From: harry@cv.hp.com (Harry Phinney)
Subject: Re: Summ. of Frame Materials for Review
Organization: Hewlett-Packard Company, Corvallis, Oregon USA
X-Newsreader: Tin 1.1 PL4
Date: Tue, 1 Sep 1992 01:53:35 GMT

tomk@netcom.com (Thomas H. Kunich) writes:
: In article <1992Aug31.203137.10654@hpcvusn.cv.hp.com> harry@cv.hp.com (Harry Phinney) writes:
: >In this part of the country the nervex-lugged 531
: >Peugeot PX-10 bikes were regarded as flexi-flyers.
: 
: Yup, that's the bike. Try comparing it to the alternative metal.

My point was, many people consider your bike to have a very soft riding,
flexible frame.  You described it as having rock-like rigidity.
People's perceptions are not always accurate.  In this case, either both
groups have described characteristics they cannot accurately determine,
or somebody is wrong.
 
: Ah, but in a turn, if you strike a large bump and the bike tries
: to 'stand-up', I assume that the stays have flexed.

Huh?  I don't get it.

: I am _not_ arguing metalurgy here.

You _are_ arguing metallurgy by claiming that one steel alloy is stiffer
than another.

: I have never seen a frame of any
: metal permanently deformed from simple riding. This indicates that 
: the yield strength of mild steel used in this manner is sufficiently
: strong to not be an issue.

As long as one is willing to accept the additional weight, this is very
true.

: There is the property of tubes that they are stiffer in relationship
: to mass than round stock. Doesn't this bear any relationship to
: the discussion? And since high strength alloy tubes have progressively
: thinner walls does this not have a measureable effect?

Yes.  The thinner walled tube will be measurably lighter, and measurably
more flexible.  Assuming a constant geometry (both round tubes with
identical ODs) a thinner wall will reduce the stiffness of the tube.
Due to the OD^4 - ID^4 relationship to moment of inertia, the stiffness
will decrease less than the weight, and thus the lighter tube will
indeed be more efficient (i.e.  its specific stiffness will increase),
but will still be less stiff in absolute terms.

: Yes, there are people who believe that their frames have gotten more
: flexible with use. I am told that this is most common with bikes
: made of Prestige tubing that do get more flexible. I haven't measured
: them nor seen any tests to prove it so I'll take that with a grain
: of salt until I see it proven or _disproven_.

You are not likely to see any tests prove this.  Steel is probably the
single most studied material in use today.  It does not now, nor has it
ever, changed modulus due to use or abuse.  Even cold working the
material and heat treatment do not change the modulus of elasticity.
You might want to re-read the quote I posted about this.  The only way
that a frame might get more flexible is if it cracks, and the crack
grows large enough to significantly affect the stiffness of the
structure.  In all cases I've seen, the frame breaks before the
stiffness change becomes apparent.

: If this discussion is correct, the additional $600 difference between
: a Cromor frame and a TSX frame of otherwise identical geometry is
: spent to reduce some 2 or 3 ounces of weight. Is this really the
: case?

If the weight reduction is truly that small, then it is possible that
the TSX frame will be more robust, as it's higher yield and tensile
strengths may allow it to take more abuse.  It will not, however, be
stiffer than an identical frame built of thicker walled tubing.

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