Why choose aluminium in a design instead of steel?
--------------------------------------------------

By far aluminium's biggest selling point as a material is its density
(weight). It comes out top of the table in this important property.
0.098 lb/in3 compared to 0.162 for 3/2 S titanium and 0.283 for steel.
So, if aluminium is one third the weight of steel, why aren't the frames
one third the weight of steel frames?

the reason is modulus (stiffness, remember?) Aluminium is roughly less stiff
than steel by the same amount as it is less heavy, aluminium 10 (106 psi)
compared to steel 28.5 (106 psi). Another property that levels the playing
field is strength. 6061 T6 aluminium (I'll get to the different aluminiums
later) has an ultimate tensile strength of 45,000 psi, which, compared to
an average CrMo steel strength of 160,000 psi, has a lower yield strength
of 40,000 psi. This brings us to fatigue strength, not a big advantage for
aluminium. It has no fatigue limit, meaning that any and all stress put
into the material will eat away at its life (remember that steel and
titanium won't fatigue below their fatigue limit). Its failure mode is
pretty drastic as well. Remember those yield and ultimate strengths that
are so close together? Not much room between when it yields (deforms) and
when it will break, so failures are more often breaks and thus a deal more
dramatic than just bending...

But aluminium's best attribute, density, comes to rescue it from its
shortcomings of fatigue strength and low modulus. Its density allows us to
apply more materials to the problem without a big weight penalty. By over-
sizing the tube diameter and/or directing its profiles to the problem areas,
you'll overcome the material's low modulus and stiffen things up. By
stiffening the structure to the point of no flex, you overcome the fatigue
problems.
This is why aluminium frames are generally the stiffest frames around, but
also the reason why there are design limits.

To use what kind of aluminium?
------------------------------

There are three main classes or series of aluminium used in the construction
of cycle frames: 7000, 6000 and 5000. These numbers are for identification
only, and have nothing to do with 'rating'. They simply have different
alloying elements and thus different characteristics. The major alloying
elements for the various series mentioned are 5000: magnesium, 6000: magnesium
and silicon and 7000: zinc. The most common 5000 series tubing is 5086.
This is what Columbus Altec tubing is. The high magnesium gives it good
corrosion resistance and it has good weldability.
It can't be heat treated, and this is where a division of opinions arises.
Proponents say this alloy is very tolerant to welding and has good post-weld
strength without artificial ageing (this can't be done to it anyway).
Opponents say the lack of post-weld heat treatment or thermal ageing means
it doesn't regain enough strength. My experience is that with good design,
5086 can make a good frame.
The most common 6000 series is 6061, usually with a T6 thermal treatment.
This was THE aluminium tube, because it's the most used structural aluminium
in the aerospace industry, so is available in a great many sizes and shapes.
Tough, weldable and with a good strength (for aluminium), its major drawback
for small builders is that it must have a complicated post-weld heat treatment
This is usually done outside the builder's facilities and can run into
problems with alignment and control of the process. And get it wrong, and
things could get scary out on the trail or on the road.
This heat treatment process renders the frame virtually irreparable in the
normal manner, welding, because the repair would wreck the heat treatment
and the repair area cannot be heat treated alone again. The 7000 series is
gaining in popularity, as it has all the usual benefits but requires ageing
only after welding to bring its temper back. This can, in most cases, be
achieved at ambient temperatures over days or weeks or be done artificially
in an oven in a matter of hours. It can also be repaired. Excellent examples
of the 7000 series are Pace and Easton frames.

Aluminium's limitations
-----------------------

So as one of the major framebuilding materials, why isn't aluminium used by
custom builders more? Its limitations mean that any design must be worked
out carefully. To get the most out of the materials, this leaves little
room for changing things for the customer's desires or whims. Aluminium also
needs processing and manipulation not found in traditional framebuilding. As
a material, aluminium lends itself more to a limited production process.
Aluminium delivers a certain type of ride that doesn't suit everyone. Some
love it, some hate it... and that's what makes the world interesting.
But with the growth in the use of suspension systems, you can probably expect
to see more aluminium bikes. Its light weight and super rigidity is a good
platform for full suspension to work from, and the double butting now coming
into aluminium may help to overcome some of its problems.
But as far as longterm strength goes, forget the labs. Let's see what happens
when these things are in the hands of the hard riding consumer for a few years