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Wheel Building - Spoke Length

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Wheel Building - Spoke Length

Old 03-17-16, 07:44 PM
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Wheel Building - Spoke Length

Hello folks,

I'm stumped and any information would be greatly appreciated. I'm a mechanic at an LBS and I've built a handful of wheels but this one has me hitting a wall. After taking measurements from the hub (Shimano Internal 3 Speed Coaster Brake) and rim (DT Swiss 545d) and entering that information into a spoke calculator, the recommended spoke length (using double butted for this build) was too long after the lacing the rim (3x lacing pattern, 36h rim). Before there was any tension on the spokes, they were bottoming out in the nipple. I used multiple spoke length calculators and took multiple measurements and got roughly the same results. Here are some questions I have:

1. Are there any tricks to determine (at least a rough) spoke length with a given hub and rim before lacing the whole rim up?

2. How many mm incorrect can a spoke be while still being usable (+/- 2mm, +/- 3mm,...etc.)?

3. Why would the given numbers from the spoke calculators be off by such a significant amount (possibly 5-10mm), given that everything was measured correctly?

I'm sure I forgot to include something but any help would be awesome, thank you!
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Old 03-17-16, 07:51 PM
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The calculators are simply that calculators. They produce excellent, consistent results, so don't look there.

This is a classic case of GIGO, (garbage in, garbage out) and any error is the result of incorrect data entered, or conversely that you built differently thn what you entered.

Giving you credit for being able to measure a rim ERD closer than that, and knowing that the other dimensions are less critical, I can come up with two likely possibilities

1- you didn't measure, and took the rim ERD from a spec chart, and it's just plain wrong
2- you can't type and input wrong stuff
3- you calculated a 4x wheel and built it 3x.

Of these, unless you're a total idiot, I suspect the 3rd is the most likely, and suggest rechecking the data being careful about the number of crosses or spokes

BTW - as a rule, most of the cases of poor spoke length calculation are to the short side, because of measuring the rim and failing to allow for the nipple heads.

Also, for future reference and to answer your last question, spokes should end between the screw slot and top of the nipple. That's a tolerance of 1mm or so. However there is a some cheat room of about 1-2mm shorter, and longer up to the limit of overrun that the nipple allows. This overrun is important, especially to folks like me, because it extends the upside tolerance and allows planning for a longer rather than shorter spoke.

Overrun allowance is a function of the thread length in the nipple since spokes are pretty standard at 10mm. I used to buy nipples that had 7mm of thread and allowed 3mm overrun, but those aren't made any more, and many I see don't allow any overrun at all, which doesn't please me at all.

Lastly, if you suspect that you only need another millimeter or two you might be able to find nipples that allow that much overrun, and use those rather than buying and lacing shorter spokes.
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Old 03-17-16, 08:01 PM
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My guess is this is a steel hub and the flanges are significantly narrower than the customary alloy hub. This would effectively narrow the flange-to-center distance or seat the spokes a little crooked, effectively letting the spokes fit a little longer than their nominal length. Either way, a shorter spoke would be appropriate. I believe the spokes on the last and only internally geared hub I laced turned out to be on the long side, too.

As far as variance, I've been comfortable as long as the threads are all concealed by the nipple, and, on the other side, the ends are no more than a millimeter or two clear of the nipple at the other end, and shorter if it's a single-wall rim. That probably works to a 1-3 mm variance.
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Old 03-17-16, 08:06 PM
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If you are good at math the formulas are in the Bicycle Wheel by Jobst Brandt. I have gotten lazy in my dotage and use Spoke Calc. with good results.
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Old 03-17-16, 08:14 PM
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Originally Posted by davidad
If you are good at math the formulas are in the Bicycle Wheel by Jobst Brandt. I have gotten lazy in my dotage and use Spoke Calc. with good results.
Spokecalc uses essentially the same formulas, with the only difference being a bit of fudge or adjustment of a millimeter to compensate for things like elongation under tension, length lost to the interweave, flange thickness, etc. It's those small fudges, along with differences of opinion about the ideal target length with respect to the top of the nipple, that account for various calculators which use the same formula.

As I posted above, don't blame a computer for what are ALWAYS gigo errors.
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Old 03-17-16, 08:17 PM
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The 3 Nexus Inter-3's that are listed in the 2015-2016 Shimano specifications book list 3 different PCD's.
59, 65 & 74mm.
You probably looked up the wrong one.
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Old 03-17-16, 08:22 PM
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Originally Posted by Bill Kapaun
The 3 Nexus Inter-3's that are listed in the 2015-2016 Shimano specifications book list 3 different PCD's.
59, 65 & 74mm.
You probably looked up the wrong one.
There is center to flange distance, aka CTF, and flange diameter (to spoke hole circle), so WTF is PCD? and does the world need another acronym to keep people confused?

In any case this is another example that bears out my opinion that you should trust nobody (except Mom) and measure everything yourself. (see 3rd line of my signature.
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Old 03-17-16, 08:27 PM
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1. At first I used the manufacturers measurements from Shimano and DT Swiss and that's how original spoke length was determined. Once the wheel was laced and it was determined to be the wrong length, it was disassembled and actual measurements were taken. These almost exactly lined up with the results from the manufacturers specifications.

