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Wheel Building question - Spoke lengths

Old 11-18-13, 07:57 PM
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Wheel Building question - Spoke lengths

Greetings!

I can't find an answer to this, so I thought I'd seek ya'lls guidance...

I've built a few wheels over the years (3 or 4), but usually just to replace a cracked rim with a new one of same type, so I didn't have to do any measuring or math or anything else that required thought (as long as I paid attention to what I was doing).

Well, that's sort of where I'm at today. However, I can't get a replacement OEM rim (they'll only sell complete wheels), so I'm looking for something similar after market. I have the OEM rim specs, and measured the existing rim & hub anyway just to compare values. Most values were within 1mm...

...Except for the spoke lengths. The spoke lengths I calculate are somewhat shorter than the spec'd/original spokes - by about 7mm.

Now, the original spokes did seem to be about 2-3 mm longer than needed - the spoke ends (before I disassembled the wheel) poked past the slot end of the nipples, so that could be part of it. But, I still can't come close to the spec'd spoke lengths.

The only thing I can think of, is that the OEM rim is assymetrical (rear wheel - spoke holes offset towards NDS). However, I can't see that that would make more than 1mm difference in spoke length.


If helpful, here are the specs (mm):

Code:
                         OEM    | measured
                      ----------|-----------  
Hub flange diameter  :   47     |  47 (both)
Center to Flange,  DS:   17.75  |  17
Center to Flange, NDS:   30.25  |  30
Rim ERD              :  596     | 597
Rim Height           :   21     |  21
Rim Spoke Offset     :    2.5   |  --


Number of spokes     :   24
Lacing               : 2 cross

                         OEM    | calculated
                      ----------|-----------  
Spoke Length,  DS    :  294     |  287      (299 if 3 cross)
Spoke Length, NDS    :  295     |  288      (300 if 3 cross)
(DS = drive side, NDS = non-drive side)


The rim I'm considering is a Kinlin XR19W, as it has the same ERD as the original rim. I intended to reuse the original spokes. However, that was before I ran in to my spoke length issue.

Anyway, basically I'd like to know why my calculated spoke lengths are so off the sped'd values.

So, any glaring/obvious issues/errors/mistakes??

Thanks for any help!
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Old 11-18-13, 08:06 PM
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Well, after looking at a picture of the wheel, I think I know the issue:

The wheel is a Bontrager Race. And while the rim has evenly spaced spoke holes (they've ditched the paired-spoke design), the hub does not. That is, the hub's spoke holes are still "paired". So, that is probably why the diff in spec'd -vs- calculated spoke length. Maybe. I guess. I don't know. I'm just grabbing at spokes...

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Bontrager_rear_ds_whole.1s.jpg (97.6 KB, 17 views)

Last edited by Self Evident; 11-18-13 at 09:59 PM. Reason: minor typo (I *don't* know)
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Old 11-18-13, 08:23 PM
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You already have enough info to get the right lengths.

Start by measuring the original spokes and subtracting the excess length to get what you'd use for the same rim and nipples. (note that with unmatched rim/hub drilling, the original wheel might be using different length spokes, so check a few.

Bow consider the rim diameters, by measuring the actual diameter to the edge. Then lay something accross and measure the drop to the nipple seat or top of the nipple (either is OK, but the same for both rims), to compare the ERDs of both rims, add/subtract to the prior adjusted spoke length and you should be good to go.
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Old 11-18-13, 08:36 PM
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Originally Posted by Self Evident
Well, after looking at a picture of the wheel, I think I know the issue:

The wheel is a Bontrager Race. And while the rim has evenly spaced spoke holes (they've ditched the paired-spoke design), the hub does not. That is, the hub's spoke holes are still "paired". So, that is probably why the diff in spec'd -vs- calculated spoke length. Maybe. I guess. I know know. I'm just grabbing at spokes...

My spoke length calculation spreadsheet has the tool necessary to figure out the modified crossing factor needed to use a paired hole hub in a traditionally drilled rim:

https://www.mrrrabbit.net/wheelsbyfle...plications.php

Figure out the "new" crossing - it'll have a decimal value...

...plug it into the spoke length calculator of YOUR choice...

Just make certain that each pair of spokes at the hub start going away from each other - not crossing each other.

=8-)
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2. I like anyone will comment in other areas.
3. I do not own the preexisting concepts of DISH and ERD.
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Last edited by mrrabbit; 11-18-13 at 08:41 PM.
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Old 11-18-13, 08:40 PM
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Also the published ERD for the XR-19W is erroneous - 597mm.

I measured 5 just a while ago to the bottom of the screwdriver flat of a 12mm standard profile nipple and got on average 595mm.

You really should have the XR-19W ON HAND and measure for yourself to the target location of choice in the nipples you intend to use.

=8-)
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5000+ wheels built since 1984...

Disclaimer:

1. I do not claim to be an expert in bicycle mechanics despite my experience.
2. I like anyone will comment in other areas.
3. I do not own the preexisting concepts of DISH and ERD.
4. I will provide information as I always have to others that I believe will help them protect themselves from unscrupulous mechanics.
5. My all time favorite book is:

Kahane, Howard. Logic and Contemporary Rhetoric: The Use of Reason in Everyday Life
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Old 11-18-13, 10:34 PM
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Thanks for the quick reply, gentlemen - very helpful info!

I now have, naturally, another question...

This is for mrrabbit (or someone familiar with his spokechart spreadsheet) - would you mind explaining the the PAIR-HOLE-HUB-CHART tab of spokechart_v2-0.xls? Particularly, the "Paired Hole Degrees" value? It seems fairly "self-evident", but there are two possible values that I can see, so I am not sure which it needs.

Does the "Paired Hole Degrees" cell need the angle between each pair of holes? That is, my hub has 24 holes, 12/side in 6 pairs. That would produce 60 degrees between each pair of holes.
(angle "a" in pic below)

Or, does Degrees need to be the angle between the holes of the paired holes themselves, which are much closer together, and have a much smaller angle?
(angle "b" in pic below)

The "b" angle (gestimated ~ 5 to 10 degrees) seems to produce an Adjusted Crossing that subsequently produces a spoke length closer to the spec'd values. But, still shy by 3 or 4 mm.



What I'll probably do is just order the rim and see if it works. If not, then I 'll order new spokes. Then another set of a different length. And then another... (I probably won't really do that ... probably...)

In any case, thanks again!
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Last edited by Self Evident; 11-18-13 at 10:44 PM. Reason: corrected pic
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Old 11-18-13, 11:06 PM
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The degrees between the paired holes themselves...my experience so far has been in the 6-10 degree range.

=8-)
__________________
5000+ wheels built since 1984...

Disclaimer:

1. I do not claim to be an expert in bicycle mechanics despite my experience.
2. I like anyone will comment in other areas.
3. I do not own the preexisting concepts of DISH and ERD.
4. I will provide information as I always have to others that I believe will help them protect themselves from unscrupulous mechanics.
5. My all time favorite book is:

Kahane, Howard. Logic and Contemporary Rhetoric: The Use of Reason in Everyday Life
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