Re: Softail frames

Have you ever taken a flight on an airplane and watched the wingtips move up
and down? How do you suppose they make aluminum do that?

That said, I see Lightspeed isn't selling their titanium Unicoi anymore.
Maybe there just isn't a market for 1" of rear wheel travel.

-Dion

"juicemouse" <[email hidden]> wrote in message
news:[email hidden]...

Quoted message said:

I wouldn't go anywhere near one. Speaking as a mechanical engineering
student (but certainly not an expert/authority), aluminum's lack of a
fatigue limit seems to make it a lousy choice for a component that will
be springing back and forth constantly every time you go over a bump or
stand up on the pedals. Elasticity isn't what concerns me, as you can
just make the shape of the chainstay whatever you need it to be to
achieve whatever spring rate you're looking for. The big problem here
is fatigue, which is when a material that is repeatedly subjected to
varying stresses wears itself out and fails catastrophically. I've
never heard of an aluminum alloy that didn't exhibit this failure
mechanism. Now if either company shared some data which showed that
their 6069 aluminum or aluminum "based in scandium" exhibited a fatigue
limit, or at least wasn't subject to fatigue for many many cycles at a
realistic stress, I might be interested. Otherwise, these frame
designs seem better suited as disposable race bikes. Using aluminum
for a springy component like this simply because it's "affordable" and
"lightweight" seems like a terrible idea for most users.

Carbon fiber reinforced plastics, on the other hand, don't have fatigue
problems (as far as I know) when they're constructed properly. Another
plus is the ability to fine tune the performance of the component by
fiber orientation and ratio of fiber to resin. I think it's
interesting (but not necessarily meaningful) that Cannondale, with
their considerable aluminum experience, chose carbon fiber for this
application. It's definitely not the cheapest route, but it'll get the
job done over and over and over again.

By inspecting them obsessively every flight, and replacing them at
regular intervals which are calculated based on how much fatigue the
components have accumulated (with a good margin of safety). We learned
about this in school, too. I also know about it because I'm a pilot,
and the preflight inspection and keeping up with regular preventative
maintance is key to flight safety. Aluminum is commonly used in this
application because of the fact that it's a lightweight engineering
material, an extremely important property in aircraft. Also keep in
mind that the most popular commercial airliners were designed decades
ago, and composite construction just wasn't viable back then. It's
interesting to note that composite structures have been creeping into
these aircraft as they have been retrofitted with new parts over the
years and as new models of the same basic design are built. Also
interesting that the newest Boeing airliner, the 787 (previously known
as the 7E7) will be built of an almost entirely composite structure
(although I'm not sure what structural components are not composite,
such as the wings). Check out this link, especially the comments made
down at the bottom of the article:

http://seattlepi.nwsource.com/business/207474_fuselage12.html

On Tue, 19 Apr 2005 09:11:10 -0700, "Dion Dock" <[email hidden]>

Quoted message said:

Have you ever taken a flight on an airplane and watched the wingtips move up
and down? How do you suppose they make aluminum do that?

They do it reluctantly, because there's no other material with a
better strength/weight ratio that's economical to use, and they
require *frequent* inspections for cracks. Bike frames don't have to
be subjected to such flexing; there are other, better solutions to the
problem of providing rear bump compliance. In aircraft, getting all
of the flex out raises the weight too much.

Quoted message said:

That said, I see Lightspeed isn't selling their titanium Unicoi anymore.
Maybe there just isn't a market for 1" of rear wheel travel.

That's probably the biggest factor.

Remember, anytime you're flexing a part that's not appropriate to
subject to such stresses, you're shortening its life. Assumptions
have doubtless been made about how much the frame can withstand in
that regard; it remains to be seem whether those assumptions were
valid.

Given the small benefit vs the large risk, I find this feature lacking
in virtue.
--
Typoes are a feature, not a bug.
Some gardening required to reply via email.
Words processed in a facility that contains nuts.

--On Tuesday, April 19, 2005 9:41 AM -0700 juicemouse

Quoted message said:

By inspecting them obsessively every flight, and replacing them at
regular intervals which are calculated based on how much fatigue the
components have accumulated (with a good margin of safety). We learned
about this in school, too. I also know about it because I'm a pilot,
and the preflight inspection and keeping up with regular preventative
maintance is key to flight safety. Aluminum is commonly used in this
application because of the fact that it's a lightweight engineering
material, an extremely important property in aircraft. Also keep in
mind that the most popular commercial airliners were designed decades
ago, and composite construction just wasn't viable back then. It's
interesting to note that composite structures have been creeping into
these aircraft as they have been retrofitted with new parts over the
years and as new models of the same basic design are built. Also
interesting that the newest Boeing airliner, the 787 (previously known
as the 7E7) will be built of an almost entirely composite structure
(although I'm not sure what structural components are not composite,
such as the wings). Check out this link, especially the comments made
down at the bottom of the article:

http://seattlepi.nwsource.com/business/207474_fuselage12.html

Actually, the inspection cycle for airplanes is designed around the crack
propogation properties of the particular alloy. Engineers say "this big a
crack is a failure," figure out how long it takes such a crack to develop
from microscopic, and use that as the time between inspections. Thus, a
crack that begins to develop immediately after one inspection reaches
critical size immediately after the next inspection, where it would be
replaced.