Ben C wrote:
> On 2008-04-28, Peter Cole <peter_cole@verizon.net> wrote:
> Well my hypothesis (which perhaps you don't regard as plausible :) is
> that this bedding in reduces the bending moment on the spoke.
>
> It doesn't matter whether the spoke bends or the hub deforms. The result
> is a spoke closer to the flange so less moment.
>
>> Bottom line is that the deformation, both the majority that comes from
>> initial tension as well as whatever smaller contribution that may come
>> from over loading, doesn't predict the direction or degree of bending
>> stress in the tensioned spoke.
>
> I don't understand what you're saying here. Surely for all spokes the
> effect of hub deformation is going to be to reduce moment on the elbow?
No. Hole deformation will change the elbow support angle slightly. It
will always enlarge the angle, if the angle was too big to begin with,
it will increase the moment. If the angle was initially too small, the
angle may be improved, but there still may be a bending stress.
>> As I said before, these threads have gone through an evolution that
>> began with a denial of the presence of residual stress and have morphed
>> into a grudging admission with the dismissal for irrelevance. Residual
>> stresses are present, they may or may not be a factor in any given spoke
>> failure, but that's impossible to know without knowing all the operating
>> stresses. No postmortem is going to tell you whether the failure was
>> caused by a raiser, bending stress, residuals or any combination
>> thereof.
>
> You would expect to see more failures starting in the regions of tensile
> residual stress (i.e. inside of inbound spokes) if residual stress from
> manufacturing is a factor.
>
> There are other factors of course, but there ought to be a statistical
> bias.
Who says there is or isn't? If spokes are bowed at the flange and the
spoke line is not corrected, the bending forces introduced as the spoke
is tensioned will swamp any residual stresses. Those spokes will
generally fail at the outside of the bend. There are those who refuse to
correct a bad spoke line then cite these failures as evidence of the
irrelevance of residual stress. It's comical, really.
> [...]
>> These (endless) arguments follow the same pattern. Something is taken
>> out of context and a straw man is created. Like spoke tension and wheel
>> strength. The FEA leaves no doubt about what happens when a wheel is
>> overloaded, it doesn't matter that an unspoked rim will support body
>> weight without collapsing, the loads we're talking about are the several
>> g's that happen when the wheel hits a pothole, that's when strength
>> becomes important. Normally tensioned wheels loose spoke tension at
>> about the same magnitude of deflection as rim damage starts, looser
>> tension will mean impact damage will happen earlier. As the loaded area
>> of the rim becomes slack, the rim looses lateral stiffness while still
>> under compression and therefore is susceptible to buckling.
>
> I follow your account of that. But I reserve judgment on whether a
> tightly spoked wheel's rim may yield before its spokes go slack.
Jobst did an FEA. It's in the book. A similar FEA is here:
http://www.astounding.org.uk/ian/wheel/3c_rim.html
A loaded spoked wheel doesn't deform into a uniform oval, it develops a
flat spot. As the length of the flat spot grows (with increasing load)
the bending stress on the rim increases. When that stress reaches a
critical value the bend is permanent. The rate at which the flat spot
grows with load is related to the initial spoke tension. That's it. That
you can ride around on a loosely spoked wheel doesn't say anything about
what happens when it meets a pothole.
>> Instead of focusing on those important issues the threads degenerate
>> into a critique of spoke bed cracking, despite the fact that this is a
>> problem for only a few rims and the straw man is raised that Jobst
>> advised builders to exceed manufacturer's specs.
>
> Fogel was probably right when he said there were no manufacturer's specs
> in those days, and that the method of going to the taco point and back a
> bit was good advice for the rims of the day.
>
> It is worth pointing out though that that is not a suitable method for a
> modern deeper-section Mavic rim. I'm not claiming Jobst ever said it
> was, but the details get lost and people over-tension their rims.
>
> I probably would have myself if it hadn't been for jim beam's
> explanations of why it causes fatigue.
Again, if you haven't read the book then you don't have a leg to stand
on. You're just repeating a misquote. Jobst (correctly) refers people to
his book. That's the "advice" you should take. The "details" don't get
"lost" if you buy the book.
>> When several sources are found that confirm the anodization fatigue
>> connection, all that is dismissed with talk of anisotropy and
>> extrusion flaws -- factors (obviously) still there whether anodization
>> is or is not.
>
> That is a much harder one to call. Some of these debates can be well
> understood with basic mechanics and understanding of stress/strain and
> S-N curves, which are simplified macroscopic views of how materials
> behave.
>
> But the effects of anisotropy and anodization on fatigue life is getting
> much deeper into the structure of metals. Yes we know in the most
> general terms they both can be factors but that's a long way from
> understanding it enough to know how to apply it to bicycle rims.
Nonsense. The "defenses" of anodizing boil down to an assertion that the
rim extrusions are so crappy that it doesn't matter if they take an
additional reliability hit from anodizing. If you have to derate spoke
tension below what all the other wheel components can tolerate just to
prevent socket cracking then you've just got weak sockets. Of course if
you don't understand the benefit of high spoke tension you don't think
you've given anything up.