The reason for wrong spoke length
With some real-life cases, this article shows why one should not easily trust online wheel building dimensions.
Believe it or not, this article topic has been on my list for a long time now. Have you done some wheel projects and spoke calculations, I am sure you have already encountered this concern. “Garbage in, garbage out.” I still remember this opening statement of an old, but still popular blog article, Measurements for wheel building - Taking your own measurements with confidence. It is that simple, expecting correct results (output) from poor-quality inputs (in that case wheel building dimensions) is ludacris. Read here also another article: Wrong spoke length - Effect of wrong measurements on spoke length calculation.
Wrong spoke length is in the most cases the result of poor-quality measurements, which can be obtained first hand by using a variety of measurement tools and wrongly getting the data on your own, or as the leading theme of this article suggests, simply easily following publicly accessible, mostly online based components’ specifications.
Are you one of them? If yes, can you be blamed? Hmm, no. But nevertheless, read further to learn how to avoid making some common mistakes using online data.
The quality of online data
So, does that mean that all online wheel building measurements are of crap quality?
Hahaha, no! I am not here to judge the quality of online measurements or other databases. That said, plenty of spoke calculators even include such data as they make the process easier. But still, probably the most common problem is that most databases over time, obviously, become obsolete. Not refreshing, updating and validating the data leads to discrepancies and incorrect specifications, which users still use after some time.
With launching of the App SpokeCalc, which is inclined towards creating databases to organize a wheel builder’s life, I thought multiple times of creating a public database of components, but then again, I almost always ended my thoughts with the problem stated above. Who would take the responsibility for the correct data? Put it on manufacturers or simply on a self-regulating wheel building community were one of my ideas, but let’s leave that aside for the purpose of this article.
The next thing is terminology. Although 99.9% of measurements are widely accepted wheel building terms with clear and meticulous definitions, some manufacturers still use confusing or misleading designations of some measurements. Terms like effective rim diameter, pitch circle diameter, spoke offset or flange to flange should be clear and uniform and yet you find values that are different than reality.
Cases of confusing dimensions
When talking about confusing wheel building measurements almost always there is one or another problem with the infamous ERD. Although effective rim diameter has a clear definition as Park Tool’s article suggests, it is a relative term. Some time ago, I explained this concept in the article Measuring rim ERD - How do different spoke nipples affect rim ERD measurement, using different types of spoke nipples, one can easily manipulate this dimension. To avoid that, manufacturers usually use standard nipples to get such measurements. But still, you can more often find ERD confused with the nipple seat diameter. On the other hand, this measurement is the inner diameter of the rim where nipple heads lie on the rim. Certainly, a more precise and objective measurement.
As it happens, I recently used an online ERD measurement from a rim manufacturer’s page. There it said, ERD: 580.9 mm. Ok, in a hurry I accepted the value and ordered spokes based on the spoke calculation with this measurement before getting the rim, and only later found out they were at least 2 mm on the short side. Once I got the rim it turned out that the 581 mm was actually the nipple seat diameter instead of the effective rim diameter.
Similar confusions happen also with wheel hubs, and to make things worse, especially the straight pull hub types. Such was my last wheel project where I was measuring the straight pull hub pair. Very early I noticed some inconsistencies in the online terminology with PCD dimension. Take a look at the next image, where it says that the PCD is 31.5 mm and the default lacing pattern for this hub is radial.
Not having to search harder, the next google result gave me the following image. Looking at the bottom technical drawing of the same hub, it is clear that this false PCD measurement is the outside diameter of a hub shell at spoke holes. Measuring myself, I got the correct pitch circle diameter which was more like 26 mm.
The next case would be the deriving flanges to center dimension following the flange to flange and the offset dimensions. Having these two dimensions and you can fairly simply get what you need for the spoke calculator. Again, among the top ranked google search results for the rear hub model, I found that Flange to flange is 67.8 mm. Looking further and the technical drawing below clearly stated that this measurement was outside-to-outside of flange ears dimension.
This isn’t very problematic if flange ears were symmetrical. But having to encountered that issue before, the non-drive side flange is usually slightly narrower and therefore you cannot simply predict the centreline. Again, having also the offset would not give as the accurate assumption what left/right flange to centre dimensions are. Luckily, these two measurements aren’t super crucial for final spoke calculation as the article Wrong spoke length - Effect of wrong measurements on spoke length calculation., suggests.
Having mentioned problems that arise from discrepancies, inconsistencies or even loosely stated online data, maybe even more common problem lies in the missing information for the wheel projects. This is the case for straight pull hubs, where usually one must turn half of the internet around to find some basic spoke offset values.
Good practices
So, after all this fuss about the online data, one can get intimidated even in searching for correct measurements better yet using them.
From my experience, I tend to rely mainly on my own measurements. Even with some stupid mistakes, which happen from time to time happen, I am the only one to blame. So, after selecting components for the wheel building project the first thing is obtaining hubs and rims to actually measure them. In that way I get inputs for the spoke calculator. Then and only then I start purchasing spokes of desired length.
But even during the above-described measuring process I still google for component’s dimensions and specifications to double check or refine (if very precise) my own measurements. That said, I just love technical drawings of hubs or rims. Simply bullet points with wheel building measurements on a web page aren’t enough, but if dimensions are clearly stated on technical drawings, such documents are of higher validity.
Try to combine different measuring methods and double check if you can with online measurements. If relying heavily on the online data, at least repetitively use trustworthy, valid pages.
Final thoughts
The purpose of this article, clearly, isn’t to completely criticise online databases of components’ measurements. But before copy pasting the data, one should double check with several resources or, better yet, take measurements on their own. Mistakes are not only costly but they take the precious time of rebuilding wheels and reordering spokes. Remember, the preparation is of 80% of the whole process, so take time and question every spec, you are not getting on your own!
Enjoy wheel building!
About author
Aljaž Trenta
Author and founder"As a cycling enthusiast, bike mechanic and self-taught web designer, combining several of my passions and skills to build SpokeCalc was a great fun for me."