Ron Ruff said:Quoted message said:The ruler-across-the-spoke-bend might get around the problems of the
margin-of-error and where-to-measure-D-from.
I'd recommend making L a bit shorter than the unsupported span... ie
inside of the spoke crossing and the nipple. Mark a point near the
center where you will attach the weight, then two more the same
distance away on either side. The straight edge will intersect the two
end points. The weight needs to hang perpendicular to the straightedge,
so adjust the tilt if necessary... but a slight error in this is not
critical.
Yes, hard to measure accurately... but I think this is the best
approach for figuring tension without a meter. I bet the results will
be very close to what you get with the Park gauge.
Dear Ron,
Until you actually hang a lot of weights and try to measure the
tension on squeezed spokes, it's hard to appreciate the annoying
practical problems involved. A few pictures and pointers may help (or
scare people off).
Here's a quick setup with a handy wheel in a fork, ready for a 60-lb
weight to be dangled, but with no complicating clamp on the spoke pair
on the far side of the wheel:
http://i18.tinypic.com/2ykc3o0.jpg
Click on the lower right in Explorer for the full-size views.
Notice that the ropes cross each other. The upper spoke points up to
the right at 21~22 degrees, and that the lower spoke also points up to
the right at 8~9 degrees.
(Or so the onscreen protractor says. The camera distorts angles a bit.
The pictures were taken in the dark to illuminate the spokes better.
Some level edges can be seen in the computer nonsense beyond the
wheel.)
Now let's dangle the 60-lb weight:
http://i14.tinypic.com/34t7961.jpg
The wheel has rotated clockwise about 20 degrees. The spoke ropes no
longer curve around each other. You use some Kentucky windage to get
the offset close and then adjust a little more until you're happy.
Unlike the shooting range, however, each shot involves heaving a 60-lb
weight back onto a stand.
With the weight, the upper spoke's hub-half points down at 5~6 degrees
below level, and its rim-half points up at 6~7 degrees.
The lower spoke's hub-half points down at 8~9 degrees, while its rim
side points down even more steeply, about 20 degrees below the level.
The rim-halves of the two spokes now diverge at 26~27 degrees.
There's just room for the Park gauge, though the camera distortion
makes it look as if the post is in the rope.
That's a DT spoke ruler. Let's look at it more closely:
http://i16.tinypic.com/33uygk1.jpg
The bends are somewhere inside the white rope's knots. The spokes are
2 mm thick. If you look up above the glare on the ruler, you can see
how small a millimeter is relative to the knots hiding the bends.
It would be darned hard to get 0.5 mm accuracy with this approach,
which seemed likely to yield only half the accuracy of a Park gauge
A single knot instead of a pair of half-hitches would let you get
closer to imagining where the bend is, but of course a real spoke
squeeze is spread out across several fingers. (I fooled around with a
Terminator-style hand-rig inside a glove, but there's no room for a
spoke gauge, and the silly thing never worked well, anyway.)
Real-life spoke-squeezing by hand might produce a different tension
than a thin rope because the squeeze is spread out over a broader
curve. My purely ignorant guess is that a broad hand-squeeze of 60 lbs
might produce a smaller tension increase than a narrow rope applying
the same force, but it might be the other way around, or even have no
effect at all.
The advantage of the two half-hitches is that they're easier to adjust
when you're setting things up (twenty times, if you measure up to 100
lbs in 5-lb increments). The wider knot provides more friction to
discourage the weights from stealthily slithering toward the hub and
are also less likely to leave permanent bends in the spokes.
Some kind of small, stiff Y-rig could hook over the spokes and expose
the bend hidden inside the knot. Then some fancy caliper work would
get around the rest of the rope to whatever ruler you were trying to
fiddle into the mess.
Or you could use strong, thin wire or even fishing line to expose the
bend.
But it seems enormously easier to use the normal industry tools
designed to make this task easy, repeatable, and reasonably accurate.
The Park tension gauge fits in ordinary 700c wheels with this rope
trick. I don't know if the ropes would leave enough room for the
Wheelsmith tension gauge (which works the same way as the Park, but
looks as if its dimensions are different), or enough room for the DT
Swiss and FSA designs (which use a different approach to achieve the
same result as the Park and Wheelsmith gauges).
Like you, I now expect that if D is measured accurately from the
correct starting position (somewhere in the thin air between the two
current end points of L) to the bend, then the equation should agree
quite closely with an accurate tension gauge.
Thanks again for the exasperating work of getting around my
misunderstanding of the results.
Cheers,
Carl Fogel
