The funny thing is that when buying a new rim, with just a little search luck, there should be a rim specification available with rim ERD exactly stated. But, should you trust it as an input for a spoke calculator to define the correct spoke length? Thinking critically, how do they even come up with that number in the first place? Does a rim have only one, true rim ERD?
Obviously the answer to the last question is a no-no. But I thought it would be interesting to experiment a little and find out how different spoke nipples affect rim ERD and how does that correlate to the final spoke length. So, one rim, several different spoke nipples. What would be the rim ERD outcome? And even more, how does factory rim ERD compare to my readings? Read on in this short article.
For the purpose of this little experiment in measuring rim ERD, I took one of my 38mm deep carbon clincher rims, with factory stated rim ERD of 569mm. This should be our reference point when comparing different measurements.
Then I gathered some pieces of every spoke type nipple that I had in my drawer at the time. All the black nipples are aluminium, yet all differently shaped. Below them there is a DT Swiss aluminium Pro Lock Squorx nipple with Torx base. Let’s name them with numbers, #1 being the top left one and so on. And finally, #5 nipple is a regular 12mm brass nipple.
Just looking at them, it shouldn’t take much time to assume that their different shape obviously defines the ending point of a spoke when a wheel is fully assembled. But to what extent. Will rim ERD actually be that different across different nipples? So, let’s get to juicy details.
HOW TO MEASURE ERD
A big part of determining the correct spoke length is using the correct “Effective Rim Diameter” (ERD). Measuring Effective Rim Diameter can be done with different techniques. In one of my popular articles, Measurements for wheelbuilding - Taking your own measurements with confidence, I cover taking all measurements for wheelbuilding including measuring rim erd. The image below shows the main concept of rim erd.
This conceptual image of the rim erd measurement goes along with the Park Tool's definition, where the ERD is that point in the rim where the end of the spoke sits (given that we have spokes of an ideal length). In other words, near the end of the spoke nipple. It is sensible that in the conquest of building strong and durable wheels, you should be aiming for the best possible thread engagement of your nipple. For more information read my article on Wrong spoke length - Effect of wrong measurements on spoke length calculation to see how important are specific wheel hub and rim measurements including measuring rim ERD.
Sure, in these days rim ERD can be found online, just like other wheelbuilding data. Sometimes these are accurate and sometimes not. I personally advocate taking your own measurements, especially measuring rim ERD. Why?
Sheldon Brown has a fantastic article on Spoke length, where it also shows a photo of cross-sectioned spoke nipples. And these are just different lengths of the same spoke nipple type. What about different spoke nipple shapes? You see where I am going... Different size and nipple types offer different thread engagement. And this is the underlaying idea of this "measuring erd with different spoke nipples" experiment.
MEASURING RIM ERD
On the photo below, there are my tools for measuring rim ERD. When I am measuring ERD on my own, I always take two spokes of the same length, a caliper gauge and a pair of spoke nipples that I will be using for the project itself. More about the process of taking measurements of your rim and measuring rim ERD in one of my articles (Measurements for wheelbuilding - Taking your own measurements with confidence ).
Tip: when measuring rim ERD, you should always use two spoke nipples of a type you will be using later on.
Here is what you should be looking at when measuring rim ERD using this technique. Two spokes on the opposite side of a rim, with spoke nipples winded up on them. Furthermore, J-bend spokes allow you to press them towards each other and take readings of a diameter at the same time using a digital caliper gauge.
So, back to our 5 spoke type nipples, what were my results? Remember, some nipples have longer base (ending) and the tip of a spoke should go a little further in.
First were regular brass 12mm nipples. In general, nipple length shouldn’t affect the rim ERD, it just offers more support for the spoke itself and makes it easier to thread a nipple onto a spoke on rims with a deeper wall. With 47mm on my Mitutoyo caliper gauge, rim ERD would be right at 567mm.
Next to test were aluminium nipples, #1 on the first photo. With somehow deeper back end, they allow spoke to thread a bit further than on a regular type spoke nipple. And result? My readings were 2mm off the first try. Rim ERD should be right around 569mm. Very close to what “factory claims” were.
Tip: when dealing with weird shape spoke nipples, minimise the risk of calculating the wrong spoke length with taking your own measurements of rim ERD.
Ok, let’s continue with the third spoke nipple type (#3) that I tested. These are also black aluminium nipples, but with a square base, allowing a 3.2mm square nipple key to attach from the back of the rim for truing without damaging black coating of a spoke nipple. Looking at them closer, they have a thread to the very end (base) of a nipple, meaning, spoke could very well penetrate through it. But with these nipples, this wasn’t the case. The thing is that the Pillar company makes a cone shaped nipple. This means that the diameter of a nipple at the very final threads is a bit smaller for better grip of a spoke so it doesn’t come loose so easily. Therefore, I couldn’t thread my spoke right to the base of this nipple type, I actually came around 1mm short. My reading? 53.50mm, which is a full 6.5mm larger rim ERD.
And here were my final spoke nipples - DT Swiss aluminium Pro Lock Squorx. Judging from the shape, my results should be closer to the previous, #3, type. But here is the catch. If the Pillar aluminium sqare nipples were cone shaped, not allowing a spoke to penetrate through, these have a plastic inner coating for vibration dampening, however, you can (with moderate force) thread a spoke to the end of a nipple and even past that point. This is just an interesting finding, not affecting any ERD measurement. But, this fact alone explains why my reading was a full 55mm. All in all, 1.5mm larger rim ERD from the previous, similar shaped spoke nipples and almost 8mm longer than regular type nipples.
Ok, let us look at this situation with some common sense. One could very well argue that there is no need for a spoke to go that long past the base of a nipple and inside the rim. That is actually true. With #3 and #4 – (extended shape) - spoke type nipples, a wheel could be just as strong and spokes firm, if spokes were 2 or even 3mm from the very end of a spoke nipple. But, again. This is just an assumption, because there is also another consideration to take into account - a number of contact threads between spoke nipple and the spoke itself… But this should be a subject for future articles, not in a scope of this little experiment.
Before we end, what about washers? On some rims they are actually recommended. These are 0.9mm thick. Of course, there is no doubt they will change the rim ERD.
Tip: Adding washers underneath spoke nipples will make an actual rim ERD larger.
To extend this article even further, we have seen how different spoke nipples affect actual rim ERD. And quite noticeably. But what about final spoke length? Changing only rim ERD, with hub measurements and lacing pattern being a constant, I entered these numbers in SpokeCalc (link) and I came up with the situation below. Compared to rim ERD change when using different spoke nipples, the actual spoke length difference was smaller, around 4mm in total for non-drive side of a wheel. Again, understandably as coefficient of change is not 1.
Ok, with all that in mind let’s return to the original, factory stated rim ERD measurement and try to sum up. As I mentioned, the rim ERD of this carbon clincher was supposed to be 569mm. So, using two 260mm spokes, the difference, shown on my digital caliper gauge, should be 49mm, right? Looking at my readings, this is somewhere in the ballpark of the rim ERD I got with type #1 and type #2. This tells me that most of the time, you are good to go with factory rim ERD measurement, only when using regular type spoke nipples. If you are planning to use some weird shape spoke nipple it just isn’t worth risking taking numbers you find online. Trust your own measurements of components you will be actually using and you should be fine.
NOTES ON RIM ERD MEASUREMENT
In my newly released total wheelbuilding tool, App SpokeCalc, I have also included an option to make notes regarding spoke nipple shape and length for measuring the rim ERD. So, when you are adding a new or editing an existing rim, you can make a record about a spoke nipple that was used in the process of measuring rim ERD. You never know when this little detail will save your day!