Couple quick answears on bars

[jdbubb@ix.netcom.com (Dan Bubb)]

First off, composites properties are substantially different from
aluminum and the same things cannot be said when including them in a
comparison between aluminum and steel.
Composites have stiffness higher than steel and weight lighter than
aluminum. Composite stiffness to weight ratio is much greater whereas
steel and aluminum are approximately equal. I'm not sure how they got
brought into this discussion although they would be excellent in this
application.
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. For an equal weight and
aluminum piece will have three times the cross section as the steel
piece and since its stiffness is 1/3 per unit area its deflection will
be the same.
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.
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. 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.
Perhaps when you use the word "compliant" relative to chromoly tubing
the correct term should be "undamped"?
There is a reason that no professional racing car uses an aluminum
swaybar and this application is substantially the same.
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.
Dan

Neal Tovsen wrote:
> 
> Ahhh...the same sorts of things could be said about
> advanced composites. Yet F1 cars use aluminum and
> composites more than steel, because that logic doesn't
> take design into account. Comparatively oversized,
> thin-wall aluminum tubing can be both stiffer and
> lighter than steel. Autotech does this with their
> strut tower bars. The concept was pioneered in the
> bicycle industry by people like Cannondale.
> Cannondale, Trek, and similar bikes are STIFF and very
> lightweight. Conversely, I've seen (and ridden) some
> cheap aluminum bikes that had steel-like tubing
> diameters with thicker walls to compensate. As you
> suspect, they weren't any lighter than a decent steel
> bike.
> 
> I think the stiffness of a well-built aluminum bar
> would actually work better than steel for this
> purpose, in addition to being lighter. Chromoly bikes
> can be made to be almost as light as aluminum, but
> they tend to have much more compliant (some would say
> "spongy" or "springy", others would say "lively")
> characteristics. Aluminum bicycles are often
> characterized as being too stiff. Well, that't good
> for us. We want a bar that is RIGID.
> 
> If I only had a TIG welder...
> 
> Neal
> 
> --- Dan Bubb <jdbubb@ix.netcom.com> wrote:
> > Aluminum really wouldn't be a good choice for this
> > part.
> > Aluminum is ~1/3 the weight of steel, but it's also
> > <1/3 as stiff as
> > steel. So, the weight to stiffness ratio is roughly
> > the same and you'd
> > have to really bulk up the aluminum part to make it
> > stiff enough to make
> > it worthwhile.
> > Dan
> >
> > Bob wrote:
> > >
> > > Stock  is 3/4''with .125 walls this can be changed
> > if people want to cut
> > > weight, haven't got a weight yet
> > > Kicking around the thought of a handfull of tig
> > welded aluminum ones.
> > > Price still haven't got that figured yet
> > > Mounting to the same place as Kevins with a plate
> > on top to spread  presure,
> > > will come with all bolts.
> > > Hope to have all things figured out tomorrow
> > > Bob
> > >
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