[email protected] wrote:
> Matt O'Toole writes:
>
>> "If you're riding on an aluminum fork you should have replaced it
>> eons ago. Aluminum forks become mushy and flexible over time. I've
>> never thought aluminum is a good choice for a fork. The only time I
>> had a bike with one, I worried so much that I got rid of that rig
>> after only two months. The fork was way light and it was good on
>> rough pavement, but it seemed like an accident waiting to happen."
>
>> Would someone please give this "authority" a brief mechanical
>> engineering lesson? I don't have the requisite letters after my
>> name to make him listen.
>
> I agree on not riding aluminum forks, they being a springing element
> of the frame and aluminum doesn't make a good spring nor does it have
> a reasonable fatigue life in that function.
eh? aluminum frames are elastic and subject to all kinds of flex - how
is it that the great jobst brandt, fatigue expert extraordinaire,
doesn't seem to understand that wild speculation in one department is
negated by oversight in the other?
> However, aluminum or
> metals in general do not get mushy with use. Some get more durable
> from work "hardening" but the elastic modulus remains the same.
you are correct that elasticity remains the same, and that u.t.s can
increase from work hardening, but frames and forks are not subject to
work hardening in use. if you want to grasp at straws for property
changes, u.t.s can increase with age in aluminum systems because
precipitation hardening can continue at room temperature, but that's not
cold work.
and it's not "hardening" in quotes - the metal really does harden.
u.t.s increases in proportion as it does so.
>
> As I have often mentioned, in winter bicycle shops advanced sales of
> bicycles and tires because the frames got mushy and the tires needed
> aging. These myths belong in the trash can along with many other old
> saws of bicycling.
red herring.
>
> Hardening is a misnomer unless the term hard is qualified. The yield
> stress is increased in "hardening" but the springiness of the metal
> remains unchanged. A high speed steel drill has essentially the same
> elastic modulus as coat hanger wire of a DT spoke, the modulus coming
> from iron, the principal component of the metal.
it's true that the modulus for most steel alloys is pretty much the
same, but the "springiness" of the steel in terms of the deformation it
will withstand before onset of plasticity most definitely changes as a
material hardens.
you should go to a library and "refresh" your memory on the basics jobst.
