spoke length selection; how to make life simpler?

[Brucey]

These days most bike shops that service everyday utility bikes don't end up building a lot of wheels; it simply isn't worth it. Many bread and butter bikes can have a cheap, terrible, factory-built wheel replaced with a new cheap, terrible, factory-built wheel for a little more than (say) the retail cost of a half-decent rim for a touring bike.

The exceptions are of course 'value hub' wheels; IGH wheels, wheels with brake hubs, generator hubs etc. There is a lot of value in the hub and many of them have steel flanges, so can easily turn into a spoke-breakage disaster area. Many such wheels need respoking, or get trashed rims and need rebuilding. If it is to be done at reasonable cost, it is best to re-use the hub.

These hubs often suffer badly. In the death throes of the old build, the remaining spokes in rear wheels are often loose enough that they are moving around in the hub. Steel flanges often end up with egg-shaped holes in them. I have even seen hubs where steel flange material has been deformed as if it were butter on a knife; jagged burrs ~1.5mm long can be formed in this way, which is something that you wouldn't believe unless you had seen it.

One of the bike shops near to me does a fair amount of 'value hub' rebuilds and when it is a well-used bike that is worth about £100 (tops) it is a hard sell to expect a repair bill of more than about 1/3rd of that to be acceptable. So the recipe is to do the rebuild quickly using inexpensive parts where possible. Inexpensive plated 13G spokes are the order of the day for rear wheels; a wheel built with these will usually last a couple of years of the worst 'utility hell' with just one or two breakages (that don't normally render the bike unrideable if it is a brake hub) and that is if you are unlucky. A wheel's worth of these spokes costs peanuts if they are bought in bulk. Similarly inexpensive 14G spokes don't seem to last, mainly because the flanges are thin, spoke washers are not used, the spoke holes are kind of jaggy, and stress-relief is an expensive luxury. Non-eyelet rims are drilled out for full size 13G nipples but often thin-walled 13G nipples (same OD as 14G) are used instead.

Building almost undished wheels with single-walled rims and 13G spokes is a bit weird if you are used to double-walled rims and stretchy DB spokes instead; I've only ever done a few like this, but basically you can bully a quite badly misshaped rim into position using the spoke key alone. 13G spokes are about twice as stiff as a lightweight DB spokes and this is a good part of the reason. If you overdo the tension the rim will crack, and if it isn't enough the nipples will back out. [I have often wondered if the angled nipples help here; if they are cocked in the rim, maybe they are less likely to back out...?]

Now, preamble done with, getting (eventually) to the matter in hand; the weird thing is that whenever I've been into this particular LBS there are relatively few packets of spokes open, yet several such wheels are built every week. They say there is no point in getting older if you don't get any wiser, and the guy that runs the place is a good case in point. Nearly all the wheels he deals with are 36h 700C rims and realistically, there are probably only about four ERDs to contend with.

But surely the huge variety of hub flange sizes/spacings on IGHs requires a plethora of spoke lengths? Well, it turns out not really, not if you are cunning. The trick is to use x4 spoking. This isn't quite tangent on 36h but this still works out almost perfectly so that flange size variation doesn't change the spoke length much. Basically a 30mm change in flange size alters the required spoke length by less than 1mm. And once you have long spokes like that, changes in flange spacing become less significant too; another 10mm of flange spacing? No problem; it is only about 1mm on spoke length even if it is all one side. Something like an SA rear hub (dyno or brake) with different flange sizes and a fair amount of dish ends up using the same length within 0.5mm each side, if you build it x4.

Now, elsewhere, it is popular to build large-flanged hubs x2 these days, the argument being that the reduced angle of the nipple reduces the chances of spoke breakage at the nipple end. That may well be so, but these 13G spokes are always going to be breaking at the other end, if they are going to be breaking anywhere; failures at the nipple end in such wheels are relatively uncommon. So, if I was building a single wheel I might do it differently, but if I had to do that kind of job with different hubs on a regular basis, as quickly and cheaply as possible, I might just build them x4 for the sake of an easy life.

cheers

[barrym]

Moulton do this certainly on their B-o-A built bikes to rationalise spoke stocks. No other reason. Didn't understand the implications when it was explained to me at a factory visit. Interesting post.