Saturday, 5 March 2016

Building an unusual wheel

The bike before (above) and after wheel rebuild.
Sheldon Brown said that building a bicycle wheel is the ultimate test of any bicycle mechanic's masterhood. If that's true then building a wheel with different number of holes in the rim and in the hub will make you Dalai Lama of bicycle mechanics. That is the main reason why I decided to build a real wheel using 24-hole hub, 24 spokes, 32-hole rim and different cross paterns on the drive and non-drive side. See some intoductory details on my ultralight page: (look for "Cracked Easton rim"). Other reason was that it's hard to get spare road rims at reasonable price. I guess that time is in shoratage nowdays, so nobody bothers with rebuilding a wheel and buys new wheelset instead. But not me! I have plenty of time and I can't imagine any better way of using it than making a wheel. The more unusual it is, the better.

So what we have here is an old rear wheel with 24 spokes (minus one broken spoke) and cracked rim that we want to replace. We also have a new rim with 32 holes. How do we go about rebuilding the wheel with a new 32-hole rim and old 24-hole hub and old 24 pokes?

The old rim has an effective rim diameter (ERD) 595.2 mm, and the new one has ERD 591.2 mm. Old wheel has radial lacing on the non-drive side of the hub (NDS) with 276 mm spokes and 2-cross lacing on the drive side (DS) with 286 mm spokes.

First, we have to choose the lacing pattern so that old spokes will fit in the new rim. There are a number of spoke length calculators, but probably none that allows for different number of holes in the hub and the rim. Using the spoke calculator with ERD of the new rim, the input of 32 spokes and different lacing patterns, I got the closest spoke lengths with 1-cross pattern on NDS (277 mm spokes) and 3-cross pattern on DS (288 mm spokes). However, with the input of 24 spokes the spoke calculator shows that I could use original lacing pattern: radial on NDS (275 mm spokes) and 1-cross pattern on DS (285 mm spokes). 32 hole rim has a hole every 11.25 degres, and 24-hole rim every 15 degrees. The maximum difference is 3.75 degres. This is not a big difference, so the ideal spoke length will be a milimeter or two shorter or longer then calculated length.

New 32-hole rim taped to the old 24-hole wheel.
So, despite all this complicated introduction, the procedure that I used was rather simple. I taped the 32-rim to the old wheel so that the two spoke holes to the left of the valve holes of the both rims were allinged. Then, going from this hole in a clockwise direction, I moved one spoke after another from the old rim to the nearest spoke hole of the new rim. We can calculate that, with such procedure, every fourth hole in the new rim, counting from the first alligned hole, will be empty. In total 8 holes. I covered these holes with a little patch of duct tape, as a protection from dust and water. Then I used the usual procedure to true the wheel.

Every 4th hole is without spoke.
The longest segment between two spokes of the new wheel has 22.5 degrees, opposed to the 15 degrees in the old wheel. As the new rim has considerably bigger wall thickness, I am convinced that this will not be a problem. Besides, the new rim miracuously happened to be 50 g lighter than the old one.

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