From: harry@cv.hp.com (Harry Phinney)
Subject: Re: Summ. of Frame Materials for Review (enough already)
Organization: Hewlett-Packard Company, Corvallis, Oregon USA
X-Newsreader: Tin 1.1 PL4
Date: Wed, 2 Sep 1992 22:19:31 GMT

tomk@netcom.com (Thomas H. Kunich) writes:
: Maybe this medium
: doesn't lend itself to careful explanations?

You seem to be seeking text books on physical metallurgy, strength of
materials, and the history of bicycles.  I for one am unwilling to post
entire text books to the net.  Someone else earlier posted a reasonable
bibliography which you might want to peruse.

: Older frames, constructed of weaker alloys of steel,
: flex like crazy when you try to ride them in a hard, fast,
: technical downhills. And newer frames constructed of high
: strength alloys don't (or at least, shouldn't.) This is the
: truism we generally all subscribe to.

You apparently subscribe to this "truism", but I do not.  I have ridden
stiff older bikes, flexy new bikes, and many many other sorts of bikes.

: I had sort of hoped
: that the same people who quoted those numbers would also
: tell us that the Young's Modulus wasn't very important and
: offer the proof of aluminum frames with a modulus of about
: 1/3 to 1/2 of steel! But no such luck.

I think if you had read the earlier messages carefully you may have been
able to understand this situation.  There is nothing unimportant about
the Young's modulus of an engineering material.  The stiffness of a part
is determined by the geometry of the part and the Young's modulus of the
material from which it is constructed.  If you built identical tubes
from Al and steel, the steel tube would indeed be approximately three
times stiffer than the Al tube.  The steel tube would also be
approximately three times heavier than the aluminum tube.  This is the
primary motivation for using large diameter tubes in aluminum frames.
As has been previously been discussed, a larger diameter, thinner walled
tube will have a higher stiffness to weight ratio.  Since Al is
approximately one third the density of steel, one can afford to use
nearly three times as much Al (in terms of volume) as one would use in a
steel frame.  This means that for a given tube diameter, the Al tube can
have a wall approximately three times the thickness of an equal weight
steel tube.  This allows a designer to utilize a larger diameter Al tube
while still avoiding buckling ("Coke can") problems.

: Older frames were designed and constructed to run on roads
: different from what we use today.

This is true, and can in some cases explain why an older bike may be
more flexible, as it may be built with a longer wheelbase, and overall
longer tubes.

: Also steel tubing was originally of quite poor quality. So
: frames were built of heavy tubing and it was stiff.

To what time are you referring?  High quality butted chrome-moly and
manganese-moly tubing has been around for many decades.  The frames to
which you referred in an earlier posting were both built at a time when
high quality tubing was obviously available, and you claimed they had
virtually identical geometry, and yet you also claimed the heavier bike
was more flexible.  I still do not believe that a bike with heavier
gauge tubing of the same bulk material (e.g.  steel) and the same
geometry will be more flexible.

: Because they were very stiff, if you hit a bump it was
: transmitted to your body which was thrown about. This is
: what caused the frame to flex. The forces of that heavy
: body bouncing about across the axis of a bicycle that was
: built rigidly in one plane only.

This doesn't make sense to me.

: So, of course, this also explains why, generally, titanium
: bikes are so treasured and why carbon fiber, which is an
: extremely stiff material, isn't becoming nearly as popular
: as one would expect.

I think this is bogus.  What about aluminum?  Also, note that while the
individual carbon fibers are indeed _very_ stiff, nobody rides a frame
made solely of carbon fibers.  To fashion a tube or other structure the
carbon fibers are embedded in a matrix of epoxy or polyester resin.  The
stiffness of the composite material is not determined solely by the
stiffness of the fibers, but is also strongly affected by the stiffness
of the matrix material and the geometry (e.g.  fiber orientation, fiber
density) of the material itself.

Harry Phinney   harry@hp-pcd.cv.hp.com
Newsgroups: rec.bicycles.tech