Harvey, I haven't reread my Materials textbook either, but I'm almost sure that steel exhibits safe maximum stress - can't remember the precise term. That is, as long as the magnitude of the stress is held under a certain level, a steel structure will never suffer a stress fatgue failure. But I'm also pretty sure aluminum has no such safe maximum stress. In essence, a steel structure will never suffer a fatigue failure if the load is kept within some limit, but an aluminum structure will eventually fail after enough stress cycles, no matter how small their magnitude. I'm not disputing, however, that large stresses on aluminum will cause it to fail sooner than small stresses. Thus an ALAN cyclocross frame really beat about over rough courses might fail in a few years, while one ridden on fairly sedate club rides only might last a lifetime.
Of course one can dispute that a failure after really hard use is really a fatigue failure at all. It might be that really hard use actually exceeds the aluminum's yield strength in an area of the frame, creating a small crack, or at least exceeds the elastic limit, causing deformation of the structure. In either case, the frame would then be subject to further damage in that area. The eventual failure might be hard to differentiate from a fatigue failure, especially since a failed bicycle frame is not going to be subjected to the kind of post-failure investigation that occurs in the case of an airframe failure.
As to tube size, ALAN's conventional OD was partly compensated by thicker tube walls, thus the 25.0 seatpost. Probably not as good a technical design as Gary Klein came up with using oversized tubes, but Vitus also used conventional OD tubes, and it seem to work OK for a rider as powerful as Sean Kelly.
Regards,
Jerry Moos Big spring, TX
Harvey M Sachs <sachshm@cox.net> wrote: I'm not convinced that either Dale or Jerry is correct. Dale said, "Anyway, if you accept the fact that aluminum, in almost any variation, when used in bike frames will have a distinctly shorter lifespan as compared to steel,"
And Jerry maintains that: "most members with an engineering/metallurgy background will agree that fatigue failures in aluminum are related to the number of stress cycles a structure is subjected to, not simple age."
I don't have time to re-read Wilson/Papadopoulos tonight, but my memory is that current understanding is that aluminum failure is much more likely to be related to relatively small numbers of much-beyond-design strains than to cumulative large numbers of small strain cycles causing fatigue.
>From what I recall of the analysis in Wilson/Papadopoulos, it would seem to be fairer to say:
1) Any frame that is improperly designed or constructed with respect to its materials and joining methods is subject to failure. Wrong alloys, wrong brazing, wrong annealing,...
2) Conversely, properly designed and built frames of steel or aluminum should last indefinitely.
3) The failure style of different materials can differ. Steel tends to fail more "gracefully," with cracks growing and material bending, while aluminum often snaps without obvious warning (campy cranks...). The failure styles may differ enough to influence decisions by prudent buyers.
We may well see premature failures of some brands/models/designs of aluminum frames. Engineering learns from failures, painful though that may be. I would be quite sure that there are some out there that skate too close to the edge, and are simply imprudent designs. There are some with material defects, and some with joining defects, and they will be concentrated on specific brands and models. I don't know whic hones, but I'm willing to be they will be off-topic. :-)
FWIW, I have had esthetic objections to the Alan design simce seeing one for the first time. Alan chose to sacrifice tube diameter for a conventional appearance. The relevant moduli of steel and aluminum dictate that similar behavior will require much larger diameter tubes if you want to work in Aluminum. Their commercial and racing success suggests that riders aren't very demanding about this similarity in "stiffness" and other properties, but it just doesn't look right to me.
harvey "I knows good designs when I see it" sachs mcLean va
+++++++++++++++++++++++++++ Jerry Moos wrote:
Not to dredge up past discussions, many of which featured spirited debates, but most members with an engineering/metallurgy background will agree that fatigue failures in aluminum are related to the number of stress cycles a structure is subjected to, not simple age. SO... an aluminum frame that has gathered dust in a garage for 20 years since purchase will be essentially as good as new, while one that has been ridden 200 miles a week for the same twenty years will probably not have much life left.
Dale Brown wrote:
We have covered this topic before but it may have been a few years ago, so I will likely repeat myself a bit here..
"In the old days" I sold quit a few of these bikes..
Most labeled as Guerciotti, but also under other brands including Alan especially as cyclocross bikes...
Anyway, if you accept the fact that aluminum, in almost any variation, when used in bike frames will have a distinctly shorter lifespan as compared to steel, these Alans, in balance, did amazingly well. Yes, after use some cracked and a few have had the bonding fail, but many are still out there and I know of no catastrophic failures.
Time has matched on and I think they are not a modern product, but just wait a few years and all these welded aluminum bikes will start cracking too. Actually, I think I am seeing evidence in the bike shop that this is already happening.