Jarrett2

I've been doing wheel truing for friends in the last year or two. Since I've started, I've seen this on a few wheels now. One spoke on the NDS will seemingly suddenly become completely loose and cause the wheel to become out of true.

What's the cause on this?

dabac

Usually b/c it was lower tensioned than the others to start with. Load changes when riding then caused it to become slack enough to unscrew.

Or riding - hitting a pothole etc - causing a flat spot in the rim. Will usually influence more than one spoke though.

fietsbob

Lack of regular service checking your wheels.

Bike Gremlin

Well laced and tensioned wheels, with good quality spokes and nipples, shouldn't have the spokes come loose.

When building wheels, make sure the spokes are not left twisted (so they unscrew later), that they are shaped to fit the hub at the hub flange and stress relieved. Also, spokes should be laced to proper tension - they shouldn't be too loose when the wheel is built.

If this is all done properly, it is uncommon for spokes to go slack and loose. But of course, it is wise to check tension from time to time, especially in the first few weeks after building the wheel, and fix if there's any problems.

Broctoon

Take a close look at the hub flange where that spoke attaches. If it has cracked, this will cause the spoke to loosen, and that hub will be unsafe and unrepairable.

FBinNY 

Over the last decades there's been an overemphasis on tension, but that slightly misses the key point, which (IMO) is spoke elongation.

Tension and elongation are related, with that relationship defined by the gauge. Obviously you want high tension, but if the spoke is too strong (ie. 2mm plain gauge) you can't get decent elongation except at very high tensions. That means you can't easily get high enough tension and elongation on the left side without having overly high tension on the right.

So, if you build with plain gauge spokes, especially if you also have a fairly light rim capping the tension, you're going to have issues with left spokes loosening. My solution is to try to avoid this in the first place. Other options include using a stiff grease on the threads which helps prevent unscrewing. If dealing with a finished wheel, I use a adhesive between the nipple and rim. I never use an adhesive between the spoke and nipple because it makes problems for future service.

Bike Gremlin

+1 good point on that - butted spokes will make it a bit easier to get them all to decent tension so they work within their elastic elongation range, not coming slack, even on the non-drive side.

IMO, using thread locker to stop the nipples from unscrewing is like using hair dryer on a ship that's got a breach in it's hull - any spoke coming loose during wheel use is a weak spot, leaving other spokes more stressed and having them become fatigued and break sooner. The better solution is using spokes and rim combination that allow for enough tension so all the spokes work within their elastic deformation range.