From: tomk@netcom.com (Thomas H. Kunich)
Newsgroups: rec.bicycles.tech
Subject: Re: Summ. of Frame Materials for Review (enough already)
Date: 1 Sep 92 23:30:42 GMT
Organization: Netcom - Online Communication Services (408 241-9760 guest)

I have been wheedling and finagling to get someone to
explain why the better frames are built of very high
strength steel alloys or the modern crop of high tech
materials.

Being a devil's advocate only seems to work when someone
else in the conversation not only knows what they're
talking about, but also has the ability to teach it in a
fairly comprehensive yet simple manner. Maybe this medium
doesn't lend itself to careful explanations?

Well, the answer to all of this is that things are often
deceiving.

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.

But this isn't because the newer steel is stiffer. As we
have seen from all of the messages (and good lord knows the
flames via e-mail) the Tensile Modulus of Elasticity is
approximately the same for all steels. 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.

Older frames were designed and constructed to run on roads
different from what we use today. Originally the roads
were, at best, what we would consider secondary roads
And at worse they were little more than the mountain trails
we seek to experience as single track.

Also steel tubing was originally of quite poor quality. So
frames were built of heavy tubing and it was stiff. So the
forks became long swooping affairs with lots of rake and
the chainstays became long torsion bar affairs -- all this
to cushion the otherwise incredibly stiff ride.

Bicyclists, and builders in particular, are pretty
tradition-bound people. So once the bicycle was established
it changed only slowly. As better alloys hit the market
they were slowly incorporated into the bicycle, not as a
strengthening device, but as a weight-saving device. Slowly
the heavy safety bicycle became more and more a
light-weight, easily driven mechanism that, with the
improving road conditions, began to increase speeds
dramatically.

With smoother roads, builders shortened the chainstays and
pulled most of the rake out of the forks so that road bikes
came to resemble track bikes in their geometry and weight.
This made the designs very much stiffer, laterally in
particular.

So what has this to do with the arguments at hand? Plenty.

Because the alloys were weak the frames were overbuilt.
Because they were overbuilt they were incredibly stiff.
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.

Newer bikes try to balance the stiffness of the _design_
with the flexibility of the steel alloys. The new alloys
can bend much further and undergo greater insults than the
older steel tubing. So it can be made much thinner and
therefore much more flexible than weaker alloys. And all
with increasing reliability and fear from catastrophic
frame failure. An insurer's dream come true.

So the newer bikes aren't stiffer at all! What has happened
is that the _design_ of bikes has increased lateral
rigidity while the thinner tubing has significantly
decreased vertical stiffness. The frame acts _more_ like a
spring which eases the impact of bumps on your body and
allows you to maintain control of the bike in extreme
conditions. And this is exactly opposite of what would seem
to be intuitively correct.

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.

And, yes Virginia, there is a good reason to buy high
strength alloy steel frames. You are getting your money's
worth if the bicycle is properly designed.