.
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.
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