# Bicycle Mechanics - Spocalc / measuring ERD

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View Full Version : Spocalc / measuring ERD

halfwheel
12-26-12, 01:30 PM
So, I'm planning on building a set of wheels calculating spoke length using Spocalc, the Excel program by Damon Rinard (http://www.sheldonbrown.com/rinard/spocalc.htm). I'm going to calculate ERD manually, but Spocalc's directions for ERD has raised some questions.

In calculating ERD, Spocalc states: "Effective Rim Diameter (ERD) is the diameter on which you want the ends of the spokes to lie. Most people prefer it near the end of the spoke nipple." Really? The end of the spoke nipple as they show in their diagram, as opposed to the bottom of the driver slot? What if it needs to be tensioned a little more, wouldn't that push the spoke past the end of the nipple?

So my questions:
1. Which is correct, bottom of slot or end of spoke nipple?

2. If end of nipple, I can understand why people recommend rounding down in spoke size, but what if your calculation is right on? Along that thinking, would it be wise to round up if calculating from bottom of slot?

3. Any obvious problems if I use bottom of slot using Spocalc?

4. What if I measure actual inside rim diameter? Is there a recommended amount to add to rim diameter to compensate for length of spoke that pushes through the rim to achieve necessary depth in the spoke nipple? Or is that based on characteristics of the specific spoke nipple, thickness of rim material, etc?

On one hand, I want to stay consistent w/ Spocalc's methodolgies to achieve the "correct" spoke size, on the other hand, I'm not quite convinced I agree w/ what it considers "correct" (which led to question #3).

fietsbob
12-26-12, 02:59 PM
There have been lots of prior posts on this , You may wait a while for someone to re plow that plowed field,
or you can do some searching for prior dialogs..

some people find Google finds things posted here, better than the site's own search.

FBinNY
12-26-12, 03:32 PM
There is no one correct spoke height in the nipple, it's a range. You want the spoke to engage the head the way a screw would a nut. Proper mininum engagement would therefore be about 2mm, which is usually close to ending at the slot. More engagement doesn't hurt, but on some spoke/nipple combinations you may run out of thread at about 1mm above the head.

So that's roughly your working range, from the bottom of the slot, to 1mm above the top of the nipple. Personally I consider it perfect if the spoke ends between the bottom of the slot and top of the nipple.

As for rounding, it depends on what you consider important, and the spokes, nipples and rime you're using. I place a high premium on good thread engagement, so I consider the bottom of the slot an absolute minimum. That means if I calculate for that, I'd round up. To give myself a margin for error, I'm careful to use spokes and nipples that will thread about 2mm or more above the top of the nipple, giving me a 3mm target zone.

That's OK because 99% of my wheels are double wall rims so I'm not concerned with a spoke sticking up. OTOH, if I were building a single wall rim I'd set the top of the nipple as the absolute maximum, and err down from there.

As I said there's no one perfect answer with everything being worse. As long as you know what you want, and work consistent to those objectives then all is good.

BTW- on your first few wheels, make a careful comparison of where you planned to end up, vs where it actually came out. This will tell you the bias in the calculator (or your measuring methods) and you'll have a correction factor that will have you hitting the target 100% of the time, (as long as you do everything the same way and use the same calculator).

mrrabbit
12-26-12, 08:42 PM

Watch that video...

Then you'll understand:

1. Why you need to test the actual threads and actual nipples you intend to use.
2. What happens to ERD when those change.

...and more importantly:

3. Why YOU the wheel builder effectively drive the numbers used for an ERD in order to arrive at a spoke length that (fingers crossed) ultimately works.

This is also why you will hear the common refrain here that it's best to do your own measuring. You can't trust most other parties ERD's since most DO NOT PROVIDE A REFERENCE FOR THEIR NUMBER.

=8-)

estasnyc
12-26-12, 11:27 PM
There have been lots of prior posts on this , You may wait a while for someone to re plow that plowed field, or you can do some searching for prior dialogs..
Be patient. I just ordered a set of spokes today for my first wheel build. I'm reusing an old Miche front hub to be laced to a Sun CR-18 rim.

