Chalo wrote:

wrote:

It's possible to design an aluminum bottom bracket axle that's safe for
even someone of Chalo's size, but it would have to have a larger
diameter than most (or all) current designs. Like not 19-22 mm but
rather something on the order of 50mm. Whether such a large diameter BB
is desireable is a separate question.

As a rule of thumb, a tubular structure made from aluminum can be made
equally stiff to a steel tube of similar wall thickness if its diameter
is increased by a factor of 1.4. Furthermore, if its cross-sectional
area is increased to accomodate the difference between the tensile
strengths of the aluminum and steel in question, then the aluminum part
will almost certainly prove to be as reliable as the steel part. The
principle is well illustrated by the aluminum frames that are
ubiquitous on bicycles today.

For my part, I find cranks with 3/4" (19mm) solid CrMo spindles with
fine-pitched splines to be perfectly adequate for my needs. This
suggests that an aluminum spindle of approximately 27mm diameter with a
9.5mm wall would be adequate for me, as long as it were made of some
very strong aluminum alloy like 7075-T6, 7068-T6, 7055-T77511, or a
Scandium-doped alloy. Such a spindle could easily be fitted into an
American BB shell along with some reasonably stout bearings.

Chalo Colina

Thanks for the input, Chalo. Your math makes sense to me.

FWIW, I secretly hope that a new BB standard emerges (the American (AKA
BMX) spec would work fine) such that we could use 25-30mm aluminum
bottom bracket axles with appropriately oversized bearings. One
advantage of an Al:Al interface is that both the BB and crank would
have similar stiffnesses, reducing the incidence of creaking.

People like Peter would hate it, but I suspect we're headed that way;
Cannondale, Specialized and Pinarello already offer such BBs. I guess
my secret hope is no longer a secret.

Jason

P.S. Peter, I mean no offense by this; "people like Peter" is meant to
mean "those skeptical of change qua change" rather than "people who
hate anything new."