Originally Posted by Psimet2001

Ohhh.....so close. I was with you until there.

I think I have a blog post about spoke failure modes but in general spokes fail for a few reasons: Inclusions/defects or contaminants in the metal is one. Seldom an issue anymore as alloys and quality have gotten a lot better but it still shows up in less expensive spokes often. It did happen in a run of Sapim CX Ray spokes about 2 seasons ago. We skirted any major issues with only a handful of spokes being affected but some wheel companies took a bath on it.

Next is induced stress risers. Similar to an inclusion this would be a place along the spoke where it's been nicked or otherwise had it's shape changed causing the geometry of the cross section to change. Can also result in a small crack. This is caused, as was mentioning in the post, impacts. These can sometimes not result in immediate breakage (plastic deformation to failure) but allow the stress riser to lower fatigue life. The beginning of the threads and the bend in the j-bend head are the two "included" stress risers.

Eventually all spokes will fail through normal use. that failure is what we refer to as a fatigue failure. Everyone should already know what a fatigue failure is. Bend a cheap metal back and forth until it cracks - fatigue failure.

Fatigue life is the term we use to describe how long it will take for a product to fail from fatigue. Fatigue life is expressed, usually, in terms of the number of load cycles the part has gone through.

Factors that affect fatigue life include: material properties (stainless steel has a higher fatigue life than aluminum for instance), Shape of material (the cross sectional area will affect the internal stress even with the same load applied), The maximal load in the load cycle as it relates to some material properties such as yield stress and finally the ratio of the change in load through a normal load cycle.

So......that's the ground work. Now let's take a wheel with spokes. All of them are run up to a tension or "load". As we ride them the spoke undergoes a loading and an unloading through one revolution of the wheel. This load and unload cycle is "loading cycle" that factors into fatigue life. In essence the lower the difference is between the low end of the load cycle and the high end the higher the resultant fatigue life (lower load ratio).

Stress risers can do exactly as the name implies. Engineers all know from statics calculations in materials that stress risers will increase the stresses seen in the materials at those riser locations. Traditional risers are things like holes in plate metal. Grab a piece of metal on both ends and pull and if there is a hole in the center the stresses around that hole are higher than the stresses elsewhere. As mentioned before the threads on a spoke and the head formation/bend are stress risers. Their overall load cycle is higher resulting in lower fatigue cycle life in those locations.

As I mentioned before the lower the ratio of loading to unloading is what has a huge affect on fatigue life. Spokes that are at too low of a tension for the geometry of the lacing and the rim/flanges/spokes count/rider weight will see a larger ratio of loading to unloading as a % of the total load they are under (spoke tension)- leading to premature fatigue failure. So yes....wheels that have too low spoke tension will have spokes break sooner. The cause is fatigue failure. Fatigue life was reduced by inadequate tension. So in this case the low tension causes the fatigue failure as you have mentioned.

....BUT....

fatigue failures also occur when spokes are at their correct tension. You see bicycle wheel systems with the spokes and nipples we use and the loading cycles they see are NOT systems that have infinite fatigue life. This means that all spokes will eventually fail from fatigue failure through normal use. They all have a finite life. That failure - if just from being at the end of normal fatigue life - will occur at either the thread base or at the bend in the head as those are always stress risers that reach fatigue life first. ALWAYS. If a spoke ever breaks anywhere other than the thread or the head then something else has caused the failure. Poor material, nicks, outright impacts from sticks, competitor's skewers, etc.

So low tension CAN cause premature spoke fatigue failure but all spokes will eventually fail from fatigue even when tensioned appropriately.

Here's the big kicker though: with modern alloy rims that allow us to reach higher tensions than in the past, better alloys and processes and quality control on the spoke wires and production we quite simply live in an era where if you actually start reaching the natural end of fatigue life for spokes and the wheel was built properly then you've really gotten your money's worth as you've ridden it way beyond where most people walk away from the wheelset and want something new. Most of the hubs on the market that are on most wheelsets under $600 or so (or carbon at or under $1200 or so) won't last long enough for spoke fatigue to start kicking in....then again cheap wheels are probably built poorly as well so all bets are off.

All of this goes to explain why I most likely said above that there's absolutely no reason in this day and age to ever carry a spoke unless you're touring meaning you have the possibility of hitting something that will break a spoke.

Apologies for the lengthy explanation and almost no one will read it but someone will and someone will get something from it I hope.