Sturmey 90mm
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Risto
Sturmey 90mm
I’ve just bought front and rear 90mm drum brake Sturmey hubs (the ones with substantially larger flanges on one side than on the other). The chap who sold them to me suggested that you can use spoke lengths of the same size if you have a different cross pattern on each side of the hub. This sounded jolly convenient so I’ve just calculated spoke lengths using a x2 on the smaller flange and x3 on the larger. There’s a 2mm difference in length on the rear hub and 3mm on the front. Are these differences anything to worry about? Does having different cross patterns on the same hub lead to any problems?
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CJ
Re:Sturmey 90mm
Up to 3mm difference can be accommodated - just - but you need to be absolutely precise with your calculations in order to be sure of picking the best compromise length. And choose a good brand of spokes and nipples, since it's likely that half of the spokes will not extend fully into the nipple head, which may accordingly snap off if the nipples are of poor qaulity.
Fewer crossings is not merely permissible but recommended on larger flanges. Otherwise the spoke enters the rim at a shallower angle than the nipple can tilt to accommodate, so the nipple becomes harder to tighten and there's a bend in the spoke where it enters the nipple. Neither of these are terminal problems, but everything goes together a bit easier if you avoid them.
Too few crossings, on the other hand, increases the outward force on the hub flange around each spoke hole. Some hubs are designed to take it, some aren't. Hubs with extra-large flanges usually are, and you should be okay with cross 2 on this one.
Fewer crossings also increase the stress on spokes due to hub torque - from pedalling or a hub-mounted brake. Cross 2 should be enough though.
Fewer crossings is not merely permissible but recommended on larger flanges. Otherwise the spoke enters the rim at a shallower angle than the nipple can tilt to accommodate, so the nipple becomes harder to tighten and there's a bend in the spoke where it enters the nipple. Neither of these are terminal problems, but everything goes together a bit easier if you avoid them.
Too few crossings, on the other hand, increases the outward force on the hub flange around each spoke hole. Some hubs are designed to take it, some aren't. Hubs with extra-large flanges usually are, and you should be okay with cross 2 on this one.
Fewer crossings also increase the stress on spokes due to hub torque - from pedalling or a hub-mounted brake. Cross 2 should be enough though.
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Risto
Re:Sturmey 90mm
CJ - many thanks. The only thing is that I think I'm suggesting the opposite of what you recommend. In the quest for equal spoke lengths I would be having more crosses on the larger flange than on the smaller. It sounds like I should give up this silly idea unless the angle at which the x3 spokes from the larger flange enter the rim at a reasonable angle. Is there any way of telling before building the wheel?
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CJ
Re:Sturmey 90mm
But of course! How else could you use the same length spoke?
For a graphical guide to spoke angle (etc.) see my spoke lengths page on the CTC website.
For a graphical guide to spoke angle (etc.) see my spoke lengths page on the CTC website.
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Risto
Re:Sturmey 90mm
Thanks again Chris. Your spoke length graphs are real gems. What a splendid club we have!
I've abandoned the idea of having equal spoke lengths but, on your advice that a 3mm spoke length difference is just about tolerable, I think I can get away with two spoke lengths for the whole two wheel build. For a compromise spoke length, I assume that one should err towards the shorter length, to make sure that the spoke head does not poke too far out of the nipple on the shorter run (the rims are double thingied, so a small amount of poke-through is presumably acceptable?) . Have you got any suggestions on a sensible compromise?
I've abandoned the idea of having equal spoke lengths but, on your advice that a 3mm spoke length difference is just about tolerable, I think I can get away with two spoke lengths for the whole two wheel build. For a compromise spoke length, I assume that one should err towards the shorter length, to make sure that the spoke head does not poke too far out of the nipple on the shorter run (the rims are double thingied, so a small amount of poke-through is presumably acceptable?) . Have you got any suggestions on a sensible compromise?