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Old 11-11-07, 06:46 AM   #1
Sammyboy
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Spoke Length Rounding

I'm building a wheelset that's requiring me to do all sorts of nonsense like mixing spoke lengths. What I'm wondering is, how to round. Spocalc gives me lengths to .1 of a mm, but of course, that's not how spokes work. I'm assuming I need to do this:

180.3 - round up to 181
180.2 - round up to 180
179.2 - round up to 180
175.3 - round up to 176
174.2 - round up to 175

In other words, round up to the nearest mm. Sometimes, the nearest I can get is more than a mm over, for instance, I can't find 181's, only 180, and 182. Is this going to cause me a problem?

In other news, finding the lengths I want in black isn't working out either....
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Old 11-11-07, 07:19 AM   #2
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always round up or use a longer spoke nipple
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Old 11-11-07, 07:47 AM   #3
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Is a 1.8 mm overlength spoke going to give me a problem though?
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Old 11-11-07, 08:17 AM   #4
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As long as the spoke has enough threads to keep the nipple from bottoming out, you can just file the protruding end flush with the end of the nipple to keep it from puncturing the tube.
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Old 11-11-07, 08:19 AM   #5
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up/down

.1,.2,.3,.4 I round down, especially with Revo's.
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Old 11-11-07, 08:45 AM   #6
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Sammyboy, you don't need to round unless the length is .5 or greater. those lengths are all less than .5 why round up? if your measurements were accurate, you shouldn't need to round up for those lengths.

nowheels, nipples, regardless of length, have the same number of threads.

Pompiere, how can a spoke puncture a tube if it's being laced to a rim that's double walled and has rim tape?
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Old 11-11-07, 09:20 AM   #7
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+1. Round down like you would normally do. Tenths of a mm are really small.
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Old 11-11-07, 09:51 AM   #8
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As long as the spoke has enough threads to keep the nipple from bottoming out, you can just file the protruding end flush with the end of the nipple to keep it from puncturing the tube.
Yeah but this will be a HUGE pain in the ass to do for 32 spokes.
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Old 11-11-07, 10:02 AM   #9
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If its a double wall rim, you can get away with 2mm too long
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Old 11-11-07, 10:17 AM   #10
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Sammy,

In my somewhat limited experience (five wheelsets so far) it's always better to round down than to round up. The danger in having the spoke even a small fraction of a millimeter too long is that the nipple runs out of spoke thread before achieving the proper tension. Rounding down to the nearest millimeter still gives you plenty of thread grip without any thread showing below the nipple, but ensures you won't bottom out before achieving the desired tension. My $.02.
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Old 11-11-07, 10:24 AM   #11
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Hmmm. With some daring bravery, I might get away with only two spoke lengths.....
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Old 11-11-07, 10:27 AM   #12
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Sammy,

In my somewhat limited experience (five wheelsets so far) it's always better to round down than to round up. The danger in having the spoke even a small fraction of a millimeter too long is that the nipple runs out of spoke thread before achieving the proper tension.
No the danger is running out of spoke, you can't lengthen it but you can file it down.
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Old 11-11-07, 10:38 AM   #13
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No the danger is running out of spoke, you can't lengthen it but you can file it down.
Again, I have to disagree. There are ~10mm - 12mm of thread grip on a typical nipple, and rounding down a millimeter won't compromise anything. If you round up and run out of thread - by even a tenth of a millimeter - before achieving the desired tension, you have to pull those longer spokes and add threads or replace them with shorter spokes. It's much better to round down to the nearest millimeter.

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Old 11-11-07, 07:28 PM   #14
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Scooper, you've assumed that because a spoke has about 10mm of threads than the nipple does as well.

the most common nipples are 12 mm long. so they can't have 10-12mm of threaded portion.

the common 12mm nipple has no thread on the flat portion. the only part that threaded is the round part before the head....and a one or two at the head.





operator, still posting unfounded shade tree advice huh?

the problem is _both_: not enough threads engaged between spoke and nipple AND having the spoke too long so the nipple cuts new threads into the spoke.

that you side with the filing down of 32 spokes doesn't surprise me giving your track record of advice.

also, think twice before you open your mouth as i've saved most of the worst advice you posted here.





the bottom line is that IF the measurements of the rim and hub are accurate, then there is no reason to think the spoke will be too long or too short. if you disagree with that, then you deserve the wheel you end up with.
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Old 11-11-07, 07:34 PM   #15
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.1,.2,.3,.4 I round down, especially with Revo's.
+1
Always round down. Nothing worse than running out of threads before reaching the needed tension.

Al
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Old 11-11-07, 09:12 PM   #16
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Scooper, you've assumed that because a spoke has about 10mm of threads than the nipple does as well.

the most common nipples are 12 mm long. so they can't have 10-12mm of threaded portion.

the common 12mm nipple has no thread on the flat portion. the only part that threaded is the round part before the head....and a one or two at the head.
Arrghhh!

OK, round up if you must and suffer the consequences. I give up! Uncle!

