On 11/30/2017 3:23 PM, Frank Krygowski wrote:
On 11/29/2017 11:22 PM, jbeattie wrote:
On Wednesday, November 29, 2017 at 7:58:57 PM UTC-8, 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.


They're too long, obviously. Is lacing 4X a possibility?
Otherwise, you buy new spokes. You could also find a shop
with a Phil or other spoke cutter.

4x won't work. That would require roughly 195mm spokes. I
happen to have a set of 170mm spokes (intended for the
original odd rims) and the calculators tell me x2 would need
177mm. So I'm assuming 170 is too short.

I'll see if some shorter nipples might work. If not, I'll
probably go with washers under the spoke nipple heads.

I'm more interested in knowing what might have gone wrong.
I've never had this problem occur before.

I just tried measuring the ERD myself, using a tape measure
to get circumference of the rim tape's surface (1260mm) and
a metric vernier caliper to get the "drop" from that surface
to the face of the hollow rivets on which the spoke head
rests (3.8mm). Circumference divided by pi minus (drop x 2)
gives me 393.5mm. From
https://www.sheldonbrown.com/spoke-length.html I get the
advice to add a couple millimeter for the thickness of each
nipple head, which gives me ERD = 397.5mm. That compares
well with the online catalog value of 396, so I'm still
baffled.


Our calculator gives 185 left & right for that rear wheel.
Sorry but I didn't see any obvious error.

--
Andrew Muzi
www.yellowjersey.org/
Open every day since 1 April, 1971