why is hallow stuff stronger?!?!?!

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What?!?!?!Huh????

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total uni said:

*What?!?!?!Huh???? *

[cap locks]STOP POSTING NON-UNICYCLE RELATED THREADS IN RSU!! THIS
SHOULD BE IN JC. WHY CANT YOU REMEMBER THAT?? YOU DO THIS AT LEAST ONCE
A WEEK. AND DONT ACT INOCENT > What?!?!?!Huh????
YOU KNOW WHAT YOU DID[/caps lock]

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total uni said:

*What?!?!?!Huh???? *

(he's talking about your signature.)

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therefore is one word

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If you mean "hollow"

'snot stronger.

If you mean "hallow,"
Then it is stronger cause God wants it that way.

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I'm guessing he was talking about cranks and i can't help him.

David

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Whoah! Thread-jacking and grumpiness abound!

I'm sure the question was, "Why are hollow unicycle components, such as
cranks, stronger than their solid equivalents?"

Assuming this to be the case, does anyone who knows about engineering
want to give us a proper answer?

This is what I think: Hollow stuff isn't stronger, unless you compare
it weight for weight, rather than comparing size and shape.

If you take a bar of metal, some of it is in the middle, and some of it
is near the edge. Try to bend it, and the stuff near the edge has to
either stretch or compress. The stuff in the middle simply has to bend.
Stretching or compressing are hard work. So, if you exert a certain
amount of force to bend a bar of metal, most of your effort goes into
stretching the metal on the outside of the bend, and compressing (or
buckling) the metal on the inside.

If this is true, then the metal in the middle is contributing less to
the overal stiffness of the bar than the metal at the edge.

Therefore, it would be possible to remove some of that metal without
making a significant difference to the strength of the bar - but you
would make it lighter.

Or, you could move that metal from the centre to the edge, making a tube
with a much bigger diameter - so you get the same total weight, but
greater rigidity.

There is more than one measure of stiffness, though. (Stoppit now!) A
metal tube can be tested for how much it bends, how much it twists, and
how resistant it is to lengthways compression, or how much it resists an
impact to the side.

The weakness of a hollow tube (tautology for emphasis) compared to a
solid bar (ditto) is that a side impact can cause the tube to collpse.
It is possible to demonstrate this by standing carefully on an empty
drinks can. I weight about 145-150 pounds and can stand on an empty
drinks can. However, if someone then sharply taps the side of the can
with a stick, denting it, the can collapses under my weight, with
amusing results. Don't try this if you or your parents/guardians are
litigious.

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this IS a unicycle related thread!!!!tons of unicycle prouducts are
hallow.

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thanks for the help guys.

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Hollow stuff is not "stronger" than a solid component of the same shape
and material. However, most things are designed to be hollow because the
highest stress when bending is going to be on the area farthest from the
-moment of inertia- (the rotational center of mass of the lateral cross
section that the component is being bent). Because of this it is better
to put the material farthest away from the moment of inertia and then to
save weight and material, remove material that is closer to the moment.
If that explanation doesn't make sense, Mikefule's puts it in more lay
terms.

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Gilby said:

*Hollow stuff is not "stronger" than a solid component of the same
shape and material. However, most things are designed to be hollow
because the highest stress when bending is going to be on the area
farthest from the -moment of inertia- (the rotational center of mass
of the lateral cross section that the component is being bent).
Because of this it is better to put the material farthest away from
the moment of inertia and then to save weight and material, remove
material that is closer to the moment. If that explanation doesn't
make sense, Mikefule's puts it in more lay terms. *

Nice explanation.

Quoted message said:

-Quoted from totally little kid-
*this IS a unicycle related thread!!!!tons of unicycle prouducts are
hallow.*

I knew "tons" of unicycle products are -hollow- also they're not really
unicycle parts. You were vague. It doesn't matter. Lame thread. It was
fun for me.

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If you are still interested in pursuing this question, perhaps you'd
like to give us an example of what you're talking about. Crank arms?
Splined crank arms? I haven't seen any solid ones of those. The hollow
ones seem pretty dang strong though.

Schwinn unicycle frames are solid steel, but most other frames are
hollow tubing. Lighter and stiffer. Using more metal does not guarantee
greater strength. You have to use the material in the places where it's
needed, or else it's just dead weight. Like a twisty Schwinn (or
Semcycle Deluxe) frame.

The advantage of the Schwinn/Semcycle design is it is much easier to
manufacture. But since those unicycles are not cheaper, and only offer a
1" seat height increment, the disadvantages outweigh the advantages.

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i just noticed that kh and profile cranks are "hollow".

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Aer you comparing hollow splined cranks to like solid cotterless, like
on a torker cx.

Edit: How come it seems every time I make a post I'm on the next page.

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Tim said:

Gilby said:

*Hollow stuff is not "stronger" than a solid component of the same
shape and material. However, most things are designed to be hollow
because the highest stress when bending is going to be on the area
farthest from the -moment of inertia- (the rotational center of mass
of the lateral cross section that the component is being bent).
Because of this it is better to put the material farthest away from
the moment of inertia and then to save weight and material, remove
material that is closer to the moment. If that explanation doesn't
make sense, Mikefule's puts it in more lay terms. *

Nice explanation.

I don't want to offend, but Gilby's explanation is a liyttle off the
mark. He has the right idea mentioning areas of stress, but the term
"moment of inertia" doesn't mean what he says, and the rotational
center of mass isn't the important concept in any case.

Here's what need to know to understand the value of hollow
construction: When a part, say a rod, is subject to bending or
twisting forces, it responds by deforming (slightly, we hope). Some
regions of the rod stretch and others compress, while the areas near
the middle remain close to neutral. Stresses proportional to the
amount of stretch or compression form within the part and combine to
exactly oppose the external force. Since the middle of the rod
experiences low stresses, it contributes relatively little to the
rod's ability to resist forces.

By hollowing out a rod into a tube, you make it a little weaker, but
much lighter. This is a much more structurally efficient shape than
the original rod. When designers choose to use high quality (read
expensive) materials, they usually want efficient shapes as well. So
the tubular parts you see may well be made of better materials and be
more efficient designs.

Hope that helps.

Ken

I don't think hollow stuff is stronger, because I kronked my KH crank
arm on a SEVEN stair today. 🙁 Seven isn't that big, is it? Not big
enough to kronk a KH crank, it seems.

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hollow stuff, *isn't* necessarily stronger, but much lighter. Imagine
how heavy your uni would be if it were solid. its just much lighter and
if properly designed, only slightly weaker. try reading the other posts
in the thread for why.

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markf said:

*hollow stuff, *isn't* necessarily stronger, but much lighter. Imagine
how heavy your uni would be if it were solid. its just much lighter
and if properly designed, only slightly weaker. try reading the other
posts in the thread for why. *

Usually when you design something, you'd have specifications to try to
quantify on how much abuse the component should withstand. So it would
be designed to be the same strength.

If making a solid part, the solid part can be skinnier, but will weight
more. If doing a hollow part, it'd have to be fatter, but it'll will
weight less (assuming all else is equal). This is because, as I
explained earlier, the farther away the material is from the centroid
axis (yeah... that moment of inertia thingy I tried to define earlier),
the more stress it'll handle... similar to having a wrench with a short
handle verses having a wrench with a long handle, the farther away from
the center of rotation you exert the force, the less force is needed to
do the task.

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