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different erds from two methods for same rim

Old 12-18-15, 08:49 AM
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winston61
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different erds from two methods for same rim

I'm rebuilding an old beater that originally had steel rims. I'm using wheel master 26x1 3/8 steel rims. Sheldon's method gives me a 544 erd and Musson's method give me 538. Calculating using both numbers gives me a spoke length difference of 3mm. Can anyone offer any insight? Should I repeat measuring and calculating? Are there any obvious amateur mistakes that I don't know about? On second thought I will measure again, look for repeated results. Thanks to all.
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Old 12-18-15, 09:11 AM
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You've answered part of your question already. Measure at three or four points around the rims and average for each method. If you have a LBS you might ask them if they have the ability to measure and if so then ask how much they will charge (and if they do so for little/no cost then buy the spokes from them). I've reversed engineered spoke lengths many times by taking a pre built wheel with the same rim and measured it's spokes and hub info then ran those through the formulas to get the ERD or to get a different hub dimension's spoke lengths.

Experienced wheel builders have gone through this already and have decided on one ERD measuring method and spoke length formula that they have learned produces a certain length that they then add their special "correction" factor to come up with the best length. Additionally different batches of rims of the same model can have the ERD vary slightly. Some spokes stretch a bit more then others and so the after build length can vary some. Published ERDs can be different actual points on a rim or within a nipple too. The Op is at the beginning of this learning curve. Andy.
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Old 12-18-15, 09:13 AM
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544mm sounds way to small for a 590mm tire size on a "cheap steel" rim.
A single wall rim will be about 10-20mm less, so shouldn't be less than about 570-580mm.

Take 2 spokes of known length.
Screw nipples on to the screw driver slot.
Run them in opposite side holes from each other and measure the "gap" between elbows with a caliper.
EXAMPLE-
Using 250mm spokes.
If the gap is 70mm....
2*250 +70=570mm

BTW-
If buying Wheelmaster rims, isn't an entire wheel cheaper than just the rim & spokes individually?
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Old 12-18-15, 09:18 AM
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You've possibly gotten into my objection about ERD. As used today, the ERD is the rim's "effective" diameter measured at the tops of the nipples (or where the spokes theoretically will end). This isn't an actual dimension of the rim itself, which when measured where the nipples sit, will be about 6mm smaller (2x the height of a nipple head).

So you heave to be consistent an measure what your spoke calculator calls for. FWIW most calculators use the current ERD values (including nipples) and will give a spoke length that has the spokes ending at same diameter as the ERD entered, but not all. Be sure to enter a value that matches what is asked for (ERD, vs. diameter at nipple seat) to get consistent results.

FWIW - I suspect that the longer calculation will turn out to be the right one.
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Old 12-18-15, 09:18 AM
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Originally Posted by Bill Kapaun View Post
..Run them in opposite side holes from each other and measure the "gap" between elbows with a caliper.
...
In this case the "gap" would be the outside dimension between the insides of the two j bends, like where you'd measure spoke length from, right?
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Old 12-18-15, 09:19 AM
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yes, you are right about cheaper to buy, but I'm semi-retired and looking for ways to pass the time. Thanks


Originally Posted by Bill Kapaun View Post
544mm sounds way to small for a 590mm tire size on a "cheap steel" rim.
A single wall rim will be about 10-20mm less, so shouldn't be less than about 570-580mm.

Take 2 spokes of known length.
Screw nipples on to the screw driver slot.
Run them in opposite side holes from each other and measure the "gap" between elbows with a caliper.
EXAMPLE-
Using 250mm spokes.
If the gap is 70mm....
2*250 +70=570mm

BTW-
If buying Wheelmaster rims, isn't an entire wheel cheaper than just the rim & spokes individually?
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Old 12-18-15, 09:32 AM
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Originally Posted by winston61 View Post
yes, you are right about cheaper to buy, but I'm semi-retired and looking for ways to pass the time. Thanks
Then get a better rim such as-
https://www.amazon.com/SunRingle-CR-1.../dp/B001CJXBHC

Strong enough for tandem and far superior braking if you have rim brakes.

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Old 12-18-15, 10:28 AM
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Originally Posted by Looigi View Post
In this case the "gap" would be the outside dimension between the insides of the two j bends, like where you'd measure spoke length from, right?
Yes, but you would deduct the diameter of 1 spoke. (you measure center to center)
It's only another 1mm, but...
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Old 12-18-15, 02:07 PM
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gonna roll the dice

Gonna go with Sheldon Brown. I re-measured several times with more than one set of eyes and another hand running numbers. Came up with 545 which will give a spoke length of 265mm. I'll buy the spokes from my not so local bike shop and ask them to check as well. Jeez, what a pita.



Originally Posted by winston61 View Post
I'm rebuilding an old beater that originally had steel rims. I'm using wheel master 26x1 3/8 steel rims. Sheldon's method gives me a 544 erd and Musson's method give me 538. Calculating using both numbers gives me a spoke length difference of 3mm. Can anyone offer any insight? Should I repeat measuring and calculating? Are there any obvious amateur mistakes that I don't know about? On second thought I will measure again, look for repeated results. Thanks to all.
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Old 12-18-15, 03:04 PM
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265mm spoke length is far more common for a 559mm ISO rim then a 590mm one. Again if possible have who ever sells you the spokes do the measurement and length calculation. That way the ability to return the spokes is kept open if they did something wrong.

Is it possible that you have miss read the rim size? Have you actually test mounted a 26x1 3/8" tire yet? Andy.
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Old 12-18-15, 03:29 PM
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Originally Posted by winston61 View Post
Jeez, what a pita.
You were seeking a way to pass the time.
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