# Why are manhole covers round?

This was a question on a trivia quiz in a bar I frequent. I believe the answer they gave was wrong, and I want to see if the Dopers know the answer.

Questions that you already know the answer to can go in IMHO, no?

So that they can’t fall into the manhole.

Next.

Cecil knows, sort of. http://www.straightdope.com/classics/a1_247a.html

Because the holes themselves are not square or rectangular?

Damn. I guess since I screwed up I didn’t remember seeing it in the archives, I should give up on this thread and give away what I was getting at.

The correct answer is “because the manholes are round.” No joking. All this business about rectangles and triangles addresses the question “why are manholes round?” The question was “why are the covers round?”

Good, someone got it right, before I got around to posting it. TheOnlySaneOne wins!

“Why are manholes round,” then?

Well, because the manhole covers are round, obviously. Otherwise they wouldn’t fit.

And the eternal question lives on – which came first, the manhole or the cover?

[sub]yes, I know, I’ll stop.[/sub]

True, I’ve spent about a week hauling them up for the company I work for. Course its not like we get the proper equipment for hauling them up. The rectangular/square ones are a bugger when they slip, especially if they’re the sort that split into two triangles with only one point to hold onto them with lifting rods :rolleyes:

So they fit the circular hole and then you dont fall in the ground. COuld you honestly imagine a triangular one??

there is another shape that is wide enough not to fall in, called a Reuleaux triangle, and here is a picture of it…

http://www.maa.org/mathland/10_21_fig1.gif

I don’t know if I did the img tag correctly, so just go to this page if the image doesn’t show up, or if you want more information about the shape.

-TJdude825

Having taken a engineering design course, we actually listed as many reasons as possible why the cover (and, subsequently, the hole) are round:

So the cover won’t fall in
Minimum amount of material for the amount of space
Easy to make
Easy to transport (they can stack nicely on top, but not as easy as squares, I’ll give you that)
Easy to move out of the way (they roll)
Easy to construct from a manufactoring viewpoint
Sharp corners are dangerous

This is a truncated list, as we had many more reasons. But you can see that round things are better for a multitude of reasons.

I disagree. Since the cover is the operating concern here, the reason the cover is round is because it’s the shape that won’t fall into a round manhole.

The shape of the manhole doesn’t matter at all, except what will allow the least chance of a cover filling in.

You can’t define the equation (round manhole cover = unable to fall into manhole) without including the “manhole cover” part.

Sure I can, we have triangular covers (and holes) here in New England. They wouldn’t fall through the holes either.

With a round cover, it does not have to be aligned precisely to fit. Any way you lay it, it will fit. If it were square, or another shape, it would have to be aligned perfectly to fit.