Couple quick answears on bars

[nealtovsen@yahoo.com (Neal Tovsen)]

--- Dan Bubb <jdbubb@ix.netcom.com> wrote:
> First off, composites properties are substantially
> different from
> aluminum 

Yes. Of course they are. I was merely trying to make
the point that you can't compare apples to oranges. An
aluminum bar wouldn't be designed the same as a steel
one. Nothing more than that.

> As far as design is concerned, comparing an equal
> weight of aluminum to
> steel in straight compression will yield almost the
> same deflection
> irrespective of cross section properties.

Yeah...but this project isn't dealing with much in the
way of compression forces. That probably only applies
to the extreme ends of the bar where it turns upward.
I would contend that either material will function
adequately in compression over a short length.

> In bending design can make a difference mainly
> because there is a
> practical minimum wall thickness before the piece
> will exhibit local
> buckling or is too prone to damage from unexpected
> impacts. In this case
> the aluminum piece can generally have a larger cross
> sectional moment of
> ineria making it correspondingly stiffer in bending.

Agreed (as far as I know anyway)

> The other consideration comparing aluminum to steel
> is the relative
> strength. Unless you compare the lowest grade steel
> to 7075-T6 or
> 2024-T3 aluminum, steel is always much stronger than
> aluminum. 

Yeah. Relative strength is the 1/3 bit you were
talking about, right? Agreed.

> In this
> case where the bar has a substantial bend and
> therefore relatively high
> stresses an aluminum piece will reach its yield
> point for a given load
> before a steel piece.

I didn't quite follow that one. By "bend", are you
talking about deflection, or the shape of the bar? And
are we comparing weight-for-weight, size-for-size, or
what? Again, let's make sure that comparisons are
framed properly.

> Perhaps when you use the word "compliant" relative
> to chromoly tubing
> the correct term should be "undamped"?

Well, that isn't really what I meant either. I meant
that a steel bicycle is more likely to flex more over
a given bump, where an aluminum bike wouldn't flex
much at all. I wasn't talking about the tendency for
either of them to oscillate, and I don't think that
really presents an issue in our application.

> There is a reason that no professional racing car
> uses an aluminum
> swaybar and this application is substantially the
> same.

I don't think the application is the same. A swaybar
goes through as much as 30 degrees of torsional
movement, and it's purpose is to act as a spring. Our
bar, though of course it has a spring value, is not
intended to act as a spring, and should experience
bending forces much more than torsional forces. And in
both, it will not experience much deflection. In fact,
the less deflection it allows, the better it will do
it's job.

> But, I don't want to influence anybody here. Please
> make your aluminum
> equivalent and let us know how it works in
> comparison to the steel
> piece.

No intent to get personal here. Just some friendly
debate. Hell, I've already admitted that I'm probably
not qualified to go very deep into this discussion! No
need to leave with the ball...

Neal

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