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### ERD question

Posted: Wed Jun 18, 2014 5:56 pm
Hi guys

I need a little help to figure out the exact ERD. The manufacturer says 548 mm ERD.

I´ve cut two spokes to an exact length of 200 mm each. Well maybe 200,5 mm each:

When I insert the spokes in opposite spoke holes in the rim, the spokes are 151,5 mm apart from each other on both front and rear:

Rear rim

Front rim

That should give me:

200,5 + 200,5 + 151,5 = 552,5

The only thing I´m worried about is, did I measure the spokes correct? I can´t figure out if I´m supposed to measure the spoke from the top of the nipple? or from where the nipple goes through the rim.

In other words, do I have to measure the spokes length with or without the nipple head.

I´ve read the section in Rogner Mussons book over and over again, but can´t find the answer.

### Re: ERD question

Posted: Wed Jun 18, 2014 6:25 pm
http://www.wheelpro.co.uk/spokecalc/ --> rims

Roger Musson wrote:Take two spokes cut down to 200mm. Glue on a nipple so that the top of the spoke is flush with the bottom of the slot in the nipple.

### Re: ERD question

Posted: Wed Jun 18, 2014 6:25 pm

### Re: ERD question

Posted: Wed Jun 18, 2014 6:49 pm
Exactly. But what I´m a little bit concerned about is, that different nipples have different size nippleheads, which will make Mussons advice inaccurate, because the measurement of the ERD will depend on the type of nipple, you are using... Or am I missing the point??

### Re: ERD question

Posted: Wed Jun 18, 2014 7:05 pm
I use Mussons spokecalc and his book to build my wheels. I've found that his way of measuring ERD to be overly complicated. There is an easier way:

Insert a spoke through the rim, and thread it into a nipple. You don't need to cut the spoke. Insert another spoke onto the rim on the opposite side of the first spoke, and thread it into a nipple. Use a rubber band, and join the two spoke ends. Measure the total length from one end of the spoke, where it enter the nipple, to the opposite end of the other spoke (L). Measure length of the nipple (N), and add the values to get ERD.

L = length from one end of spoke/nipple interface to other end
N = nipple length

ERD = L + 2N

### Re: ERD question

Posted: Wed Jun 18, 2014 7:09 pm
That sounds right. ERD as used by the Sheldon Brown and DT Swiss calculator etc. should be exactly the diameter made by the very end of the threaded tips of the spokes once they are installed. What type of nipples and washers used affects how far you want the spokes to reach.

### Re: ERD question

Posted: Wed Jun 18, 2014 11:57 pm
Here is how I do it, with no issue at all after building many, many wheels.

ERD measurement tool making

1. Cut the spokes to 200mm. If you cut them to 200.6mm, or whatever, file them down until they are EXACTLY 200mm. This is not hard.

2. Screw on any standard nipple (12mm, 16mm, pink, blue, neon black, whatever), making sure to screw it down EXACTLY to where the top of the spoke is completely flush with the slot cut into the top of the nipple; this is all that matters. The length of the nipple is irrelevant, as this is just how long the rectangular part is that the spoke key engages with; the heads will be the same inside the rim.

3. When you are happy you have it in the right position, make a visual, mental note of where the top of the spoke sits, then open your super glue pot. Unscrew the spoke slightly, drip in some superglue down into the nipple, then quickly screw the spoke back up to the perfect, flush position you had it at before. Leave it for a few minutes, to set fully.

Measuring

1. Cut little bits of white electrical tape, and number then 1 - 24 (or however many holes you have on your rim). Put these on the rim at each hole.

2. Get a notebook and pen. Write the following; 1-13=, 2-14=, 3-15=, 4-16=, 5-17=, 6-18=, 7-19=, 8-20=, 9-21=, 10-22=, 11-23=, 12-24=

3. Get a steel ruler.

4. Put the two spoke/nipple sets through opposing holes in the rim, and lay them onto the steel ruler. Make a note of the length (200+200+whatever the measurement on the ruler is). Write this into your notebook at whatever nipple hole crossing you used.

5. Move the spoke-nipple sets to the next opposing nipple hole crossing round, measure again and record. Do this all the way round, then average. That is your ERD.

Then, use one of the online spoke length calculators to get your spoke length. I prefer the WheelProUK one.

You can even set up a spreadsheet in Google Drive and record your various rim measurements for later use, if you want to do things extremely thoroughly. I have a single spreadsheet in Drive, which I have all of my wheel builds on, which I record ERD, spoke tension round the wheel, tyre inflation / fitting spoke tension reduction, etc. like the massive nerd I truly am.

### Re: ERD question

Posted: Sun Jun 22, 2014 9:06 am
Slagter wrote:Exactly. But what I´m a little bit concerned about is, that different nipples have different size nippleheads, which will make Mussons advice inaccurate, because the measurement of the ERD will depend on the type of nipple, you are using... Or am I missing the point??

Some manufacturers (MAVIC eg) measure the ERD from the inside of the rim at the nipple seat. This makes the number independent from the used nipples, but you have to know the measurement of the nipples. Most of the time this method gives an ERD wich is ~3mm shorter.

The problem is, that the same term is used for different things and therefor you can never be sure wich method is used. This is one reason why people advice you to measure the rims by your own and don't rely on manufacturer data

Most publicly available spoke calculators work well with the Musson method but you should avoid spoke calculators wich do not explain how they want the ERD to be measured.

### Re: ERD question

Posted: Sun Jun 22, 2014 11:07 am
Orfitinho wrote:
Slagter wrote:Most publicly available spoke calculators work well with the Musson method but you should avoid spoke calculators wich do not explain how they want the ERD to be measured.

That is a good point.

### Re: ERD question

Posted: Sun Jun 22, 2014 11:40 am
Orfitinho ; That´s exactly what I´m concerned about. So either I have to measure the the ERD from the nipple seat or I have to measure it with the same kind of nipple, that I´m going to use for the build. But as far as I understand your point, I still have to know, how the spokecalc uses the numbers.

Which spokecalc do you recommend?

### Re: ERD question

Posted: Sun Jun 22, 2014 12:43 pm
The calculator of Roger Musson is simple and works well.
The Spokomat offers a lot more information about the intendet build. (Setup->Sprache changes language from german to english). I don't use its database of components.

### Re: ERD question

Posted: Sun Jun 22, 2014 12:43 pm