Is 2mm Too Big A Spoke Length Error?
 

Just a quick wheelbuilding question, please.

My daughter's bike has Normandy high flange hubs and Mavic MA40 rims. 36 hole, 3 cross. I do not recall what length spokes I used, but spoke calculators say 296.3 mm and I always round down, so likely used 296 mm.

I am toying with the idea of lacing in a generator hub. To keep the cost down I went looking for such a hub with dimensions similar to the Normandy. I came up with the Sanyo H27 which also happens to be among the least expensive genhubs.

According to the spoke calculators I checked, rebuilding this wheel with the Sanyo hub requires 294.1 mm spokes.

I "think" that 2 mm is not much difference and there is a very good chance I can re-build the wheel using the Sanyo hub and the same rim and spokes. What do you think - am I being too "optimistic"?

Dimensions per calculators (I haven't personally checked, yet)

Sanyo H27
LFD 66
RFD 66
LFO 28.5
RFO 28.5

Normandy hi-flange
LFD 63
RFD 63
LFO 33.7
RFO 33.7

We don't know what kind of penetration you aimed for. If you shot for bottom of screwdriver flat, 2mm more should be fine. If you shot for free of the nipple, then it's probably sketchy. IMO 2-3mm past the nipple is all you get.
But you can always put a washer between nipple and rim to buy yourself another mm.

Good question. I will pull off the tire and rim strip and see what penetration I got. The washer idea never occurred to me.

It may or may not work.

Aiming for the nipple tops I'd expect to run out of thread, and for the slots it wouldn't surprise me on at least one or two spokes.

DT spokes (9.5mm of thread) bottom in 12mm DT nipples (8mm of thread to the top) 2.5mm above the slot bottom and 1.5mm past the top. which gets you right to the limit aiming for the slot.

Other brands are similar.

You're starting with 2mm of error, could get close to 0.5mm of stretch from butted spokes, and may be even farther off due to how some one else measured the rims.

Measure ERD before you start - put spokes in opposite holes, thread nipples on to your target depth, measure across the elbows with calipers, and add twice the spoke length. Repeat in a few spots and average because rims can be slightly egg-shaped.

If you're close to working or not use nipple washers which increase ERD by 1.5 - 2mm.

The target for where spokes end is between the bottom of the slot and top of the nipple. This bullseye is about 1.5mm wide. However, working range runs from about 1mm below the top of the slot, to wherever the threads tun out at full engagement. This upper limit depends on the specific spoke and nipple combination, some running out very close to the top of the nipple, and others allowing an overrun of 3-4mm or so.

So your answer depends on where the spokes end now, which way the 2mm is, and whether you have the room in that direction.

BTW- if you're going longer, and the nipples don't allow, other nipples might. Or you can extend the possible depth using a 2mm reamer to remove a few threads at the small end.