Asymmetric rim design and spoke offset bed
I covered asymmetric rim design and offset spoke bed (OSB) measurement in one of the most popular articles on my blog – Measurements for wheel building, Taking your own measurements with confidence. To understand the concept, I then wrote “The main logic behind these rims is that they tend to reduce your wheel’s dish, making the difference between left and right spoke bracing angles a bit subtler. This should result in a more even tension between drive and non-drive side of the wheel.« All in all, asymmetrically offset spoke holes help equalize the spoke tension of a wheel build, making it strong and more durable. The OSB concept is clearly shown in the cross-section profile drawing below.
Note: you can read more about different rim designs and their effects on the spoke length in the following article: Rim designs, How different rim designs affect spoke length.
It was a great opportunity for some new blog content as I had one such rim at hand for my next wheel building project. Specifically, it was the carbon mountain rim WTB CZR i30 with reinforced spoke holes and an asymmetrical design. Built to endure all the beating up, this durable trail/enduro 29” rim featured a factory asymmetrical spoke offset of 5 mm which is significant enough to give us identifiable difference in spoke length.
Also, because of altered bracing angles of the spokes on each side, the spoke tension should change in favour of longer-spokes side of this wheelset. That said, the cross-section profile of this rim is shown on the drawing below.
So, the main question was how would the spoke length vary compared to a conventional rim with spoke holes positioned in the centre of the rim. Put differently, how in this specific case, the OSB of 5 mm affects the spoke length and spoke tension ratio.
OSB and the spoke length
In my other article – Wrong spoke length, Effect of wrong measurements on spoke length calculation – I dived into researching the degree of different measurements, including a spoke offset bed on the final spoke length calculation. With the coefficient being somewhere between 0.06 and 0.12 mm, it meant that for every millimetre of spoke offset, the spoke length would change by that much. Altogether, this isn’t significant by itself, but in case of rims with greater OSB, like the article concluded, “getting couple of those measurements wrong and errors will sum up to a greater overall error. «
Taking this knowledge into consideration let’s now switch to our actual data of this wheelset and see how significant effect does the OSB measurement have on the final spoke length calculation. Below are the screenshots of front/rear wheel data and spoke calculations.
As we can see from the results below, we are in the ballpark of the findings of the previously mentioned article (K = 0.06 / 0.12). Specifically, by adding a 5 mm OSB to our calculations, the spoke length of the non-drive side of the front wheel is longer by approximately 0.5 mm, while drive side spokes become a bit shorter (0.6 mm).
Similar are the findings for the rear wheel. Longer side spokes, which are in this case positioned on the non-drive side of the wheel, are shortened by almost 0.6 mm, while drive side spokes calculation shows approximately 0.5 mm increase in spoke length, having a 5 mm OSB of course.
Again, the difference of the spoke length is not really significant, but… In cases when you can only order odd or even spoke lengths, disregarding the offset spoke bed measurement can trick you in rounding in the wrong direction.
Asymmetric rim design and spoke tension
As said above, the spoke offset also affects spoke tension ratio. So, by how much does the spoke tension between drive and non-drive side balances out (equalizes), having a rim with such significant spoke offset?
If the findings implied that the difference in spoke length isn’t really notable, the difference in spoke tension ratio between drive and non-drive side of spokes is dramatic. As we can see from the screenshots, with such an asymmetric rim design, we can improve spoke tension of the front wheel’s drive side spokes from 70 % up to 96 % of max rim tension.
Similarly in case of the rear wheel, except here we are improving the tension ratio in favour of the non-drive side spokes. Having bracing angles more equalized, spoke tension of the non-drive side spokes changes from the original 63 % up to 90 % of maximum rim tension.
As the real case data from this article suggest, rim design matters. Such was also the premise of the Cannondale’s asymmetric integration concept, that includes this knowledge, but alters the whole geometry of the rear wheel. As shown above, by choosing an asymmetric rim design, you won’t get significantly different spoke lengths, however it will notably improve the tension ratio of the wheel making it more durable and also allowing you to put less tension on spokes.
Happy wheel building!