Measuring ERD--different methods give me differing results
 

So I am slowly going nuts. Here's the story, a continuation of an earlier saga:

I ordered some Ambrosio Nemesis rims. Of course I needed spokes for them, so I looked up the ERD. It is commonly thought to be 617 according to various internet sources (including spocalc). I also asked the shop where I bought the spokes--they told me the ERD is 614. I received the rims a few days ago and went to my LBS with them. They measured using some rim rods and told me that the ERD is 614.

But wait, there's more! I just sat down and measured them myself. Using the method where you thread nipples onto two spokes, pull them taut and measure the distance between the nipple ends then add twice the nipple length, I got an ERD of 617.

Then I learned about the way Jobst Brandt apparently measures rims. He sticks a spoke into a rim hole that already has a nipple seated in it and measures the distance between the spoke head and the outside of the rim, then measures the diameter of the rim--outside to outside-- and subtracts twice the first distance. Guess what: the ERD using this measuring scheme was 614.

That is, until I learned that ERD is measured to the bottom of the screwdriver groove on a nipple, meaning that using Jobst Brandt's method I should actually add 1.5 mm per nipple back (this is the measured depth of the groove on the nipples I am using), but using the first method it seems that I should be adding only 10.5 mm rather than 12 (though an illustration of this method shows that you should add the whole nipple). This simply reverses the measurements--Brandt's gives me 617 and the first method gives me 614.* edit: this paragraph is actually wrong. See end of post.

For a certain ERD length, is spoke length calculated as to be flush with this groove taking into account that people measure ERD encompassing the whole nipple? A cursory glance at the spocalc formula leads me to believe that there is no such correction. Based on these exercises it seems to me that I am in the ballpark and it doesn't really matter if I ordered spokes based on an ERD of 614 or 617. The LBS told me not to sweat it, and upon emailing the show I bought the spokes from they said the same thing. On the other hand, people on the internet seem to think--contrary to these shops and Sheldon Brown--that if spoke length is off by more than 1 mm from the calculated measurements, you are screwed.

Another idea is that I should measure the ERD according to the instructions of the spoke length calculator I'd be using. In this case, the instructions on Damon Rinard's spocalc page (the two spokes pulled taut + length of whole nipple) are right on the money. If anyone actually reads all this, please share your thoughts!

edit: actually, in my caffeinated and clear thinking state, I now realize that I am wrong about "correcting" Brandt's method. I would have to subtract 3 more mm if I had measured into the groove of the nipple.

Could be wrong but I believe Jobst aims for the screwdriver flat.

Sutherland's method typically aims at something a tad less that the top of a nipple. I.e., you end up somewhere between the flat and the top - closer to the top but not quite. This is why when rounding results - rounding down keeps you safe. Using 12mm nipples, it all works out just fine.

For myself I aim for the screwdriver flat:

1. First I measure the inside diameter using a meter stick with a closed right eye on the left and a closed left eye on the right.

2. The I take an inverted nipple and mark it on a drill press or lathe:

- 0 is located at .5mm below the screwdriver flat.
- 1, 2, 3, and 4 are located out along the nipple barrel.

3. I stick that in a hole adjacent to the joint. (Covers joint sleeves) Get a reading.

4. I stick that in a hole away from the joint area. (Non sleeve area...) Get a reading.

I then add it all up. When I do this - I find myself matching Mavic's ERD's rim by rim - they too are aiming for the screwdriver flat.
Since I have a spoke machine - I don't have to deal with rounding - I can go as-is with the calculation results.

Aiming for the screwdriver flat gives you a 1.0MM "underhead" and about 1.5MM "overhead" should you get something wrong.

The 617mm figure for your rim is from Sutherland's - hence you should round down. I wouldn't be surprised if I measured it and came out with 615/616mm when aiming for the screwdriver flat.

Most important thing to remember is:

There is no ERD per se. ERD is "EFFECTIVE" rim diameter. In other words - if the diameter you used worked without issues or hacks or band-aids - then that is YOUR effective rim diameter.

I know a wheelbuilder who when people start arguing what the ERD is for a given rim - he takes off his glasses and does a face palm. Literally...

=8-)

notice that most published data is "subject to change without notification".
buy rim and hub first, then buy the spokes.

Or you can buy the ERD sticks and get the proper answer in one go.

I just finished building some mavic open sports.There were at least 5 different ERDs for this rim on the net.I used the first method you mentioned on your post to get the ERD and ordered the spokes accordingly.It turned out perfect.

I think that I will take that to heart, haha.

AEO: I would have waited except that Record hubs and Nemesis rims are not exactly an uncommon build. I knew that I was taking a bit of a risk. The place I ordered from--Wheelbuilder.com--would allow me to exchange the spokes if I did not use them. At this point I don't think that I will bother though.

If you plug both dimensions into the Spoke calc I think you will find that there is little or no difference in the spoke length.

614 v. 617 = approx. 1.5mm difference in calculation result. Impact is greatest on 14 and 16mm nipples and old Edco nipples.

