Like any other emerging technology, electric bicycles have their rough edges to iron out. With hub motors, one of the issues that might occur is wheel strength. Namely, my rear wheel came with the standard spokes you find on most bicycles, only shorter because the motor makes the hub wide. In my case, the motor weighs about 6-7kg, so the problem with using such spokes is that the motor gives them a heavy beating and they often snap. Although the motor represents a very small part of total weight (6kg, vs. 110kg bicycle + driver), this weight is directly hung on the spokes and there is very little amortisation. If you’ve ever chopped wood with an axe and the axe gets stuck in the log you’re chopping, you know that if you turn the axe upside down, the sheer mass of the log splits the log when the axe lands. That is (more or less) the effect the electric motor has on the spokes.
After going through 3-4 sets of standard spokes, I have decided to try something a bit more robust: 3.5mm motorcycle spokes. The problem with much thicker spokes is that none of the existing holes are big enough for the spokes, so I was quite worried that I would damage the motor or wheel rim by widening the holes. Since I had no “Plan B” for my failing spokes, I gave it a shot anyway: here is how it went…
First of all, I took the motor off of the bicycle. This did not go as planned. To take it off, its electrical cord has to be disconnected as well. Because I drive in all weather, the connectors are exposed to water, dust, snow, ice, salt, vibration and, of course, electrical current. My connector crumbled in my hands as I disconnected power…ah well, at least I could now take the wheel off of the bicycle. I replaced the connector with a different one and it seems to do the job well.
Back to the subject, I discovered that my very (cheap and) simple carpenter’s table would serve me very well as a platform to work on. It holds the wheel in place without allowing it to rotate or lean sideways. Since many of my old spokes snapped when I tried to unscrew them, I simply took my pliers and cut the remaining spokes in half to speed up their removal. If your pliers are not that good, it takes quite a bit of force, by the way. The important thing to note here is that it is best to keep a few spoke clusters in place at various sides of the wheel (see picture below) because it is much, much easier to work with the wheel when the hub is fixed than when starting from scratch.
Broadly speaking, it is best to remove half of the old 4-spoke clusters from various sides of the wheel, use a drill to widen the holes on the wheel rim and the hub rim, put new spokes in place and repeat the whole cycle with the remaining spokes. These are a few tips and tricks that might prove useful:
- remove clusters of 4 spokes from various sides of the wheel, never removing a cluster whose neighbouring clusters are not in place
- drill the inner rim hole in the direction of the spoke, not towards the wheel centre
- use a low-speed electric screwdriver to screw the nipples onto the spokes to speed things up, but perform the final tightening using a manual spoke key
- spokes are tightened not once and for all, but a little bit at a time one after the other, so each spoke is revisited several times
- put a new, hard protection stripe between the rim and the inner tube to make sure that the sharp edges of the rim do not puncture the inner tube
- if your spokes came with the nipples screwed on, you can save 10 minutes of work by using an electric screwdriver to remove all the nipples before you start using them
There is one more recommendation that I thought of too late, but which I consider very useful: consider alternating spoke head orientation on the hub rim. All my spoke heads are pointed outwards (as they were originally) and this seems to have been a bad idea: the spokes are brushing against each other so the wheel creaks as it rolls. (Update: the problem got so bad that I spent another 2 hours to correct this mistake. Looks much more promising, but I have yet to take it for a proper test ride…)
For anyone considering doing something similar, here are a few numbers and facts:
- 26″ double-walled rim with a very high ridge
- Crystalyte V48 hub motor
- spokes: 2mm old spokes, 3.5 mm new spokes (180 mm length)
- drill bit diameter: 10mm (12mm?) for the outer rim, 6mm for the inner rim and 4mm for the motor rim
- new high-current connector: XT60, ordered on-line
All in all, I almost couldn’t be happier with the result: the entire procedure took less than
3 5 hours, the wheel looks much more robust now and while there’s a chance that it might cause the rim to fail sooner, the standard spokes simply didn’t work and I am able to drive with the new ones reasonably carefree: I don’t believe I will have to worry about this problem again for years to come.