"David Rees" <
[email protected]> wrote in message
news:
[email protected]...
>
> "jim beam" <[email protected]> wrote in message
> news:[email protected]...
> > your reference to "structural metal" is also misleading. the
"structural
> > metal" of a bike is not one, regardless of alloy unless it's made of
the
> > cheapest mild steel, for which you can rely on an endurance limit.
no
> > aluminum component has an endurance limit, not do high strength
steel or
> > titanium alloys. so, compare apples with apples - quality bike
materials
> > have no endurance limit, composites or not.
>
> Are you sure high-strength steels and Ti alloys don't have endurance
limits?
> My understanding of 'endurance limit' or 'fatigue life' is a material
cycled
> within it's elastic range for a defined number of cycles. I thought
steels
> and Ti could be cycled alomost indefinitely within this range whereas
Al
> could not, hence the extremely stiff Al bikes we have today -a design
> requirement necessitated by the nature of the material. Ti and carbon
would
> seem to be ideal choices for someone wanting a forgiving ride because
more
> flex can be allowed due to their endurance limits, and to a lesser
extent,
> steel. With steel, it seems that elongation of less than ~6% was the
culprit
> causing the failure of some '80s Excel tubing made of much
higher-strength
> (~200ksi?) steel, not lack of endurance limit.
>
>
Here's an interesting web site from an organization that tests bike
frames and components. It's translated from German:
http://www.efbe.de/
The following excerpt discusses the results of testing 12 different
frames back in 1997:
"The fact that aluminum and carbon frames in this test lasted longer
than the steel frames is not in our estimate a question of the material,
but the design effort. Not the material, but its skillful use gives the
result. However, the manufacturers concentrate their design efforts in a
logical way on frames with good potential for light weight
construction - and those are made from aluminum or carbon, and only
rarely (because of low rigidity) from titanium."
http://www.damonrinard.com/EFBe/frame_fatigue_test.htm
What they were suggesting was their results were more reflective of
frame design and construction rather than material.
Excel tubing had a reputation for premature failure. So have all of the
other brands of extremely thin wall steel tubing. I think that it's more
a function of overheating during brazing than material failure.
Most steel frames are assembled with brass or bronze brazing rod that
has a melting temperature of over 1600°F. Silver brazing alloys used in
premium frames have brazing temperatures of 1200°F to 1600°F. The
re-melting temperature of all brazing alloys once they've turned solid
is even higher. The critical or transformation temperature for most
alloy steels is around 1900°F.
It's really easy to overheat thinwall steel tubing in just a few
seconds. Cheap bikes are made with tubing that can have a wall thickness
of over 2mm. Heavier gage premium tubing such as Columbus SP has a wall
thickness of 1mm at the butted ends of the main tubes. Some ultra light
tubing has wall thickness of less than 0.5mm at the butted ends of the
main tubes.
One of the hardest areas to control heat is where the seat stays attach
to the seat tube because the built up fillets require more time to fill
in with braze material. I've had to ream out the seat tubes on a lot of
pro bikes because of a slight bulge inside the top of the tube due to
overheating.
When Reynolds first released their thin wall 753 tubing, builders were
required to use a special brazing alloy and submit braze samples to
Reynolds before they would sell them tube sets.
Most thin wall tubes are made of special high strength alloys. As easy
as it is to overheat the tubes it's just as easy to cool the brazed
joints too quickly causing increased hardness and potential cracking.
Reynolds included warning messages will all of their tube sets warning
about drafts while brazing.
Over the past 30 years I've seen a number of steel frame failures. The
most frequent tubing failure that I ran across on quality frames was the
at the downtube, headtube joint or lug. The cracks started at the
underside of the downtubes and worked their way up to the top. The
second most frequent failure was the right chainstay 3" to 6" from the
bottom bracket. The third most common area was the seat tube at the
bottom bracket. Steering tube failures were fairly common on 1970s era
bikes. The steerer broke in the threaded area or at the crown or the
braze failed in the crown and the brake bolt was all that held the fork
on.
I saw a the head tube break off completely on some cheap welded
department store bikes and old cheap made in Taiwan bikes, also on a
fillet brazed Schwinn Collegiate.
Chas.
