ERD - Effective Rim Diameter

The main purpose of the Effective Rim Diameter (ERD) is to decide where your spokes will end.

Then, there will be some geometric calculation involved and most of the spoke calculator will do that just fine. Especially the one from Roger Musson on the Spoke Length Project.

We use 12 mm standard slotted nipples as a reference for ERD. It is useful to do so because you can then compare your measurement with value from the rim manufacturer and with your other builds. Do it, sometimes you make mistakes...

You probably already know that you MUST measure ERD before ordering or cutting your spoke, because every rim is different.

There are three cases where ERD might provoke an headache.

You found them a lot in the Andra Series and other strong rims from Ryde. The nipple is forced in one direction to accommodate more crosses of the spokes. If you measure the ERD with traditional method, you will overestimate by several millimeters.

Then you might think that you can trust the ERD for the manufacturer... Nah, that will not do. you MUST measure it. Especially with Ryde, I use an ERD with a difference of 4 mm from the value they publish, that's a lot !

The trick is to measure the inside rim diameter and then measure what is missing from the inside of the rim to the slotted part of the nipple. For that, use a 12 mm standard nipple and mark where the inside of the rim stand. Then, use a caliper to measure to the slotted part of the nipple - where you want your nipple to end. Do it on several rim holes, it is not consistent... Then you store this value.

When you build with a rim with oriented holes, you then just need to measure the inside diameter of the rim and your stored value of the rim.

ERD oriented holes rim = Inside Rim Diameter + rim specific value.

We use 12 mm nipple as a reference for ERD. But for some nipples it will not work. For examples, head driven, 14 and 16 mm DT Swiss, oversized 13G or 12G nipples. With those nipple you need to add or decrease the length of your spokes for a good build.

You will find on the website from Melody Wheel that they measured every nipple they sell and provide how much you need to adjust the spoke length. Useful ! and thanks to them ! Because that's a lot of work and we will see that it is used for other puposes.

If you don't find any values, you will need to measure it yourself with the nipple of your choice and determine where you want the spoke to end. You want it to support the head of the nipple, but not squeeze too much on the thread. You can make your own nipple adjusted rod to measure the ERD directly (If you use the Roger Musson method for ERD measurement) with different nipples.

In doubt, I prefer too long spokes and hurt the thread a bit, than too short. The nipple head might break if unsupported, especially with aluminum nipples. The thread can have a bit of damage without hurting the build, in fact it will lock the nipple on the spoke, which is a positive thing.

You also could make your own nipple adjusted rod to measure the ERD directly.

I found the nipple adjusted rod method useful for special cases like oversized nipple on oversized rim holes, where a 12 mm standard nipple does not make any sense anyway. But, this ERD is specific to the special nipple you use for this build. You cannot compare it to the manufacturer value or use it in other build with different nipples. So I only use it in those cases. I prefere to use a standard ERD for "classic" build and adjust the spoke lenght latter depending on the nipple I use.

This reflects my generalist view on wheelbuiding. I build and repair all kinds of wheels with all kinds of nipples, so I need a robust and repetitive ERD for my spoke length. It would be totally different in a production series, for example.

Now, that's a tricky subject...

When building small wheel like 20" for cargo bike, you might use oversized spokes like Sapim Strong or E-strong with a rim oriented rim like the Andra series from Ryde. You can then cross the spokes 2 or even 3 times depending on the diameter of the hub. You use Sapim Polyax Nipple or DT Squorx with their Pro Head and you can go up to an angle of 25° at the rim spoke interface. Nice.

But, spoke crosses will bend the spokes a lot because of their diameter and their short lenght. It will change the spokes length needed to reach where you wanted, you might end with too short spokes ! You'r ERD is meant for straight spokes, not bend one. On a cargo, you don't really want too short spokes and break the head of the nipple or too few thread.

Why is this subject tricky ? Because it depends... It is not always a problem, it is difficult to just say : add 1 mm and that's it. When I build a cargo wheel, it is very often a rebuild from a failed industrial cargo wheel. The hub and the rim are still OK and I reuse them for a new perfect build. So every time it is different.

But anyway, here is a trick that might help you :

Cross spokes, but without interlacing. Spokes will be straight. Without a bend, your spoke will not be too short. It will also be more comfortable to build, as you don't need to bend them by hand...

Interlacing is useful when you brake a spoke, it does not jump around. And maybe for stability and to share the spoke effort, but not everybody agree. Anyway, with very short, oversized and stiff spokes, I am not sure if it is really relevant.

Their is another post about building small wheels. Read it if you want to have fun building those wheels, instead of tears...