This it the type of project that is only justified if absolutely necessary. Not a good idea to try for anyone without a lot of wheel building experience. My project qualified, and I have built hundreds of wheels over the years when I worked as a bike mechanic, so I set down to figure out how to do it.
The internet was not a help. Some plans for other hub/rim mismatched projects used complex mathematical equations that went way over my head. My searches did not turn up a single person who documented doing this particular combination. There were just a few quotes on forums saying it couldn’t be done. I doubt I am the only one who has done it, but what follows is how I figured it out. This method could be used to figure out other hub/rim mismatched combinations too, since you just makes a drawing and measure the spoke lengths off of it.
The hub is a 28 hole Schmidt SON XS dynamo hub. The locknut to locknut dimension is narrow- 74mm wide instead of the usual 100mm. It is laced to a narrow Sun CR18 20″ (406) 28 hole rim presently. I wanted to use a wider rim.There are lots of wide rims available in the 36 hole format, but none in the 28 hole format. Unfortunately, they do not make a 36 hole version of the SON XS hub. I use it on my folding bike and I absolutely want to keep this narrow hub instead of getting a 100mm version in the 36 hole. It makes the folded size one inch narrower, and for me thats a big deal.
I chose the Sun Ringle big baller, about 33mm wide. It will allow my tire to hold lots more air, and I will be able to run the tire on lower pressures without having it feel squirrelly, and with less risk of pinch flats.
On a large sheet of watercolor paper I mapped things out. The new 36 hole rim was traced on the paper and each spoke hole was marked. The big circle drawn is the ERD (effective rim dimension), which is where the end of the spoke is supposed to be. I drew the 70mm hub flange circle in the middle and made equal spoke hole markings for the 28 spoke hub. Then I marked off every fourth or fifth hole as a place where no spoke would go (on the drawing it is a S with a circle around it). Missing spokes would be equally spaced apart so that wheel strength is unlikely to be an issue. That left me with 28 spoke holes left. Easy enough so far!
Then I drew a line where the first spoke should be. It would be a cross 2 pattern. I looked at my existing wheel (28 spoke cross 2) to start the drawing, and estimated a good spoke angle for the first one. After that, it was quite straightforward to draw the rest of the spokes in.
Skipping spoke holes caused some spokes to be longer and others to be shorter. When I was done, I measured all the lines and came up with 10 different lengths from 168mm to 186mm. Thats all even numbers- if it was odd number, I just rounded it off to even.
Then I added 2mm to the spoke lengths to take into account the hub flange to center of hub factor. I used the wheel building spoke calculator
on the UBI site to get an idea of how much extra length would be needed.
Building this way, some of the spokes would reach a little farther to the opposite side of the rim. Every other rim hole is staggered sideways a little so that the spoke coming from the hub on the same side is a bit closer to the rim. This difference in length is too small to affect the spoke size with this rim, so that issue did not concern me.
In the above picture, you see the wheel laced up with length labels on each spoke. The big white dots on the rim cover the spoke holes where no spoke will go. All this labeling makes the process of building it less confusing. The wheel is round, true, and snugged down so I can see how well I did on predicting the spoke lengths.
About half the spokes were too long. 2 to 4mm too long. I think this was because I was not careful enough about measuring the lines I drew. Now that I knew the exact lengths I needed, I had Sellwood Cycles
(my favorite local bike shop) cut me the spokes I needed to fix this.
|Big Baller rim on left, Sun CR 18 rim on right. The wider rim will support the tire better when at lower pressure, and increase the air volume for a smoother ride.
All finished, tight and true. The rim is very strong (triple box section design) and 28 spokes are more than enough to hold it together. The tire is 3mm wider mounted on this rim compared to the narrow CR18 rim, and holds a greater volume of air. Replacing a rim when the time comes will now be much easier, as there are lots of 36 hole rims on the market and I can find them in most bike shops. The brake track (rim sidewalls) should hold up longer on this sturdy hoop, so I will use the front brake more- without concern of wearing out the rim. The 28 hole rim was always a special order affair to replace, and I was reluctant to brake with the front brake out of concern of wearing the rim out. I have been using this wheel for several years now, maybe 20K miles, and have replaced it twice due to normal wear on the brake sidewall. I tape a new rim next to it, loosen up all the spokes, and transfer the spokes to the new rim.