A number of valid points, but I'd like to raise one point in regard to the comparison of bicycle to aircraft parts. Both make extensive use of aluminum (or aluminium if you will), and we all know that aluminum is subject to stress fatigue cracking. But this has nothing to do with the age of the components per se. Unless I was asleep in Engineering Materials class those may years ago, a 40 year old pair of NIP GB bars is no more likely to fail than the day they were made. It is the number of stress cycles endured and the magnitude of those cycles that determines when a stress fatigue failure will occur.
Aircraft are subject to more thorough operational data logging than almost any manufactured item. As a result the number of miles flown is well known. Maintenance logs also record the service miles of all the components of an aircraft, so it can be known with good accuracy what the number of stress cycles endured is. While fuel load, passenger load, airspeed and altitude do vary, these are are kept within specified limits so a fair estimate of the magnitude of stresses can be made.
With old bicycle components, one seldom knows how much use they have had. Even in the rare instance of the original owner, how many of us carefully log every mile ridden and when each component was replaced? And even if we knew that, has anyone done an analysis of the stresses placed on various components during use? Further, the stress will be affected by rider weight and riding style, and therefore will vary much more widely than the stresses on a given aircraft. Finally, has anyone ever seen a figure for any bicycle component of how many stress cycles of what magnitude it can endure? Or even a simplfied figure of how many miles of use with a typical rider of some specified weight? If such data exists, I've never seen it.
It may be that the stress fatigue limits of most bicycle components are such that they are unlikely to be reached in a human lifetime, even if a bicycle was ridden by an individual every day from age 15 to 95. And even if a bicycle component was made in the 1890's, it has probably spend more of the interim gathering dust on shelves than on the road accumulating stress cycles. Because of all this, I have the strong suspicion that stress fatigue failure in bicycles is almost entirely a myth, and that failure of bicycle components is due to mechanical damage or poor design rather than to stress fatigue as such.
Regards,
Jerry Moos Houston, TX
hey <B50@veloemail.com> wrote: I am sorry that I'm not replying to specific posts, but I was busy last nig ht at a local launch of a lot of new off-topic stuff from SRAM. Those stude nts of the development and history of the derailleur may be interested in S RAMs revisit of some old and proven French ideas.
On to topic. Someone asked a ways back whether the "death fork" bikes were marketed only as Lamberts. I think the question was answered implicitly but not explicitly. Cast iron evidence is always helpful, and I recently saw a very original Aerospace complete with pinned aluminium fork (semi-death fo rk?) and Viscount transfers. Components were labelled Lambert. It is now ri dden by a very small Japanese woman who probably won't stress that fork bey ond its rather mean design limits.
Writing of design limits, I can't help but take issue with the argument tha t broken Campy cranks are down to inexperienced and ham-fisted owners and m ake-a-swift-buck dealers. OK, some things are broken from missuse. In the c ase of cranks I'd say a worn out taper, stripped pedal thread and stripped puller thread are all evidence of mechanical ineptitude (hands up whose tri ed to pull a crank with the washer still in its hole. Doh!) But a broken cr ank? Hmmm. Oh, the rider didn't notice the hairline crack and fettle it wit h a rattail file. The rider was too big, a real meathead, not like those n ice, spinning, mechanically tuned velo racers. Has anyone seen a track spri nter lately? Maybe the owner or shop can be blamed for not replacing an obv iously beaten up or very old crank (I recently saw a NR crank snapped throu gh the middle like a carrot, not at any obvious stress risers) but an awefu l lot of breakages occur before due time and while the owner's attention is elsewhere. If a part breaks when being used in a forseeable way because of a strength limit imposed by a design fault (such as a stress riser) then t he failure is down to the product's designer and manufacturer, not its end user.
To temper this I'll admit that high end bicycles are often built for a limi ted life; one season in the case of professional level racing machinery. At this level I think our industry is more akin to the aerospace industry tha n the auto industry. In cycling weight is always an influentual factor in d esign. How influentual depends on the purpose of the bike, but given this g roup's predilections we can assume that for most of the machinery we own or covet it was a vety important design factor. In the Aerospace industry and the related aviation industry strict time parts replacement regimes are en forced. Yet I have just built a bike with a pair of forty year old Aluminiu m GB bars. In the aviation industry they'd have been scrapped decades ago. I think it is my responsibility to use those bars with some caution and I'm not planning any hilly rides for them. And if they do break? I don't think I'll go running to Gerry Burgess with a warranty claim!
David Gordon Wilson has written considerbly on the topic of design, product failure and safety. Chris Juden, technical officer of the CTC is another s ource of illuminating research. Those interested in the professional approa ch to product failure rather than my ill-considered ramblings should refer to Bicycling Science (3rd edition), by Wilson (MIT 2004).
Happy pedalling Michael Toohey, Rangiora New Zealand
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