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Discussion Starter · #1 ·
I'm prepared to build my first wheel. I'm doing a lot of reading (including Brandt and Musson). Everything I've read says to measure ERD yourself, so I will. I'm just baffled that the companies that produce the rims and, I would think, have tolerances on the order of a few microns can't be trusted to give an ERD accurate to within a millimeter.

It seems to me they would be able to do this with great accuracy. Mike T gives some great info on how to practically do this. He kindly provides some great tips (Measuring ERD) His methods imply several measurements--measuring two spokes and then once placed in the rim of interest measuring the distance between the spoke ends. Thus three measurements and potentially three additive errors (if you are unfortunate enough to slightly underestimate or overestimate all three).

I'm sure it's the best way to do it, I just don't understand why manufacturers can't provide a more accurate ERD. Are rims that inherently dimensionally unstable ?

I'm more a mathematician than an engineer...so practical explanations would be appreciated.:)
 

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A wheelist
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Sometimes rim specs change as running changes in manufacturing and you have to trust the makers/marketers to update their information. If they don't you lose. I built wheels with some Ryde rims last year and within 5 minutes of getting the rims out of the box I knew they had effed up the ERD. I e-mailed them and they admitted the error**. They changed it on their site but the last time I looked they hadn't changed in in the .pdf info sent to retailers. So who can you trust?

**I did some measurement sleuthing and found they they had given the useless (to us anyway) NSD (nipple seat diameter) figure as the ERD. They were about 5mm out. I forget just how much.

Measure 3x, buy spokes once. Be ultra accurate.
 

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When you measure ERD i think you mean to the rim bed?

I've read also ERD can be considered as the bottom of the nipple slot, or the top. Then i'm never sure where the mfr or vendor measures it, unless i do it, and buy spokes later, but that doesn't answer it if you're replacing a rim with something different.
 

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Russian Troll Farmer
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Effective Rim Diameter is measured from where the nipple seats, not the bottom of the tube bed (except in single-wall rims). I'm sure SOMEBODY has a database somewhere....
 

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Effective Rim Diameter is measured from where the nipple seats, not the bottom of the tube bed (except in single-wall rims). I'm sure SOMEBODY has a database somewhere....
ERD is the measurement between the end of a spoke and the end of another spoke diametrically opposite in a wheel with correct length spokes. The ideal spoke length is to the bottom of the nipple slot.
 

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.............I just don't understand why manufacturers can't provide a more accurate ERD. Are rims that inherently dimensionally unstable ?

I'm more a mathematician than an engineer...so practical explanations would be appreciated.:)
The problem arises because as wheelbuilders we require precise rim diameter measurements to accurately calculate spoke lengths while the rim manufacturing industry follows greater tolerances emanating from fundamentally not considering the rim the precise instrument we would like it to be. Basically, not their problem how we accuratelly calculate spoke lengths. To that add that there is no standard on how to measure ERD that is universally adopted by all manufactures.
The stated ERD is a nominal length that is basically useless for what we do. It changes based on how one measures it, on how radially true the rim is and on how worn the extrusion die is before replaced.

If you want accurate measurements, measure the rims yourself AND, equally as important, follow the ERD measurement procedure indicated by the specific spoke lentgh calculator you plan on using.
 

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ERD is the measurement between the end of a spoke and the end of another spoke diametrically opposite in a wheel with correct length spokes. The ideal spoke length is to the bottom of the nipple slot.
Good to know.... is this the dimension thar Kinlin or H+Son or Brandon gives for their rims?
 

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Good to know.... is this the dimension thar Kinlin or H+Son or Brandon gives for their rims?
I wouldn't know and I don't care. I will never accept someone elses rim or hub measurements. They have nothing to lose from bad measuring, typos, incorrect listing, lack of updates and probably a few other ways of giving out bad info.

The dimension I measure is the one needed by the spoke calculator I trust - Roger Musson's. How do the other calculators calculate the ERD? Do they show you? If not, even the ERD you measure might not be right for their calculator.
 

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Discussion Starter · #10 ·
Hang on a second, guys ! I'm going to measure 3 (or more--I have OCD) times and have someone else check before I order spokes. Do worry. But that's not my question.

I wondering why manufactures can't provide accurate dimensions. Let's face it,they aren't building the rims to act as pizza molds! They are building them so people can build wheels with them. In order to do this builders need to know how many millimeters from the bottom of the slot of one nipple to the corresponding part of a nipple on the opposite side of the wheel. They should be able to provide this number (they could even give a few numbers corresponding to some of the most common nipples) to within 0.5 mm. They aren't stupid and they know why people are buying the rims--to build. Why do they not provide this number ?
 

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Why do they not provide this number ?
They DO. Heck some even have it printed on the rim decal (I think Pacenti did/does) and most of them have it on their site. But you have to take a leap of faith to trust that number. What if the rim extrusion was changed as a running change to give a thicker nipple bed (I know of a rim that did) and what if George in "packing" was told to update the website but he forgot?

Or what if you're looking up an ERD on a spoke calculator's database and you picked the wrong rim? Maybe you looked up the Pacenti SL23 and you chose V1 specs but you really had the V2 version? Or someone inputted the database 549 when they meant to type 594?

There are just too many ways for you to get the wrong number and to buy 52 CX-Rays (at about $150) that you can't return, as they will be used spokes when you find out the boo-boo, is something that is completely avoidable.

