[CR]forks, cranks and breakages

(Example: Framebuilding)

Date: Tue, 19 Jul 2005 02:25:09 -0700 (PDT)
From: "Michael Toohey" <B50@veloemail.com>
To: classicrendezvous@bikelist.org
Subject: [CR]forks, cranks and breakages

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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