Steel
-cheap, most common
-very good durability
-easy to weld(cheap production cost)
-flexier.  The ride is more forgiving, but wastes more energy.  I usually
recommend someone who is below 180 to ride a steel frame
-unless the frame is very expensive, it's usually heavier than aluminum
-will rust

Aluminum
-slightly more expensive than steel, but coming down in price
-some people question durability, however, frames have lifetime
warrenties, so you shouldn't let this hold you back
-slightly harder to weld/bond, but can be done
-very stiff.  The ride is rigid, power can be delivered more directly,
however, the ride is harsher
-usually lighter than similarly equipped steel bike
-come in all sorts of cool colors and won't rust when exposed to the
environment(Note, aluminum will oxidized, but this is differen't than
rust)

Titanium
-one of the most expensive frames, good stuff
-durability is very good, assuming you find the right builder
-it's very hard to bond, if the builder is experienced, the frame will be
great.  If not, you might be in trouble
-some frames can be made to have the horizontal stiffness of aluminum but
with the vertical flex of steel frames, making this one of the more
desirable material.  However, if not done properly, the frame can feel
dead, much like any other frames
-usually about the same weight as the aluminum frames, sometimes a bit
more, sometimes a bit less

Carbon Fibre
-very expensive also
-durability is ok.  The fibres can unwoven/break microscopicly by each
bump.  At the end, the unwoven spot will fail by breaking.
-it's usually molded
-some people find these frames flexy.  However, some manufactuer will use
carbon main frame only with al lugs and rear triangle.  CF tends to be
flexy at times, however, like any other material, it can be made to be
very good.  
-usually very light.

I'm sure a lot more people will add stuff/correct me.  Please do!

John Stevenson [johnstev@magna.com.au]
Preamble: there are no bad materials, only bad applications. Almost any
material you can think of can be built into successful mountain bike
frames, providing the engineer who does the design work knows the
strengths
and weaknesses of the material, how to use and compensate for them, and,
most importantly, how to translate those concepts into production
processes
that build reliable bike frames.

All materials are available in different grades, with stronger grades
usually being more expensive. Plain carbon steel, as used for department
store junkers, has an ultimate tensile strength of about half that of the
heat-treated alloy steels used in very high-end steel frames.Stronger
steels make for more expensive frames because, while the raw material is
relatively cheap, its very strength means that shaping it into tubes,
cutting and joining those tubes is more time consuming and hence costly.


Price ranges

Rather than saying a material is/is not cheap, why not do it like this:

Bike cost ($US):      0       500     1000    2000    4000    8000
                      |       |       |       |       |       |
Frame materials:         <------steel------>
                          <---aluminium------------>
                                         <--------titanium---->
                                        <----carbon fibre------>

and so on. You might like to get the right US price bands, as I don't have
easy access to that info

>Steel
>-cheap, most common
>-very good durability
>-easy to weld (cheap production cost)
>-flexier.  The ride is more forgiving, but wastes more energy.  I usually
>recommend someone who is below 180 to ride a steel frame

Bzzzt! 'wastes more energy' definitely unproven, probably unprovable,
probably untrue. There have been some very long, tedious and circular
discussions of the whole issue of the actual effect of frame flex on
efficiency in r.b.tech. Given the calibre of the minds there that have
failed to reach meaningful conclusions, I think it's safer for you and me
to leave this one alone rather than repeat folk misconceptions. I will say
that it's very hard to imagine that, say, Henrik Djernis could have ridden
steel frames to three world championships if they wasted any significant
amount of energy. The fact that world titles and world cup races have been
won on virtually every frame material under the sun implies that the
differences between materials as far as energy transfer efficiency goes
are negligible at best and nonexistant at worst.


>-unless the frame is very expensive, it's usually heavier than aluminum

Have you actually weighed a low-end aluminium frame recently. Some of them
are getting very heavy

>-will rust
>

Unless looked after.

>Aluminum
>-slightly more expensive than steel, but coming down in price
>-some people question durability, however, frames have lifetime
>warrenties, so you shouldn't let this hold you back

A broken aluminium frame waiting for a free replacement is still a bike
you
can't ride. Bit of a bummer in June.

>-slightly harder to weld/bond, but can be done

Actually, quite easy to bond, which is why it's often done in preference
to welding.

>-very stiff.  The ride is rigid, power can be delivered more directly,
>however, the ride is harsher

It should be said here that this is a property of the application not the
material. Aluminium is more flexible than steel and has the annoying
property of lacking a definite cyclic stress fatigue limit. This means
that however small the repeated stress it is subjected to, an aluminium
part will eventually fail because of metal fatigue. A steel part, on the
other hand hand, has a cyclic stress level, below which it will last
forever. To get round this, designers build aluminium frames so their
cyclic stress levels are as low as possible to maximise their lifespan. In
practice this means using large-diameter, thin walled tubes, which also
happen to be light and rigid. In theory you could also build very light
steel frames by using thin-walled, large diameter tubes, since the fatigue
limit stress of good steel is usually higher than that of the aluminium
alloys typically used in bikes. However, thin-walled steel is hard to weld
and tends to be rather easy to crush. Aluminium's lower density means it's
tubes are thicker-walled and less prone to crushing.

>-usually lighter than similarly equipped steel bike
>-come in all sorts of cool colors and won't rust when exposed to the
>environment (Note, aluminum will oxidized, but this is differen't than
>rust)

In certain environments, particularly salty ones, aluminium is prone to
corrosion. Certain aluminium alloys must be painted or they will corrode
on contact with air.

>
>Titanium
>-one of the most expensive frames, good stuff

Actually the quality now varies almost as widely as steel and aluminium.
There are some quite inexpensive titanium frames on the market, but they
tend to use lower grades of material than the 3 percent aluminium, 2.5 per
cent vanadium alloy that's used by quality builders such as Merlin and
Litespeed.

>-durability is very good, assuming you find the right builder
>-it's very hard to bond, if the builder is experienced, the frame will be
>great.  If not, you might be in trouble

Gary Helfrich will be along in a moment to claim that this is bollocks and
that titanium is only slightly harder to weld than steel and much easier
than aluminium. Also you're confusing welding with bonding, two totally
different processes.

>-some frames can be made to have the horizontal stiffness of aluminum but
>with the vertical flex of steel frames, making this one of the more
>desirable material.

Sorry, but this is just plain bollocks. Bike frames are damn nearly
perfectly rigid in the vertical plane, whatever they are built from.
There's no doubt that there are material/configuration factors that affect
how a frame feels, but these are probably to do with the way the frame
dissipates or transmits vibration, rather than the Young's modulus of the
material.


 However, if not done properly, the frame can feel
>dead, much like any other frames

I've never understood what people mean by a 'dead' frame. I suspect it's
another piece of bike culture folk bollocks.

>-usually about the same weight as the aluminum frames, sometimes a bit
>more, sometimes a bit less
>


>Carbon Fibre
>-very expensive also

Priced a Giant carbon fibre bike recently?

>-durability is ok.  The fibres can unwoven/break microscopicly by each
>bump.  At the end, the unwoven spot will fail by breaking
>-it's usually molded

Er, half. Carbon tubes are usually molded, but are then joined into frames
by bonding into lugs. Trek, Giant use this process and they must account
of the majority of carbon frames out there.

>-some people find these frames flexy.

Some people can convince themselves of anything in order to pander to
their prejudices. Which isn't to say that carbon frames aren't flexible,
no doubt some are, but this is just more repetition of 'lots of people say
this so it must be true' bike folklore bollocks.