Palmer

I hope that someone can help me figure this out. I'm building a wheel for a touring bike. Velo Orange rear hub, Velocity Dyad rim, 36 spoke, x3, DT Swiss spokes and nipples. I laced the wheel whithout any problems. However, when I began to tension the spokes, I noticed that one spoke was significantly too long. So long, in fact, that the nipple bottomed out on the spoke threads before there was any measurable tension on the spoke. Visually, everything looks fine. All of the other spokes/nipples are about where I would expect them to be, with adequate threading on the spoke for tensioning and truing, including the spokes adjacent to the problem spoke. The rim looks fine, both internally (to the extent that I can tell by peeping through the spoke hole with a flashlight) and externally. I have tried different spokes and nipples, but the result is the same.
The only thing that I can figure is that the spoke bed is somehow thinner at this one spot than anywhere else. It doesn't look thinner, and I can't think of an accurate method to measure.
Any ideas as to what I'm missing? If I can't find anything amiss with the rim, would it be safe to use a shorter spoke for this one spot? I hesitate to do that, but I really can't think of a reason why the rim would be more likely to fail if one spoke was shorter.
Thanks for your ideas.

Andrew R Stewart 

My first thought is, could that spoke be longer than the rest? Using a depth gage (a spoke held by your finger and thumb will work) can you confirm the spoke bed thickness where the "long" spoke is and adjacent spoke holes?

A common miss lacing has two spokes not alternating rim holes, side to side. This results in one spoke seemingly too long and one too short. Another common mistake is to have one flange's spokes not offset, WRT the other flange's spokes, at the hub by one hole. But this usually has half the spokes too long and half too short.

I see no issue with subbing a spoke of the proper length (and this "proper" assumes that the lacing is otherwise done correctly) in place of the "too long" one. (And this is a good learning point to suggest always getting a couple of extra spokes/nipples when building a wheel, stuff happens after all)

If you can't figure it out and have a relationship with a LBS that does wheelbuilding (hopefully where you got the spokes from) show the laced but with one too long spoke to them. This stuff can sometimes be "can't see the tree for the forest" kind of thing. Andy

oldbobcat

What Andy said.

Plus my two cents. Whatever it is, it's something that can be seen or measured if you look closely enough. Keep looking. You have a better chance of finding it than we do.

FBinNY 

Agree with above, but will add.

There is absolutely no way that a differerence in rim depth can account for that much error without being obvious as a dent.

I gather you pulled and checked spoke length, so odds favor an error in spoke sequence.

Time be for careful checking of lace pattern.

Start at hub.
Alternating inside/outside on both of flanges?
Alternating pulling/trailing on each flange?
Now the rim -. Look carefully here
Each group, ie. R/L, pulling/trailing goes to every 4 holes.

Also double check at the crosses. Not lacing over/under adds to spoke length.

Lastly, there's a common mistake, but it wouldn't account for only one spoke, so I'll go there later only if necessary.

3alarmer 

.
...sometimes a quick way to spot an error in lacing is to look at the crossings. If one of them is in a different elevation than the others on your wheel, something is off.

3alarmer 

.
...also, in a wheel with dish (you did not mention if this is the case), it's not unheard of to mistakenly swap one of the longer side spokes with a shorter one from the drive side set. I have done this a couple of times, until I started using a black Sharpie to mark the spokes for one side or the other, prior to lacing the wheel.

Bill Kapaun

What gage spoke and which side?
IF you're using a 1.5mm spoke on the DS, you may have simply stretched it beyond the yield point.
ELSE-
IF you're sure everything is laced properly, I'd just remove that spoke and one other 180 degrees opposite from the same side and compare to make sure you didn't accidentally get a wrong length spoke.
It takes less time than answering a 1/2 dozen posts here.

CliffordK

Pull the long spoke and compare the length to those next to it.

If this is a single walled rim, you can carefully file down long spokes. A double wall rim is harder to do that.

However, that won't help if you run out of threads.

I assume you have a truing stand and are truing the wheel as you tighten the spokes.