Many of us get two shipments of wheel parts -
1. The hubs and rims.
2. The spokes, after we have measured the the hubs and rims.

Inconvenient? Sure. Expensive? Yes. But nowhere near as inconvenient and expensive as ordering spokes twice.
 

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Hang on a second, guys ! I'm going to measure 3 (or more--I have OCD) times and have someone else check before I order spokes. Do worry. But that's not my question.
This quality will serve you well in building your wheels.

Inconvenient? Sure. Expensive? Yes. But nowhere near as inconvenient and expensive as ordering spokes twice.
Not to mention the lost time and frustration in having to unlace and re-lace your wheel.

As Mike and others have stated, trust nobody except yourself. A mechanic at my bike shop who builds wheels echos this advice. There are too many things that can go wrong in trusting manufacturer stated ERDs.

BTW, did you buy Roger Musson's e-book?
 

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Discussion Starter · #13 ·
This quality will serve you well in building your wheels.



Not to mention the lost time and frustration in having to unlace and re-lace your wheel.

As Mike and others have stated, trust nobody except yourself. A mechanic at my bike shop who builds wheels echos this advice. There are too many things that can go wrong in trusting manufacturer stated ERDs.

BTW, did you buy Roger Musson's e-book?
Reading it as I type ! Great book!!!


Sent from my iPhone using Tapatalk
 

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Russian Troll Farmer
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ERD is the measurement between the end of a spoke and the end of another spoke diametrically opposite in a wheel with correct length spokes. ....
Incorrect. The measurement is to the seat of a nipple to the opposite nipple seat. The spoke itself should be shorter, occasionally a bit longer (that's why they give you ±2mm usually), sitting inside the sleeve of the nipple. We are measuring a dimension on the rim, not the spoke itself.
 

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i tried to measure erd a couple times. i think it's the one thing i haven't mastered. so i just use online sources for the rim. only once did i not get it quite right. but it still worked. as it always does.
 

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A wheelist
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Incorrect. The measurement is to the seat of a nipple to the opposite nipple seat. The spoke itself should be shorter, occasionally a bit longer (that's why they give you ±2mm usually), sitting inside the sleeve of the nipple. We are measuring a dimension on the rim, not the spoke itself.
Bzzzzt. Thanks for playing but you're not right - not for any spoke calculator I ever used anyway. Roger Musson's book shows the measurement from spoke end to spoke end. Park Tool's site (they should know a bit about bike mechanics eh?) says (copy & paste from their site) "The ERD is that point in the rim where the end of the spoke sits...."

You tell those guys they're wrong. I won't.

If the calculator you use tells and shows you to measure the NSD (nipple seat diameter) and then compensates for a nipple head depth behind the scenes then that calculator takes a leap of faith to know what the dimensions of your nipples are (ahh!).

I've measured ERD by the methods on my site for a few decades (5?) and have yet to have the spoke lengths come out wrong.

YMMV.
 

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Discussion Starter · #17 ·
Bzzzzt. Thanks for playing but you're not right - not for any spoke calculator I ever used anyway. Roger Musson's book shows the measurement from spoke end to spoke end. Park Tool's site (they should know a bit about bike mechanics eh?) says (copy & paste from their site) "The ERD is that point in the rim where the end of the spoke sits...."

You tell those guys they're wrong. I won't.

If the calculator you use tells and shows you to measure the NSD (nipple seat diameter) and then compensates for a nipple head depth behind the scenes then that calculator takes a leap of faith to know what the dimensions of your nipples are (ahh!).

I've measured ERD by the methods on my site for a few decades (5?) and have yet to have the spoke lengths come out wrong.

YMMV.
I don't know spoke nipple anatomy. What do you call the part at the bottom of the nipple's slot? You are measuring from that point to similar part on opposite side of wheel, right? It's NOT called the nipple seat, but maybe that's what the poster meant?


Sent from my iPhone using Tapatalk
 

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I don't know spoke nipple anatomy. What do you call the part at the bottom of the nipple's slot?
"The bottom of the nipple slot" is what it's called. :D The ideal place for a spoke to end.


You are measuring from that point to similar part on opposite side of wheel, right?
In my world, yes.


t's NOT called the nipple seat, but maybe that's what the poster meant?

You're right, it isn't. This is the measurement Ryde gave in my earlier post. And they changed it to the real ERD - a difference of 5mm.
 

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I finally measured my Aksium ERD using Mike's method to be 594mm, which agrees with this person (at least the first time) 20 hole rim replacement - BikeRadar Forum when I was in the same situation they were.

I had the option of re-rimming with an Archetype, which I already have a set of, exactly like the link above, or XR-19W, which is within a mm of that Aksium, like this link: Why we say: "Measure your own ERD's!" KinLin xKeyMet XR-19W - Bike Forums. I wonder how BHS gets their measurements, since they have a few 'verified' numbers.
 

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Once you understand the derivation of the spoke length formula, you'll see that the erd is measured to the ends of the spoke in the finished wheel, as Mike said.

For a particular rim, the erd cannot be a fixed dimension because it needs to take into account the nipples being used. For example the erd measured with DT 16mm nipples will be 4mm less than if it were measured with 12mm nipples (although standard practice is to measure with 12mm nipples and subtract 2mm from the spoke lengths).
 
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