Ben C wrote:
> On 2007-10-03, jobst.bra
...@stanfordalumni.org <jobst.bra
...@stanfordalumni.org> wrote:
>> carlfo
...@comcast.net writes:
> [...]
>>> overloads occur often, the nipples of slack spokes can unscrew,
>>> reducing tension to affect both wheel alignment and strength."
> [...]
>> I think you just showed that no load and strength calculations of
>> wheels are made in the book.
> Perhaps you can help clarify this point then.
> What does "strength" mean? Technically we know it means breaking stress.
> Can it also be used of a structure, as opposed to of a material, to mean
> the force (or stress?) at which the structure collapses, even if
> collapsing doesn't involve anything breaking, and might not even involve
> anything even yielding?
> If it can, then it's correct to say in that sense that high spoke
> tension increases strength. If it can't, then it's not correct-- high
> spoke tension doesn't change the breaking stress of any of the
> components in the wheel. I think that's jim beam's point. Since his
> expertise is in materials he naturally takes "strength" to mean
> "breaking stress".
Have you ever seen a "broken" wheel? That is, a wheel broken from
excessive load?
A wheel will break two ways, flat spotting (denting) the rim and
"tacoing". From Sheldon Brown's glossary:
--------------
Taco
To bend a wheel so that it assumes a saddle shape. A Tacoed wheel
is more than just out of true, it has bent far enough that the spokes
have assumed a new equilibrium position and lost tension. Two spots, 180
degrees apart will be way off to the left, two other spots, halfway
between, will be way off to the right. A tacoed wheel is also known as a
"potato chipped" wheel.
<http://pardo.net/bike/pic/fail-002/img_0221.crop.jpg>
---------------
First you have to convince yourself that higher spoke tension means
better radial support. Consider the analogy of railroad tracks. When the
rim deflects enough to slack the spoke, the spoke is out of the picture,
you might as well remove it. This is like removing a railroad tie. As
the wheel deforms more, more spokes become effectively removed, and a
longer span of rim is unsupported, just like a span of railroad track.
The combination of track and tie is much stiffer than track alone.
Now consider that the wheel is still under a great deal of
circumferential compression. This is akin to putting the railroad track
under longitudinal compression. As you remove ties, the railroad track
will also want to spread (buckle).
A wheel "wants" to taco. If you keep increasing the spoke tension it
eventually will. It is constrained from doing so by the lateral rim
stiffness and the spoke tension. Imagine tensioning a wheel with no dish
(removing the lateral spoke support), it will taco more readily. When
you load a wheel radially enough to slacken spokes, the compression is
still there and the wheel will want to taco. This is often aided by some
lateral forces. Jumping a bike onto a less than straight wheel is the
classic way to taco.
The railroad track analogy is technically accurate. Without spoke
support, the wheel becomes more prone to both tacoing and denting.
Spokes, hubs and rims all operate in high cycle loading, meaning breaks
(fractures) come from fatigue. There is no way you can normally overload
these components to fracture. What happens in bicycle wheel failure is a
structural failure, the rim just deforms, buckle, dent, or both.
High spoke tensions make for a strong wheel structure. The context is
engineering, not metallurgy.
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