Mystery broken spoke!
 

Unfortunately after thousands of miles of all different types of riding, on all sorts of different bikes, I broke a spoke. I guess I shouldn't be that upset considering it was my first, but this one was a bit strange so I'd like other mechanics' thoughts on it. (I work at a shop as a mechanic)

Broken spoke in question was a rear NDS Sapim CX-ray. The wheel is a Powertap G3 to a Pacenti SL23, 24h, 2x both sides, all CX-Rays. I've built many wheels and the last few years have been racing/training on wheels that I've built. Last year I trained/raced on a set of A23's, same hubs, but Sapim Lasers (essentially a non-aero CX-Ray). I rode the crap out of those and crashed them twice. The rims bent slightly but even still I trained on them all year and into the Fall with no issues.

The wheels I'm riding now were just built about a month ago and only have a few hundred miles on them. The rim is arguably nicer than the A23 (welded seem, a bit deeper/wider profile) and overall I was very pleased with the build.

This brings me to today, where after about 30 miles of riding out of nowhere I brake a spoke. I didn't hit anything and wasn't hammering on them. Also worth noting is that I weight 140-145 lbs, which is well within the weight limit of a 24h rear wheel. Also, the spoke broke right at the bend.

Like I mentioned, I've built dozens of wheels and all of the wheels I ride now I've personally built. The spoke lengths are all right on and the tension/true to my knowledge was right on.
My only guess is that there was some uneven spoke tension which put too much stress on that one spoke. Maybe it was too little tension, but I know these wheels are sitting at the correct tension.

Any help or suggestions at all would be very appreciated! I replaced the spoke already and things seem fine. I'm hoping it was just a one time freak thing and won't happen again.

Nice sub-text.

It could have been a bad spoke.

Breaking at the J bend is most common.

I never used to do it, but everyone suggest pre-stressing the spokes. Did you do it?

I'd just replace, and see how you do from now on.

Yeah I always pre-stress.

First of I'll venture that it was a elbow in spoke (50/50 odds on that so no genius involved). IME, of the 4 types, elbow in/out & left and right flange, I've found the elbow in left side spoke the most likely to break.

We all have our pet theories about spoke breakage, but when it happens this soon, I prefer to call it a fluke and let what happens later drive my process. If no other spokes brak for a long while it truly was a fluke. OTOH, if more spokes break and.or the interval gets progressively shorter, then I start searching for possible answers.

Thank you, this seems spot on. The spoke was elbow in. Hm. I'm hoping like you said it was just a fluke.

The only way to tell is to keep riding it until the next spoke breaks. There is no conclusive way to predict that. If you get a second failure fairly soon, you can safely predict that the interval to the next break will be shorter yet. So you're options then will be to either retension the entire wheel with the existing spokes, or rebuild.

You could do those now, but may not need to.

The biggest challenge that a wheel builder has is centering the rear rim while getting enough tension in the non-drive spokes without having too much tension in the drive side spokes. Jobst Brandt in his book "The Bicycle Wheel" says that the most common spoke failure is in the rear NDS. Low tension causes excess flexure in the "J" bend at the hub flange.

The power tap hubs make for more dish in the wheel and lower NDS tension. I rebuilt a wheel for a friend a few times with a power tap hub. He bought the wheel from Performance and cracked the rim. I had an Open Pro in my bike room so I rebuilt the wheel using my rim while he sent the broken to Per. for replacement. He broke the second. so I built it up with a Sun CR18. He began breaking NDS nipples so I added tension to the drive side to be able to get decent tension on the NDS. That solved the problem.
He is a big man (over 220) and strong as an ox so he needs a strong wheel.

This is a case of either raising the bridge or lowering the water.

The irony of left spoke breakage is that they're too strong for the job. And the secret to solving the problem is to use lighter (weaker) spokes on the left. This allows them to stay within the working spring range without needing to crank up the tension on both sides. (no need to weaken right side spokes, they're strong/weak enough already)

I've been building that way for decades, and building durable wheels for people with breakage issues by building weaker wheels. I've been on the soapbox preaching less is more here on BF for almost 5 years. Over that time a number of people here have followed the advice with good success.

Of course that isn't the ONLY way. You could raise the overall tension if the rim can handle it, or use fewer spokes on the NDS, or use offset rims, or use hubs with flange asymmetry. Of course you could combine some methods to advantage. But thinner spokes on the left is an option open to anybody without having to buy anything else.

Another advantage of the less is more approach is that you not only get a tougher wheel, you get a lighter wheel, and who's going to complain about that.

That would be me! Thanks.

One other reason that Power tap hubs may be problematic. I haven't seen one since the early versions, but if the flanges are a bit thinner than typical, or the spoke holes bigger, or countersunk, then flange thickness could be the culprit.

Ideally you want spokes to leave the flange right along the face, not out from it unsupported 1mm or more away. That's especially true of inside elbow left spokes which leave at an angle away from the flange. The set angle and the distance from the flange allow the elbow to cam in the flange, opening the angle at the elbow. This is very difficult to properly stress relieve, and impossible to relieve once the wheel is tight, so those spokes will tend to break first. (on an otherwise well built wheel)

The remedy is a washer under the spoke head to effectively thicken the flange and pull the elbow tight to the flange face.