spoke length problem

[Frank Krygowski]

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.

--
- Frank Krygowski

[avag...@gmail.com]

Real time. ERD is estimate

Use a wheel beam...search here in RBT

Beam rim with hub hole n shim rim up

Then quadrant rim with 3-4 experimental spokes in 4 groups pre measured real
time with tape or steel yardstick - Wal sewing

Then arrive at 'exact' length

[jbeattie]

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.

-- Jay Beattie.

[John B.]

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.

[avag...@gmail.com]

https://photos.google.com/photo/AF1QipN29fyTVNVxB2qs9wVGfqktX2McTvE1X9SAuRq5

https://photos.google.com/photo/AF1QipPPKO7vO3P0iZY3wC48IfuocJNeMvV_TztSOdm5

https://photos.google.com/photo/AF1QipMpS6ydVxy9Bv4-_3yrH0IiSyv3rSfHjbj_XC8r

https://photos.google.com/photo/AF1QipNwAp3Nw4yd3SP4Vz0aCQneY7rkasfHxXt1MZgI

[russell...@yahoo.com]

All my experience is with buying too short spokes. I use the calculators to figure the right length. Then always buy a spoke 2-3-4 mm shorter figuring the calculators are over generous and the spokes will stretch when tightened. Went a bit overboard on this on one set of wheel and ended up with lots of threads showing below the nipple. In your situation the other suggestions of new spokes or cutting longer threads on the existing spokes is the best method. But just using washers under the nipples inside the rim would be the easiest fix and still be fine and dandy.

[Frank Krygowski]

All 404.

--
- Frank Krygowski

[Frank Krygowski]

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.

--
- Frank Krygowski

[avag...@gmail.com]

Why yawl refuse real time measurement with a simply made device is intolerably STUPID

[jbeattie]

On Thursday, November 30, 2017 at 11:37:28 AM UTC-8, russell...@yahoo.com wrote:
> All my experience is with buying too short spokes. I use the calculators to figure the right length. Then always buy a spoke 2-3-4 mm shorter figuring the calculators are over generous and the spokes will stretch when tightened. Went a bit overboard on this on one set of wheel and ended up with lots of threads showing below the nipple. In your situation the other suggestions of new spokes or cutting longer threads on the existing spokes is the best method. But just using washers under the nipples inside the rim would be the easiest fix and still be fine and dandy.

Washers are so trailer.

It is odd that the length was fine for the front but not the rear. The obvious explanation would be a mistaken flange height/spoke hole measurement. The ERD was correct for the front so it should be correct for the rear, assuming the as-manufactured rim is not out of spec. I assume Frank put the right number of spoke holes into the calculator. The only variable in the equation is the hub. No?

-- Jay Beattie.

[Frank Krygowski]

On 11/30/2017 4:53 PM, avag...@gmail.com wrote:
> Why yawl refuse real time measurement with a simply made device is intolerably STUPID
>

I did not have the rims when I ordered the spokes. Why is that not clear?

--
- Frank Krygowski

[AMuzi]

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

[Frank Krygowski]

Yes, everything was double checked, then double checked again when
things didn't fit. I'm using both dial calipers and vernier calipers on
the hub dimensions.

It occurs to me that the front wheel barely tensioned up. I don't think
I could have gotten another millimeter worth of turns out of those nipples.

I suspect that the rim ERD is a bit undersized. That's the most
difficult measurement to make, and I know from stories of super-tight
tires that rim diameters are not super precise. (The front tire was
certainly easy to mount, no tire levers required.) The most exact way of
measuring the ERD directly would require stripping all the spokes out
again, then precisely measuring the diameter using a couple spokes and a
caliper. But I don't plan on going through that trouble.

It's also interesting to me that the various spoke length calculators
don't precisely agree. Four different calculators gave 188, 186, 185.1
and 185.3. At its heart, the calculation is just three dimensional
Pythagorean theorem. The variations indicate some assumptions that are
hidden and different.

For kicks, I used Sketchup (a three dimensional drafting program) to
draw a simple model of the hub and rim. (Details on request. It's
overkill for standard spoking patterns, but could be handy for spoking
oddball wheels.) I drew in a couple spokes with the proper number of
crosses and measured a spoke length of 186.1mm. That's straight line
point-to-point geometry, and it certainly confirms the 186mm spokes I
bought. Too bad they don't fit.

This afternoon I dug in my junk piles and found some shorter spoke
nipples that thread a millimeter further onto the 186 spokes. I think
those plus washers should fix things.

At least washers in a rim are less visible than hose clamps on a stem! ;-)


--
- Frank Krygowski

[Frank Krygowski]

Thanks for checking. At least I don't have to slap my forehead just yet.

--
- Frank Krygowski

[AMuzi]

>>>> 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.
>>
> Thanks for checking. At least I don't have to slap my
> forehead just yet.
>

Because Shimano tricoaster and the trig are well known, I
suspect the actual ERD is not what we assume. There are
several rims whose dies were changed over the years with
various retoolings.