# why is hallow stuff stronger?!?!?!

this has been buging me for some time now and I demand awnsers!!!

I will answer your question if you will tell me why the mere fact of your being determines that you post there (wherever there might be).

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

YOU KNOW WHAT YOU DID[/caps lock]

therefore is one word

If you mean “hollow”

'snot stronger.

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

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

David

I’m sure the question was intended to be, “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 the metal 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 the bar lighter.

Or, you could move that metal from the centre to the edge, making a tube with a greater 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 collapse. It is possible to demonstrate this by standing carefully on an empty drinks can. I weigh 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.

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

thanks for the help guys.

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 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.

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.

i just noticed that kh and profile cranks are “hollow”.

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.

Re: why is hallow stuff stronger?!?!?!

“Tim” <Tim@NoEmail.Message.Poster.at.Unicyclist.com> writes:

> Gilby wrote:
> > *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.

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.