Cause of Broken Spoke
 

I have a bike which has wheels which I (sort of) built myself. I bought good quality hubs and rims, and had a friend who is a great wheelbuilder walk me through the process of building the wheels, using his tools and his spokes. He also calculated the spoke lengths for me. I ride the bike every day, and built the wheels 3 years ago. Every week or two I do minor maintenance, on the bike, lube and adjust, and check that the wheels are true, and I have patted myself on the back for how the wheels have remained perfectly true after 9500 miles. I have moved, and no longer have access to my friend, nor his tools.

Last night, on the way home from work, a spoke broke on the rear wheel - and I was curious as to why this might have happened. As soon as the spoke broke, the wheel was out of true (as would be expected), although the wheel seemed to be perfectly happy up until the break.

I'll be bringing the wheel into my LBS to have it repaired, and to have the hub serviced if it needs anything, so I'm not looking for advice on how to replace the spoke, rather, I am curious as to what might have caused it to pop after so many trouble free miles. Do I need to be worrying about the other 35 spokes on that wheel?

Not an expert by any evaluation, but even though the wheel has performed well over time, it is possible that the spoke tension was off. Where was the break, was it the "head" of the spoke that broke off at the bend around the flange? If so, was it the base of the head or at the bend?

What tension were you using?
Too low and they flex/fatigue and tend to break at the elbow.

You didn't state which side and where the spoke broke.

One spoke in 9500 miles?

I think you just weeded out a weak spoke.

This thread has some interesting (potential) answers: http://www.bikeforums.net/showthread...a-broken-spoke

Broken spoke is on the non-drive side, and is one of the ones that points backwards so it is under greater tension during acceleration. The break was right at the end of the threads, as it exits the nipple. The spoke broke as I was accelerating away from a stop light.

touch up tension and truing checks are always desirable ..

NB in a million miles of wire used to make spokes, not all of the wire is perfect.

Are you a light weight or a heavy rider? I am no expert, but I haven't seen many breaks at the nipple. If the tension was low the threads can act as a stress riser and contribute to the break.

Also note that you need to stress relieve your spokes so you don't have parts of the elbows which didn't make it past their elastic limit during the forming process fail early due to fatigue. Presumably the same things could happen at the end of the threads if you neglected correcting spoke lines and/or stress relieving.

I like twisting spokes around each other at their outer crossing with a brass drift (anything softer than the spokes will work like a plastic screw driver handle or old left crank arm) although you can also squeeze near parallel pairs _hard_ (gloves make that more comfortable).

I'm not really at either extreme - I am 175, the wheel is a 36 spoke wheel, but this is a commute bike with a rear rack and more often than not there is a pannier back there adding to the load. Still, this shouldn't be too much to ask of the wheel.

If the wheel seems true to the eye, is there a need for any further examination? I don't have a wheel truing stand.

Is it advisable to pay the local bike store guy to check the wheels every xxx miles just as a precaution?

Yes. If some spokes are too loose it'll go out of true and may break spokes. That may be because the wheel is tensioned unevenly or average tension is too low. Low average tension is also bad since it'll take less to cause the bottom spokes to go slack at which point the wheel is horizontally unsupported in that zone, can shift off center, and will collapse when the rim springs back - I quit letting other people build my wheels after learning that one the hard way.

Some one who knows how things work should look at new wheels, bring the spokes up to sufficient uniform tension in each side, and stress relieve if the wheels weren't built by a single individual who knows what he's doing.

I would bet against some guy at the LBS being able and willing to do that.

In addition to the formerly reputable shop that built me a set of under tensioned wheels where the front folded and rear never stayed true I let a LBS charge me $70 in labor + $40 in spokes to swap a 3 speed IGH hub for an 8 on my wife's bike because I was feeling lazy. I had to fix that when they screwed it up and neglected to put enough tension in the wheel to keep it true and spent more time than if I'd just built it myself.

Once it's right it'll stay that way until you want to replace the rim because you bent it on an obstacle or wore out the brake tracks.

Spokes break from metal fatigue after countless repeated flex cycles. Normally they break at the elbows, where there's the most flex, and least strength. But the thread also has lower strength by virtue of smaller cross section at the root of the thread.

Under ideal circumstances there's little flex of the spoke at the threads since the nipples move with the spokes. OTOH id the nipples are tightly constrained in the rim and cannot move with the spokes then you'll have flex and ultimately fatigue at the first engaged thread.

Another possibility, if you live near the ocean, or ride year round where they salt the road is chloride damage form salt. Water wicks between the spoke and nipple, then evaporates leaving salt behind. The grades of stainless steel used for spokes are very vulnerable to chlorides and when exposed over long periods, become incredibly brittle --- roughly equal to Ronzoni spaghetti right out of the box.

Regardless if cause the answer to whether you need to worry, is probably not yet. Spoke breakage is like popping corn. there's always a delay between the first pop, and the bulk, on an accelerating basis. So expect a second failure eventually, then in less time, a third, and pretty soon they'll seem to breaking daily. Because of this I always excuse the first failure, and depending on the time lag, sometimes the second, and get serious about a rebuild on the third break.

As FBinNY alluded - in general - cyclic load is the cause.

However, there are several things that can contribute to that cause being detrimental sooner - OR - later.

1. Stress relief. When done as part of a build - even el-cheapo bargain spokes can go years without breakage.
2. Tension. Too little = excessive flex per cycle = earlier onset of fatigue. Adequate tension = less flex per cycle = later onset of fatigue. Quality spokes can hold out longer - but the inevitable will happen.
3. Spoke Diameter and Spoke Gauge mismatch. The closer they are - the less slop at the flange holes - the better the heads and elbows are supported.
4. Flange Thickness and Spoke Elbow mismatch. 6.2mm elbows are intended for today's 3.2mm thick alloy flanges. Put those in some classic quality steel IGH hubs from the old days with very thin flanges - you get unsupported elbows. Hence why folks will add spoke head washers to force the elbow and initial shank run up against the flange.

The biggest cause...

5. A wheel builder who doesn't pay attention to multiple variables all at once when planning the build - such as the variables noted above.

Those are the technicals...FBinNY and others have already provided the environmental causes - both in nature and with the rider.

=8-)

With respect to MrRabbit's thorough info above, I suspect he missed this in post number #3.

Spoke breakage at the threads is almost the result of chemical damage, or thread stress at the nipple. Factors that cause classic elbow failure don't apply at this end of the spoke.

Thread stress at the nipple happens because the nipple is rigidly held in the rim. In some cases it's predictable because the nipple is not in line with the spoke cause a bend as the spoke leaves the nipple. Other cases are more subtle, where the nippli is aligned, but held so tight that it's can't flex with the spoke.

Rigid nipples are very common in aero rims, because the rim's wall thickness is greater than with shallow profiles, so the spoke hole is deeper and guides the nipple more than usual. This can be made worse by corrosion of non eyeletted rims, which shrinks the spoke hole, thereby holding nipples even more rigidly.