It's bothers me that folks try to migrate ERD from something that is dynamic to something that is static reference-wise - the very religious aspect that has come to dominate ERD discussions that Jobst Brandt noted sometime back in a USENET post.
ERD is not a standard, it does not have any standards, nor is it a whitepaper spec.
So I'll spill my spiel one more time:
1. ERD is the distance from the end of one spoke to the end of the opposing spoke in an already built and functional bicycle wheel.
Keywords: Distance, Ends of Spokes, Already Built, Functional, Wheel
Important Note: It's not about the rim. It's about the wheel. If you can't grasp this - you will always struggle with the concept of ERD.
2. When you attempt to come up with an ERD - you are essentially no different than a weather forecaster. You are trying to predict the future. In this case, you are trying to predict what that distance will be from the end of one spoke to the end of the other when your wheel is finished.
Fortunately, unlike the weather forecaster, the variables you are working with are only a handful:
- The rim
- The rim presence or non-presence of raw joint sleeves.
- The nipple type and profile.
- The nipple size and thread depth.
- The spoke thread length.
- The spoke and thread minimum and maximum insertion behavior.
- YOUR aiming preferences as a wheel builder.
The PHYSICAL variables noted above plus YOUR aiming preference is what makes ERD dynamic.
You the wheel builder ultimately drive what the ERD will be for a particular build.
That also means the same exact rim can have dozens of different ERDs - not just one.
http://www.youtube.com/watch?v=gCXFKN9JvCQ
3. The are many methods which to use to "predict" in the form of an ESTIMATE what distance in millimeters when a calculation is done will result in a workable spoke length when building a wheel.
One common one is to:
a. Use two spokes of known lengths.
b. Use two standard profile 12mm, 14mm or 16mm nipples.
c. Set the nipples on each spoke to the desired aiming point that allows at least 1mm of overhead so as to not run out of threads before achieving desired tension.
I typically aim for the bottom of the screwdriver slot of 12mm standard profile nipples for 1.5mm of overhead.
Most old 10mm standard profile nipples easily allow for aiming at the top of the nipple. But you rarely see these anymore...
d. Position in rim and grab the elbows with calipers and snug.
e. Repeat in several locations and take the average of the caliper readings.
f. AVERAGED READINGS + COMBINED KNOWN SPOKE LENGTHS = "ESTIMATED" RIM DIAMETER
4. Of course as we head off to our favorite spoke length calculators and plug in that distance as "rim diameter" - our fingers are crossed.
With luck - if the spoke length we get works - i.e., we end up with a functional bicycle wheel.
If that is the case - our "rim diameter" is judged to be "effective" - hence how we arrive at EFFECTIVE RIM DIAMETER (ERD).
5. Good practices are as follows:
Publish ERDs with a reference.
Example: 606 "~" Flat of 12mm
This means 606mm as-is aiming for the screwdriver flat of a standard profile 12mm nipple.
This way other folks can decide one of the following:
- "I'm doing what he is doing and I'll use the number as-is."
- "I'm using 16mm standard profile nipples with typical threads and reducing by 1mm on each end for a value of 604mm."
- "I'm using inverted FSA nipples with a higher aiming point on each end of 4mm for a final value of 612mm."
References are great. People can use as is - or from experience - adjust to suit their needs.
Without a reference - your published ERD number means absolutely nothing and cannot be trusted.
However, one important point remains. The same make and model of rim can change from one rim to the next. So as we always say here on BikeForums.net:
"It's always best to measure your own ERDs!"
=8-)