This much I can say: I've taken measurements for everything instead of relying on anything anybody says and when I plug the SAME numbers into the three spoke calculating websites that I know of, http://www.wheelpro.co.uk/spokecalc/ , http://www.bikeschool.com/tools/spoke-length-calculator and http://lenni.info/edd/ , I find that the difference between the shortest and longest calculated spoke length is about .3mm.

It's when I read the measurement instructions at these three websites that I can see where things can go awry.

It's not just the definition of ERD that can be different. It's also the definition of the measurement from the center of the hub to each flange. How should you factor in the width of the flange?

The only reason flange thickness doesn't appear as critical as is how to "properly" measure ERD is that it changes the spoke lengths that I've calculated get by only about .2mm at most.

One thing that I have done is to use a digital micrometer to measure the depth from the top of the nipple to the bottom of the driver slot. Turns out that the two nipples that I used for measuring ERD have two different depths. When added together I get about 2.8mm.

Now what I've done for myself is to take an ERD measurement based on using the bottom of the driver slot and use that to calculate the LOWER limit for spoke length and round UP to the nearest integer above.

I then add 2.8 to my measured ERD, run the numbers through for an UPPER limit and to see if this integer millimeter length that I've chosen is less than that and become confident that I didn't f**k up.

Give me a week for the spokes to come in the mail and for my finding out if this will be the reality.

FBinNY
12-27-12, 12:00 AM
Be patient. I just ordered a set of spokes today for my first wheel build. I'm reusing an old Miche front hub to be laced to a Sun CR-18 rim.

This much I can say: I've taken measurements for everything instead of relying on anything anybody says and when I plug the SAME numbers into the three spoke calculating websites that I know of, http://www.wheelpro.co.uk/spokecalc/ , http://www.bikeschool.com/tools/spoke-length-calculator and http://lenni.info/edd/ , I find that the difference between the shortest and longest calculated spoke length is about .3mm.

It's when I read the measurement instructions at these three websites that I can see where things can go awry.

It's not just the definition of ERD that can be different. It's also the definition of the measurement from the center of the hub to each flange. How should you factor in the width of the flange?

The only reason flange thickness doesn't appear as critical as is how to "properly" measure ERD is that it changes the spoke lengths that I've calculated get by only about .2mm at most.

One thing that I have done is to use a digital micrometer to measure the depth from the top of the nipple to the bottom of the driver slot. Turns out that the two nipples that I used for measuring ERD have two different depths. When added together I get about 2.8mm.

Now what I've done for myself is to take an ERD measurement based on using the bottom of the driver slot and use that to calculate the LOWER limit for spoke length and round UP to the nearest integer above.

I then add 2.8 to my measured ERD, run the numbers through for an UPPER limit and to see if this integer millimeter length that I've chosen is less than that and become confident that I didn't f**k up.

Give me a week for the spokes to come in the mail and for my finding out if this will be the reality.

As I said, there are all sorts of little errors that can add up or cancel out. For example most people measure spoke holes on center, but spokes are measured to the inside of the elbow, so there's 1mm discrepancy right there. Interlacing inside over outside uses almost 1mm per spoke, vs. not doing so. Then spokes elongate slightly as they're tensioned. At the same tension, lighter and butted spokes will lengthen more than heavier spokes. The conical bottom of nipples fits differently on many eyeleted vs. plain drilled rims.

The calculators do raw math, and get you close, but in some cases between some of the variables and how you measured can put you off 1-2mm. That's why you'll only get consistent results if you do everything the same each time, and develop your own guide to fudge factors based on experience. Noting how far off expected target your first wheel comes out, will help you get closed on the next, and after a few wheels you'll be nailing it with spokes ending exactly where you expected them to.

halfwheel
12-27-12, 08:33 AM
Wow, thank you all for your thoughtful responses. There's a lot of great information here, it's exactly what I was looking for. I'll also check out the links and video tonight after work.

Prior to my post, I did find a little bit of info on the subject. People would say how far they preferred the spoke, but not with a lot of dialogue as to why. I actually have built a set of wheels about 10 years ago--if I remember correctly all I did then was plug the hub model, rim model, # of spokes, and spoke pattern in QPB's calculator. It then provided lengths for DT and Wheelsmith and off I went. I didn't consider differences in tolerance then.

But anyway, I have a set of wheels in mind to build, but I think to get back into it I'll rebuild an old machine built wheel I already have and go through the measurements manually. Thanks again.