36 Inches

At a recent tournament, a team was pushing the limit in terms of horizontal expansion. I brought it up, and the refs were adamant that it was a 36 inch square, not a cylinder like I thought. Which is it?


See above q and a.

If the refs said that, they’re totally wrong. But yours isn’t quite right, either. It is neither a square prism nor a cylinder. The link @TeamTX gave is good. The actual shape is closer to the cylinder idea, but the robot can extend outside of such a cylinder by a little bit while still remaining within the allowed expansion. Think of it more like a cross between a right cylinder and a right, equilateral, triangular prism. Look here for further explanation:

It’s also like a slinky in that it can shift as you go up vertically.

The reason the rule is written this way is to make it practical. The vertical expansion tool is an easy way to check for 36" expansion without having to worry about cylinders. It’s hard to define, but really easy to check.

I don’t think that’s the case. Otherwise you could make a robot that at 4" above the floor sticks out a foot beyond its 18" cube, at 5" above the floor sticks out a foot on a side 90 degrees off from the 4" one, etc. Then you’ve got a robot making a + shape measuring 42" along each line of the +. Make more of them at different angles, and occupy a circular projection with a 42" diameter. Alternatively, you could put an tower on one end with an arm that swings from its top, and when it first swings up it expands to 30" or so horizontally in addition to some upward, so that you can aim it up and away from the end of your robot with the tower; this would allow you to be in the expansion zone and contact the high flags on either end of the field.

I made a quick Inventor Sketch of the part of their robot I was talking about.

I believe that the best way to visualize it is to look at the top of a robot as if it were a 2D image and if any point on the bot is >36 inches away from any other point on the robot then it is out of size

afaik, its actually a reuleaux shape, but that is closer to cylinder than to rectangular prism

That’s a cylinder you just described.


No, he did not just describe a cylinder. Look here for further explanation:

The robot you drew is illegal.


Any horizontal dimension is under 36". The robot would easily pass the test as long as the tool is held horizontally. In order for it too not for you would have to twist the tool. That isn’t horizontal.

What that means is you measure from any point to any other point in the robot, and you ignore the vertical dimension, thus limiting yourself to any horizontal dimension. That circled number shows you a horizontal dimension measuring more than 36".

If everyone were used to math/physics speech, we would talk about projections onto a plane, but what is written gets the same idea across to most people better.

Edit: You’re confounding measurement in any horizontal plane with measurement of any horizontal dimension.

I PM’ed the ref in question. He knows who he is if he chooses to respond. Not saying names unless he wants to reveal himself.

To expect every ref to understand all of the nuances of the game is unreasonable and not helpful. I’ve never had a volunteer that didn’t want to do a good job and even the insinuation that they could be publicly shamed is not the best use of our time.

I hosted two VRC events this year and several IQ myself. I don’t mind being in these discussions and being wrong at the end of the thread. I’m happy to be corrected, but that’s my choice to be in these discussions. I think I’m batting over 500, but I have eaten my own words several times.

From the Q&A referenced above:

I still think I’m right, but I’m open to interpretation. I have posted a new Q&A to clarify.

I used the image you made. Thanks!

https://www.robotevents.com/VRC/2018-2019/QA/219 <— I put the old one initially!

You know your interpretation allows a robot to measure nearly 6’ across in every direction, right? (To be stable you’d probably have to limit it to 5’ or so.) That robot would occupy nearly a quarter of the entire field.

A is illegal. B is legal. (And it looks like they’ve now replied to your post to state that officially.)

This isn’t the right way to use it, though it will suffice in many instances. I’ll explain how to use it properly for the dimensions being analyzed. Look at the third image on this page:

You’ll see the sizing tool should be held in a vertical plane. You can then spin it around a vertical axis so it measures different horizontal directions while remaining in a vertical plane. Check from one outer spot (meaning hold it against that spot while rotating around a vertical axis through that spot) and then translate it horizontally as needed to check other outer spots. If you can get it to go around the entire robot without touching, the robot is legal. If going around the entire robot this way cannot (not “isn’t,” but “cannot” - don’t want unsteady hands to be a factor) be accomplished, it’s illegal. Technically, the sizing tool should be infinitely thin, but that’s not possible, and it’s highly doubtful that will matter, as if it does matter a little bit of vibration or wobbling is likely to take that 35.99" robot to 36.01". Note, you might have to tilt the sizing tool a little while keeping the top bar horizontal if a robot is in the expansion zone and a tall piece blocks using a vertical plane.

@sankeydd If I understand your interpretation correctly, this side view of a robot would be legal? At no point on a plane of say “z” inches is the robot over 36" because of the rise before each extension. Correct me if I am wrong but I believe this is how both @callen and I interpret it

I put the wrong link… Fixed it… Dang Nabbit…

As @callen didn’t think the robot from @Sylvie was legal, I doubt he would think this one would be OK. If you held the tool vertically as the product page shows it would not pass. I think it’s OK though.

Even more than that…

A robot with a standard base and a beam that extends at a 45-degree angle into infinity would technically pass the 36" expansion rule as I see it… (It would fail the 18".) I see where you’re coming from and I’m not sure why I’m stuck on this. I think it was described to me once this way by and RSM but they aren’t the GDC either. I don’t know if the photographer for the website consulted with them either. I will see with the new Q&A and I am happily corrected.

Correct link: https://www.robotevents.com/VRC/2018-2019/QA/219

I did the inspections for middle school VRC last year and most teams looked at me funny when I got the 36" expansion tool out. It wasn’t an issue with any of the teams as most of them were dr4b’s and none of them would have failed any version of this test. I appreciate all of your thoughts on the matter.

As I interpret the rule. there is nothing that allows for there to be any dimension horizontally to be greater than 36 inches. It doesn’t matter where the horizontal dimension is relative to the floor. If the two legs of the 36" sizing tool are on the floor/table, then when the robot is fully expanded horizontally, NO orientation of the robot should have a dimension that would not be contained in within the dimensions of the sizing tool. Therefore, I would rule that the robot in as TarenMayer presented and the idea that AlexM_4478X presented would both be illegal. For example, you place a vertical line at one corner of the robot and use that vertical line as the center of a circle of radius 36", NO part of the robot should extend outside of the surface described outside of the vertical line swept by the circle. This should be true for any exterior point of the robot.
Although, many have stated that it is not necessarily a cylinder of radius 36", see my attached picture below.
36 size.PNG

Not only was I wrong, but it was right in the ref training videos. Feeling a little slow! I feel like I wasted their time…