southpawboston

I want to replace a rim with another while keeping the hub laced to the existing spokes. The new rim's ERD is 4mm larger. This means the spokes will have 2mm less threads threaded. Will this be a problem? It's for a front wheel, so it won't see as much load as a rear and there won't be any dish issues.

GravelMN

It depends completely on if the spokes are long enough to safely engage the nipple head (not just the nipple threads). If they were long enough that a mm or more stuck out of the head of the nipples on the old wheel, you might have enough to safely reuse them with the slightly larger ERD rim. The end of the spoke should be no shorter than a thread or two below the screwdriver slot in the nipple head. If it is more than a mm below flush with the bottom of the screwdriver slot, you might be risking nipple breakage.

FBinNY 

The image below gives you the answer.




If the spokes are now beyond the height in the right picture, 2mm less may still leave them long enough. If they're already to the short side, you don't have 2mm to give up.

However, that's in theory. The reality is that many built wheels with spokes that are too short (like the photo on the left) and they hold up OK for a long time. A nipple failure is the likely MODE of failure, if for example something gets caught in the spokes, but that's no guaranty that there will be a failure in normal use. I've seen many wheels like this fail in the usual way with spokes breaking at the elbows.

IF you are using butted spokes and keep tensions reasonable, oddds favor you. If you're using 2mm plain gauge spokes and crank up the tension they don't.

southpawboston

Ah, ok, that illustration really drives the concept home, thanks. I'll check to see how far the spoke end is from the nipple head in the current wheel. I may just pony up for new spokes.

hueyhoolihan

the last few spokes on either side may not even reach, but it's impossible for me to be sure until you've given it your all, made multiple attempts, and failed miserably.

i actually hope it works out.

jyl

Wait, will you then have misnatched rims? The horror!

Jiggle

You can use a 1.5mm spoke which will stretch about 1mm under tension and use brass nipples instead of aluminum.

Bill Kapaun

Which entails buying new spokes.
If buying new spokes, why not get the correct length?

FBinNY 

ALL spokes elongate when tensioned. The amount of elongation governed by Young's Modulus or Stress/Strain. As such, for any given spoke length, the elongation is proportional to the tension, and inversely proportional to the square of the diameter. So a 1.5mm spoke will "stretch" slightly twice as much as a 2mm spoke, or about 1/2mm more (based on the prior statement, actual math not checked). Not exactly enough to factor when calculating spoke length.

The brass nipples won't make a difference either. There's no meaningful difference in strength between brass and aluminum nipples IF the spokes reach at least 2-3mm into the heads. 15g brass and aluminum nipples are both slightly stronger than 14g by virtue of the smaller hole, but again not enough to matter when all is right.

SquidPuppet

Serious question FB. Do you mean 2-3 full turns, or 2-3 full threads? I ask because as best as I can measure, the entire head only measures 3mm.