2. I'm a very proficient typist.

3. Definitely entered 3x into all the calculators I used.

I'm aware that spokes should end at the bottom or top of the slot. Here's a question for you: on the DT Swiss spoke calculator website, after entering all the measurements, they have two different results for spoke length. They state "accurate" and "recommended" lengths, the recommended spoke length is 4mm less than the accurate length. Why?
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Old 03-17-16, 08:37 PM
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Originally Posted by aaronmichael
....
I'm aware that spokes should end at the bottom or top of the slot. Here's a question for you: on the DT Swiss spoke calculator website, after entering all the measurements, they have two different results for spoke length. They state "accurate" and "recommended" lengths, the recommended spoke length is 4mm less than the accurate length. Why?
This is a reason I've repeatedly stated here that ERD as used by most people IS NOT A SPEC.

To be a proper spec it has to conform to what in other contexts is called the 4 corners rule. By that I mean, a spec must only reference what can be measured and duplicated, and be free of any assumptions or adjustments. If any other conditions must be met, they must be included as a footnote as part of the spec. ERD includes an assumption about the thickness of the head of the nipple, which varies slightly by brand and type, and fit into the nipple seat, along with an assumption about the "right" height of the spoke in the nipple.

A true spec, would be the actual diameter (as measured or measurable) of the rim at the nipple seat, or as I call it, the "nipple seat diameter, aka NSD, to which a builder can add the allowance for the nipple head and how high into it he wishes the spoke to end. Or if they want to publish an ERD value, they need to include their assumption for spoke height within the nipple head.

Many Europeans use "my" (not really mine) approach and publish an actual diameter, but there's little consistency, so it's like negotiating a contract with the parties speaking different languages.

As for why DT has to values, and recommends using something other than an accurate (whatever that means) length, you'll have to ask them.

Remeasure your rim, add 5-7mm (2x the thickness of the nipple, less an allowance to end short of the top) and call that the ERD and rerun the calculation.
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Old 03-17-16, 09:00 PM
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You've probably already thought of this, but since no one has mentioned it explicitly... ...did you measure the spokes to confirm you're using what you think you're using?
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Old 03-17-16, 09:20 PM
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Originally Posted by FBinNY
There is center to flange distance, aka CTF, and flange diameter (to spoke hole circle), so WTF is PCD? and does the world need another acronym to keep people confused?

In any case this is another example that bears out my opinion that you should trust nobody (except Mom) and measure everything yourself. (see 3rd line of my signature.
A specification Shimano uses in their Tech Docs to denote the Pitch Circle Diameter of the spoke holes.
Their definition of Flange Dia. Is the OD of the flange.
Argue with them.
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Old 03-17-16, 09:25 PM
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Not to be snarky but check all of your measurements.

Whenever I've encountered a similar problem the answer has always been that I had made a dumb mistake somewhere. The last time that happened to me I was in the process of lacing up what I had assumed was a 32 hole wheel and discovered I was working with 36 hole components. Not only did I not have enough spokes but the ones that I had were too long. The good news is that there's a strange satisfaction that comes once I figure out my mistake. Good luck!
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Old 03-17-16, 09:25 PM
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Originally Posted by Bill Kapaun
A specification Shimano uses in their Tech Docs to denote the Pitch Circle Diameter of the spoke holes.
Their definition of Flange Dia. Is the OD of the flange.
Argue with them.
No argument, I was being semi-facetious. I've never seen "pitch circle" used that way. It's usually a gear cutting term and used for generating gears and sprockets. Pitch circle would be the circle the chain's pins lie on when wrapped around a sprocket.

I guess by extension, or by best translation of Japanese convention might be appropriate. However, in this context, I would never have made the leap from PCD to Pitch Circle.
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Old 03-17-16, 09:36 PM
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Originally Posted by FBinNY
No argument, I was being semi-facetious. I've never seen "pitch circle" used that way. It's usually a gear cutting term and used for generating gears and sprockets. Pitch circle would be the circle the chain's pins lie on when wrapped around a sprocket.

I guess by extension, or by best translation of Japanese convention might be appropriate. However, in this context, I would never have made the leap from PCD to Pitch Circle.
I used it frequently back in my shipyard boilermaker days. Pretty much anything that had 2 flanges bolted together.

What the OP SHOULD do-
Post all the numbers you used for calculations for obvious errors.
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Old 03-17-16, 09:54 PM
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Originally Posted by Bill Kapaun
I used it frequently back in my shipyard boilermaker days. Pretty much anything that had 2 flanges bolted together.

What the OP SHOULD do-
Post all the numbers you used for calculations for obvious errors.
The only ones that could generate an error of more than 1mm or so are the rim diameter, no. of crosses and flange diameter. The OP should measure those himself and compare to the spec values he entered.

for ERD he can take the actual diameter and add 6mm.
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Old 03-17-16, 10:41 PM
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Is it possible you laced this wheel 2x?
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