easyupbug 

Most steels have an endurance or fatigue limit about half the Tensile Strength.

Jeff Neese

I misspoke - sorry. Steel DOES have a fatigue limit. What I meant is that below that threshold, steel does not develop fatigue stress. That's in contrast to aluminum and other metals, that do not have a fatigue limit and will fatigue with any repeated flexing.

In normal operation of a bicycle, spokes don't come anywhere near their fatigue limit so they have a theoretically infinite lifespan.

pdlamb

Kind of like cycling clothes. Bottoms last a couple years, jerseys never seem to wear out.

I was going to argue about front wheel spokes breaking, but on second thought, if I did break one it was too long ago to remember.

FBinNY 

Going back to the OP.

I get curious about how and where these kinds of concerns originate.

Consider that bike wheels are made using the same materials as bridges, aircraft, cars, tall buildings and so on.

Now, consider that just about all the longest bridge spans are far older than this wheel, as are the US B-52 bombers still in service, along with DC-3 aircraft nearing 90 years in service.

The point is that metal structures, including bike wheels are extremely long lived, and I wonder why someone liningin this era would be worried. Unless, of course he's read too many "you die" posts here on BF.

SJX426 

I have never seen an expiration date on any steel anything. I don't get where these really ignorant concepts come from with all the evidence everywhere you look that steel lasts as long as it is protected from harmful environmental extremes. Yes, the climate will kill us all within 50 years too.

When I was a child in 1960's, there was the fear mongering that we would run out of oil in 20 years and the planet would freeze during the next generation.

Trakhak

The current temptation to blame corporations and stockholders if at all possible, even for fragile bike rims, is understandable, but it feels like a stretch here.

Can't remember whether I read this or was told it by one of our shop's sales reps, but at some point in the early '80s, an Italian rim manufacturer asked an importer's rep why Americans bought so many lightweight 32-hole tubular (i.e., sew-up) rims, since the use of such rims in Europe at the time was mostly confined to time trial record attempts, with 36-hole rims being the standard for all other types of races. The rep replied something like, I don't know, but we sure sell a lot of them!

In other words, far from forcing the change from above, the hunger for lighter rims took at least one manufacturer by surprise.

Looking back, the likely explanation is that almost all Americans riding on tubulars back then were new to the sport, especially compared to their European counterparts, and naively figured that since light in a racing bike is good, lighter must be better. If one of the guys in your team/club bought them, you'd be sorely tempted to follow suit. No coercion on behalf of stockholders necessary.

A couple of data points: my Bianchi Eco Pista, which I bought brand new, the first year they were imported to the U.S., was one of my all-time favorite bikes, but the front wheel had a distressing habit of breaking its (galvanized, not stainless steel) spokes regularly. It hadn't occurred to me to check the tension of the spokes---why wouldn't I trust Bianchi to get it right? By the time I realized that they weren't going to stop breaking, it was clear that the low tension had prematurely fatigued all the spokes, so I rebuilt the wheel with all new stainless steel spokes.

My Specialized Langster fixed-gear bike (another that I bought in the first year of production; coincidence?) was the only other bike I've ever owned that broke spokes regularly, in both the front and rear wheels, starting fairly early after purchase. You'd think I'd have learned to check the spoke tension on each new bike I bought after the Eco Pista, but no. Again, galvanized spokes (painted black), not stainless, and again, insufficient tension from the factory.

The moral of the story (other than pointing out the wisdom of checking spoke tension on new wheels): spokes can indeed last indefinitely unless abused. But wheels that are built with insufficient spoke tension, such that each spoke swings through a wide range of stresses with each rotation out on the road, are likely to experience spoke breakage that would not have occurred otherwise.

Road Fan

Ok, I totally agree with the statements regarding steel. But what about the aluminum or its alloys that the rims (mostly) are made of? Aluminum cannot withstand below-limit stressing nearly as well as steel. As the spoke tension cycles, the steel elongates within its elastic zone and harmlessly relaxes (a cycle of stress). That cycling tension is resisted at the nipple by pulling on the hole. Some Al rims have no special reinforcement at the spoke holes, some have a thicker wall thickness than at the braking surface, some depend on a washer to spread the stress, or a single-ferrule (looks like a pop-rivet), or a double rivet (tries to spread the stress between two walls of the rim, think tubulars or some of the Mavic clincher products). But regardless of the rim or flange design, an aluminum-based part does not have a lower limit for accumulation of internal damage due to fatigue.

Tensioning the spokes is a matter of balancing the static stress between the flange, spoke and rim such that the cyclic strain in those three components all remains below the fracture-prone zones of all three elements. For the steel spokes this is a huge number of cycles, so large we consider it infinite, barring unexpected over-stressing. For the aluminum and aluminum alloy parts the number of cycles and the extent of the cycles, measured by stress, must be kept below the regions (on a graph showing the range of both stress and strain for the involved materials) where crack formation may occur. So there needs to be an estimate of the useful lifetime of the wheel and perhaps the bicycle, and setting of a hard target that the aluminum-based parts need to be designed, in order to last over that time period within the usage and stressing profile.

It amazes me to imagine my 1980 Masi wheels (Wolber dark anodized tubulars) are roughly 40 years old and have not been rebuilt at least since 1985 when I got the bike!

Hondo6

Work hardening and fatigue limit (AKA endurance limit) are mutually exclusive concepts.

Fatigue/endurance limit refers to the fact that stresses below the yield limit in certain metal alloys appear to cause no change in the alloy's response to stress and can thus be repeated an "infinite" number of times. ("Infinite" is in quotes here because testing anything to ascertain whether such an infinite limit exists is problematic. However, available data indicates that this is likely the case, or that the number of repeated stresses that an item made from such an alloy can withstand is indeed extremely large.) Stresses below a material's yield limit cause elastic deformation, not plastic deformation; no permanent change in shape results.

Work hardening is the result of plastic deformation. Plastic deformation occurs when a material is stressed beyond its yield limit, causing permanent deformation.

FBinNY 

+1
Work hardening, by definition, requires that the metal is "worked", namely bent, stretched, or otherwise deformed beyond the yield limit.

Stressing below the yield point doesn't permantly distort or work the metal, and therfore doesn't work harden it.

Fatigue refers to the cumulative damage caused by repetitive sub-yield stresses.

IMO, it's impossible for work hardening to come into play on a bicycle wheel before lots of other stuff happens.

And FWIW, I very firmly believe that it's easy to properly buildvwheels such that fatigue isn't possible either.