Luns Tee said:In article <[email hidden]>,
Quoted message said:Given the same dimensions for the short spoke span in the gauge, the
different deflections must reflect how stiff or how elastic the spoke
material is.The difference in bending stiffness does affect the measurement,
but there are oddities to DT's calibration charts that are fishy to me.The 2.0mm titanium spoke is half as stiff in bending as a steel
spoke. It should deflect more than steel, and for a given range of
tensions, the range of deflections should be greater than for steel.
that's an extremely good point!
Quoted message said:
DT's charts do indeed indicate a greater range of deflections
for titanium, however note that the difference between titanium and
steel varies from the the titanium spoke having more deflection at low
tensions (smaller reading) as it should, to the steel spoke having
less deflection at high tensions, which would make no sense.My best guess as to what's going on is that there's a constant
offset being introduced into the measurement, with the steel spoke's
measured deflection being a fixed amount more than what it should be,
and/or the titanium's being less. For larger displacements (low
tension), these are overwhelmed by the actual displacement
differences, but at high tensions, the offsets are overwhelming the
actual difference.The steel spoke displacement is at most only 0.02mm greater than
the titanium in their tables. This is well within the range of offsets
observable from a bend/kink in the spoke, and is the offset that only
Jobst's design can zero for.Other weirdness in the chart: compare the columns for
1.8/1.6 and 2.0/1.8 spokes. Comparing the straight gauge 1.8 to the
2.0/1.8 spoke, or the 1.8/1.5 with 2.0/1.5 spokes should give you some
feel for the tolerances in these measurement values.Quoted message said:Possibly the companies are applying complicated physics
equations, but I think that it's likely just measuring known loads.I won't speak for what DT does, but the spreadsheet that Park
provides for their gauge does have equations embedded in it (credited to
C.S. Howat). The equation used has no basis in physics as far as I can
tell, but is based on observing that the measured curves, when
plotted on a semi-log scale, are curved somewhat like a parabola. This
may work reasonably well over some range of measurements, but wouldn't
extrapolate well, nor is it able to predict a-priori what the curve
for one type of spoke should be based on measurements for another.This isn't to single out Park though. The calibration sheets
that FSA provides also give curves that are based on measurements. As
with Titanium vs. Stainless in DT's charts, the curves on FSA's charts
sometimes cross too, when I don't think they should. This is the
result of the curves being, I think, a polynomial fit of the measured
data. While it's a convenient fit to perform in Excel, and produces
smooth pretty curves, it doesn't reflect what the data should be doing.-Luns