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Old 11-11-07, 09:26 PM   #17
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The online DT Swiss calculator rounds down, and with Revolutions will round down an extra mm.
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Old 11-11-07, 10:39 PM   #18
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I just built a wheel where the spoke ends were perilously close to poking through the nipple and puncturing the tube. My vote, round down

I did give thought to taking a rotary tool and grinding all the spoke ends but thats pretty tedious
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Old 11-12-07, 12:48 AM   #19
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I'm definitely feeling the love for rounding down. I had no idea this would be a contentious issue!
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Old 11-12-07, 01:23 AM   #20
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nipple specs

Informative image, thanx Scoop!
I wonder if Sapim,Wheelsmith and others have similar specs for a given length?(threading and under-head length)



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Old 11-12-07, 05:01 AM   #21
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+1 on rounding down. Running out of threads before tension is reached is a major PITA!.
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Old 11-12-07, 07:05 AM   #22
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Informative image, thanx Scoop!
I wonder if Sapim,Wheelsmith and others have similar specs for a given length?(threading and under-head length)
My pleasure.

I'm not aware of anything similar for Sapim or Wheelsmith. I believe these nipples were cut and photographed by Roger Musson for his book, and he uses DT exclusively.

I really didn't intend to start a contentious debate about which way to round spoke lengths, and in any case I certainly am not recommending rounding down more than a millimeter from the calculated length for 12mm nipples.

Here's more on the subject from Roger Musson in his book Wheelbuilding, 4th ed., and my own experience pretty much reflects his advice. The photos he refers to are those in my post six posts up (Figure 59 from his book).

Spokes are generally manufactured in 1mm increments but many stores only stock even sizes, and some spokes are only made in 2mm increments. When using the spoke length calculator I always treat the resulting spoke sizes as maximums, there are however other considerations and this relates to the length of nipple used.

My spoke length calculator aims to calculate a spoke length such that the threaded end of the spoke lies flush with the bottom of the slot in the nipple - about 1mm below the top surface of the nipple. Rounding the spoke length up or down is based on the size of the nipple being used.

The following discussion is applicable to DT spokes; I'm not sure if the same applies to other brands.

For illustration purposes I've produced some cross sections of 16, 14, and 12mm DT nipples shown in Figure 59. The ruler at the top is in 1mm increments. All the nipples were gently screwed down until the threads bottomed out. Once the threads bottom out the nipple becomes tighter to turn, it will easily tighten with the wrench but the spoke will be deforming the threads in the nipple and whilst generally okay I always try to avoid this by selecting the correct length spoke.

12mm nipples

This is the most common size and accounts for the vast majority of wheels I build. The only time I use something longer is on the Mavic UST rims where a longer nipple is required due to the special eyelet design (Mavic recommend 16mm but I use 14mm).

When using 12mm nipples my ideal length is when the spoke is flush with the bottom of the slot in the nipple which is about 1mm below the top surface of the nipple (this is the length given by the spoke calculator). As you can see from the photo the spoke can easily screw through an extra 2mm (1mm above the top surface) so for spoke length purposes there is plenty of leeway in selecting a suitable length. With the ideal length at the bottom of the nipple slot you can go +/- 1mm. You could go +2mm but you will encounter the nipple bottoming out (emphasis mine - Stan). I wouldn't go -2mm because you are getting too low a thread engagement and also the shorter spokes will be more problematic when lacing/tensioning because you will be getting tight spokes too early in the build process unless you modify your nipple driver accordingly.

For example if using standard 12mm nipples and I calculated 264.6mm it would be okay to go with 265 or 264 but with a calculated 264.5 I'd be going with 264 but with the 12mm nipple it's not critical.

14mm nipples

The spoke in the bottomed out nipple lies flush with the top surface of the nipple. The spoke calculator will provide a length to the bottom of the nipple slot so you must subtract 1mm to give the ideal length which gives the option of then selecting spokes in the range plus/minus 1mm without fear of encountering the nipple bottoming out.

16mm nipples

The spoke in the bottomed out nipple lies flush with the bottom of the nipple slot so this is the absolute maximum length - do not go longer. This would be the length that the spoke calculator gives you so you need to modify this length by subtracting 2mm to give the ideal length with the option of then selecting spokes in the range plus/minus 1mm.

The DT Swiss spoke length calculator automatically subtracts 4mm if you specify a 16mm nipple and subtracts 2mm if you specify a 14mm nipple, both of which I find excessive compensations.
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Last edited by Scooper; 11-12-07 at 07:28 AM.
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Old 11-12-07, 08:07 AM   #23
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Awesome. There's nothing I like more than facts!
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Old 11-12-07, 08:22 AM   #24
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Scooper, great pic but notice that there is a _slightly_ larger margin of error with the 12 mm. if you're off by about 1 or 2 mm too long with the 14 and 16 mm nipples, you'll have the nipple start to cut new threads into the spoke. those new threads will be a point where failure could occur.




dwoloz, is the wheel you're talking about using a single wall rim?



Sammyboy, if you're hub and rim dimensions are accurate, you'll be fine if you round down to the nearest whole number.
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Old 11-12-07, 08:27 AM   #25
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I wouldn't go -2mm because you are getting too low a thread engagement and also the shorter spokes will be more problematic when lacing/tensioning because you will be getting tight spokes too early in the build process unless you modify your nipple driver accordingly.
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Those photos show 20 threads on the spoke and 18 threads on a 12mm nipple with 2 threads per mm. So if you had to round down 2mm you'd have at least 14 threads engaged; that's not even close to being too low.
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