=8-)

Personally I prefer to aim for my spokes ending at the bottom of the slot. This is easier to build up with electric nipple-drivers as they get pushed out 1mm below the slot. Then I bring it up to tension and true for the final couple of turns.

Also if you use the two opposing spokes to measure ERD, I recommend you use four spokes in a + configuration. Then take two measurements at 90-degrees to each other. I've found that a lot of rims are not perfectly round and the process of tightening the nipples down will actually ovalize a round rim. I will then take the average of the two measurements.

I've since developed a variation on Brandt's method that measures everything from the rim-edge down. The variable that's most consistent between rims is the ETRTO bead-seat diameter. Find that spot on the rim and measure from there.

ERD sticks?

hang a plumb line across the rim?

add thickness of rim spoke nipple hole and inside diameter

Rohloff manual has a formula
L=[square root of] (.25x diameter to 2nd power) + 3400 - (50 x d x c )
c=.924 for 1x, .707 for 2 x

whole manual is downloadable to see the page

http://www.rohloff.de/en/index.html

I did measure at four positions on the rim, equally apart. There may have been 1/4 mm of discrepancy. I would expect these Paris Roubaix tested rims to withstand the stress of the dental floss I used to tie the spokes together ;)

I'm going to see how the spokes seat in the nipples (I'm using Sapim. Anyone have experience with those?), and decide whether to lace them up or exchange them. If there is a good amount of leeway for too-long spokes, then I'll go for it.

Removed...

-Jeremy

So using the Sapim spokes/nipples that I just got, ERD measures 619 or so. I think that means I am good to go.

You are taking an awful big risk of running out of threads before reaching optimal tension using an ERD of 619...617 is very close to the top of the nipple already.

=8-)

Good advice! A lot of people worry about the wrong problem.

Well, I got the spokes using an ERD of 617, but the way Sapim nipples sit in the eyelets of the rim (this is what I am using in my build) gives me an ERD of 619.

What I meant was that since the ERD measured with a Sapim nipple is larger, it means that I have more room to the top of the nipple for my spokes, which were bought with an ERD of 617 in mind. Does that make sense?

ERDs for traditional rims already include standard/generic 10mm/12mm nipple dimensions.

If using inverted nipples w/nylock in carbon rims - the ERD goes up a few mm - if using low profile nipples with UFO like head and very restrictive threading the ERD goes down 1-2mm...

...but that's not what you are working with, right?

Toss the 619 number you keep bringing up...go play with your PS3 or something...and get some sleep.

=8-)

;)

Wheelsmith Rim Rods.
http://www.jensonusa.com/store/produ...+Rim+Rods.aspx

Well, the 617 number was arrived at by measuring the distance between two dt swiss 12mm nipples. Sapim nipples give a number of 619--hence they must sit further up in the eyelets. That's all I'm saying! Eventually I'll have time to build the wheel and report back.

I'm a grad student. This is impossible.

1. Place coffee brewer on desk.
2. Keep coffee cup on plate warmer.
3. Right-click mouse on desktop - rest cursor on the popup dialog menu.
4. Find closet.
5. Sleep.

=8-)

Resurrecting this thread to say that the wheels came out perfectly fine and I had no problem hitting ~120-125 on the rear drive side spokes. I think that they did come out of the nipples a bit but seeing as there is a ton of glue and base tape between the tube and the spokes, I'm not very worried.

I maintain a comment/notes filed alongside each ERD in my spreadsheet - database.

ERD = 617 "-"

What would you suggest I put in the comment/notes filed ERD wise?

616?
615?

Each 1mm of ERD roughly equals .5mm of spoke length. Whatever you decide I'll edit as such in my document.

Congrats!

=8-)

:thumb:

KISS. Use the 1st method to measure ERD. Take reading at 4 places and use the average value. If you want to shave a few grams from the wheel, then round down the spoke length that you see on Spoke Cal. If you want the spoke to be flush with the end of the nipple, then round up the spoke length on Spoke Cal. The goal is to keep the end of the spoke flush or within 1 mm of the end of the nipple.

617 is what got him to the end of the nipple Furballi. That's why I'm asking for one that would likely reflect a screw driver flat ERD based upon his most recent experience. (Hence the 616 or 615 query...)

Aiming for the end of the nipple is not really good practice as it's the equivalent of backing oneself into a corner with only one way out...unless using inverted nipples w/nylock in a carbon rim.

=8-)

After spending money on spokes that were too long I succeeded with this method: Buy two spokes and measure their length, tip to crook. Get two of the nipples you intend to use. Insert the spokes into two diametrically opposed holes in the rim. Thread the nipples onto the spokes. I chose to thread just until the end of the spoke became visible through the slot - how far is a judgment call. Hand tension the spokes toward each other at the center of the rim and measure the distance from crook-to-crook using a vernier caliper. This is an outside measurement. Add the the following: measured distance crook-to-crook + length of first spoke + length of second spoke = ERD at the point the wheel comes into tension at your chosen nipple/spoke thread protrusion. Repeat at four evenly spaced points and average. Round to suit.