On Wed, 29 Nov 2017 22:58:52 -0500, Frank Krygowski
wrote:

Warning: Actual bike tech content below.

I'm building up an antique bike for a kid. After a false start with the
original rims (very light, but in a very obscure size) I ended up
replacing the wheels with 20" Sun rims on hubs that I had lying around:
a Campy front hub, and a Shimano 3CC 3 speed rear hub with coaster brake.

I measured both hubs and used Sun's online ERD value of 396mm for the
rim. I plugged dimensions into three different online spoke calculators.
It looked like I could use the same spoke length front and rear if I
went 2x front and 3x rear.

Here were the calculator results:
Sapim said front 189mm, rear 188mm.
DT said 186 front, 186 rear.
United Bicycle Institute said 185.6 front, 185.1 rear.
So I ordered 14 gauge Sapim spokes, 186 front and rear.

The front wheel built up perfectly, as usual. But the rear wheel won't
tension up. The spoke threads are bottoming.

I just double checked my hub dimensions. They seem to be spot on. And
again, everything was perfect in front. The spokes measure 186 with 10mm
of thread. The nipples are 16mm long with an unthreaded 5mm deep hole at
the end that slides over the spoke.

I can think of several possible solutions:

1) buy shorter spokes.

2) sacrifice a few nipple threads by drilling into the end of the nipple
to counterbore another 2mm or so. It's for a little kid, and I doubt
he'll be stressing the wheel very much.

3) acquire a die and extend the spoke threads. The cut threads won't
have a full thread form, but there should be plenty of strength in the
existing threads.

4) put washers under the spoke heads. It's a double wall rim so there's
room in there.

5) and next time, maybe go 2mm shorter than the calculators say? But
why? These spokes aren't stretching. I can't get any tension at all.

I'm curious if anyone's had a similar problem. If so, did you determine
what caused the error? I'm prepared to smack my forehead if necessary.

Quite a lot of shops over here no longer stock threaded spokes but cut
to length and thread (would one call them) blank spokes so assuming
sufficient full diameter size it should be no problem in threading the
spokes a little further. Assuming, of course that you can get the
proper die. I include that comment as I once threaded some spokes a
bit too far and a number of them stripped the threads out of the
nipples as there wasn't sufficient thread contact :-(

I believe that might be a better scheme then counter boring the
nipples although the usual standard for thread length is one diameter
for full strength so I would assume that something like 1-1/2 to 2
thread diameter would be sufficient even in brass nipples. Or
alternatively use washers under the nipple head.

I believe that someone else mentioned it but can you go X4 on the rear
wheel?

Re calculators. The old chap that runs (I think) the oldest bike shop
in Phuket, Thailand doesn't use any sort of calculator. He has, what I
think is, some sort of special steel ruler and he measures the rim to
flange distance with the wheel and hub laying on the floor. After he
measures things he pauses and looks wise for a moment and says
something like, "Yup, 265.4, that'll do her" and gets down the 265.5
spoke box :-)
--
Cheers,

John B.