Re: [CR]Long answer to Super Record BB question, drifts OT.

(Example: Component Manufacturers:Campagnolo)

Date: Wed, 1 Jun 2005 20:56:23 -0700 (PDT)
From: "Tom Dalton" <tom_s_dalton@yahoo.com>
Subject: Re: [CR]Long answer to Super Record BB question, drifts OT.
To: Kurt Sperry <haxixe@gmail.com>
In-Reply-To: <75d04b480506011819177a21fa@mail.gmail.com>
cc: classicrendezvous@bikelist.org

No, YOURS was a great post. I'll have to give it another read, or two, but it makes sense based on my very limited understandling of materials science. I think this says a lot:

"have remarkably similar *specific* moduli"

in other words, the Ti BB spindle will be just as stiff...once it is just as heavy....

and this also says a lot:

"so there is frequently little to be gained by substituting one for another if stiffness is the primary design case, ignoring buckling failures in thin wall sections"

Which also suggests that steel frame tubes, for example, may be more limited in terms of stiffness-to-weight than aluminum, based on other limitations on the final dimensions (i.e. beer can failure).

Just reading between the lines.

Tom Dalton

Bethlehem (where it was once all steel, all the time) PA

Kurt Sperry <haxixe@gmail.com> wrote: Great post! I've had similar musings. I think the early buzzphrase on Ti in the '70s was essentially "as strong as steel at half the weight" which was in some senses sorta kinda almost true and led to some conceptually questionable practices- speccing Ti instead of steel for parts without significantly changing the dimensions. The yield strength or UTS of the sorts of Ti alloys commonly used back then in military aerospace aplications like alpha-beta phase 6Al-4V was indeed better than the most prosaic mild steels. Part of the problems of early Ti applications I think may in part have been that those applications generally used CP Ti, a "lesser" engineering material in most senses.

Another consideration in the problems seen by substituting Ti for steel was the massive difference in the relative moduli of elasticity between steel and Ti. Many parts on a bicycle are more stiffness limited than strength limited, and a steel part replaced by a dimensionally similar one of Ti will be far "flexier" than its steel equivalent. All engineering metals save *real* exotics like AlBeMet or some metal matrix composites have remarkably similar *specific* moduli, so there is frequently little to be gained by substituting one for another if stiffness is the primary design case, ignoring buckling failures in thin wall sections. This alone explains why a stiffness case design optimized for steel will frequently see very little reduction in mass when optimized in another "more exotic" material. Marketing considerations may be more significant than engineering ones in many of these cases where "low tech" steel is replaced by "high tech" metals I'll leave out consideration of CFRP composites because that material was FAR rarer than Ti in CR list timeline lightweights. Anisometries and differing failure modes make direct comparisons there difficult anyway.

Kurt Sperry Bellingham WA

John Thompson wrote:

As for titanium use predating the SR group, consider that the SR-71, which was conceived in the era of the Chevy Bel Aire was made primarily from titanium. 40+ years later it's also still the fastest jet ever in service (that we know of). Cold War tax dollars allowed Lockheed to learn all sorts of stuff about Ti fabrication, some of which may still be secret. (Check out "Skunk Works" by Ben Rich for more).

Drifting a bit closer to topic, I think Campy still suffers from this "retain the basic design, apply the cool new material" approach. In fact, they are worse than ever, replacing aluminum with carbon all over the place with little revision in design. I guess it's good for marketing, but I refuse to accept that this does not lead to either weaker parts, or parts that are stronger (heavier) than necessary. Looking back at the second gen BB, you can see where Campy's inability or unwillingness to really change the basic BB design lead to a part that did not best exploit the properties of a new material. By the time they made it solid, the weight benefit was significantly reduced, and it was still said to be a more flexible part. A more common example would be "early" aluminum frames, like Alans, that retained lugs and the basic dimensions of a steel bike. Compare this to a Klein, and the far better bikes that this would lead to, in which the wall thickness, diameter and profiles of the tubes were much better suited to exploit aluminum's characteristics. Look also at Shimano's current crank and BB, which totally revises the entire design, but still uses the steel spindle and alloy arm. They are pursuing the best designs with the established materials while Campy is reusing old designs on new materials, which seems pretty stupid to me. Of the old square taper BB, for example, the faithful will say "tried and proven design, backward compatible, etc...." To which I respond "outdated, not opitmal for the material, thinking inside the box." Yikes, I spoke "office."

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