User Guide

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15
Is aluminum a new material?
It should be common knowledge that most
modern aircraft use aluminum exclusively for
their primary structures (internal frames and
bulkheads) and 95% or better of their exte-
rior surfaces, including load bearing skins.
The aircraft industry has been using these
alloys for several decades. The aircraft com-
panies have picked aluminum because it
offers the best combination of material prop-
erties and processing capability in order to
create high performance, light weight, robust
aircraft. So aluminum alloys have certainly
proved their long term durability and high
performance in the aircraft industry. The
occasional failure that has occurred has typi-
cally been due to a design or manufacturing
defect or improper maintenance.
What about fatigue?
Occasionally we hear fatigue failure erro-
neously described as similar to the result of
bending a coat hanger back and forth. This
example is not relevant to the durability
or reliability of a bicycle frame. When you
permanently deform the coat hanger you
are yielding it. This has no relation to
fatigue strength. Some of the highest fatigue
strength materials (like carbon composite)
will not take a significant permanent set,
breaking instead at a high force level. So
these extremely high fatigue strength fibers
would rate near zero by the coat hanger test.
What are the benefits of aluminum in
bike frame construction?
Aluminum is a great material to work with.
It’s light weight, or more accurately, low den-
sity. One cubic inch weighs one tenth of
a pound. And the lighter weight positively
affects the ride quality.
Aluminum provides a great ride, if you use
it to its optimum. The low density and high
formability of aluminum allows a designer to
tailor the stiffness of each part of the frame
through tubing and joint design. Tube shap-
ing and butting can make more difference in
the ride of the bike than the material itself.
Aluminum is very strong. It is possible to
achieve significantly higher strength proper-
ties in the aluminum structure per weight
than in steel. Part of this comes from the
basic material properties. You can use more
material, and more easily form the material,
so you can put just the amount and shape
needed into the bike.
But the largest contributor to high
strength is engineering and design. The low
density and high formability of aluminum
allows tubing with increased wall thickness,
complex shapes and larger sections where we
want to achieve high strength properties in
the overall structure.
Are all aluminum alloys basically the
same?
Some of the highest strength aluminum
alloys, particularly in the 7000 series, have
low elongation, or toughness, or resistance
to crack propagation. This is important for
overall strength and fatigue resistance. With
alloys exhibiting higher toughness less mate-
rial is needed to resist fatigue, and this can
result in a lighter bike. Like with any bike
frame material, good design and manufactur-
ing is much more important than a small
difference in a single mechanical property.
Are all 6061 frames the same?
While the choice of an alloy is important,
a more important factor is what you do with
that material. Our Wisconsin built frames use
the highest level of aluminum manufacturing
technology in the industry. To start, all tub-
ing is mitered with lasers so each tube is
exactly the right length, each joint better fit-
ting and stronger. Careful welding and full
solution heat treatment is followed by preci-
sion alignment, so our frames ride straight
and true. Without the costly attention to
detail we apply to every frame, even the best
geometry won't ride at its optimum.
What do the numbers mean?
When we discuss aluminum alloys, we refer
to a four-digit number. This is the alloy
name, based on the alloying materials in
the aluminum. A metallurgy reference would
explain precisely what elements are added to
the aluminum in a specific percentage.
The second part of describing aluminum
alloys is the heat treatment or other strength
enhancements which have been applied to
the alloy. With some alloys, special heat treat-
ments or work hardening are essential to
achieve their maximum strength. Other alu-
minum alloys attain their maximum strength
by simply cooling at room temperature, also
known as ‘normalizing’.
Since heat treatment adds extra steps to
6061 AND 7005 ALUMINUM ALLOYS