On 11/30/2017 5:17 PM, jbeattie wrote:
On Thursday, November 30, 2017 at 11:37:28 AM UTC-8, 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?

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