Crossmember Bars POLL

[nlowe79 at gmail.com (Nate Lowe)]

On 6/18/05, Allyn <amalventano1@tds.net> wrote:
> thats not how metal works. either you take it past its yield strength or you
> dont. either it bends or it doesnt. its that simple.

>From my materials engineering textbook: "Fatigue is a form of failur
that occurs in structures subjected to dynamic fluctuating stresses.
Under these circumstances it is possible for failure to occur at a
stress level considerably lower than the tensile or yield strength for
a static load." Further on it says: "Fatigue is important inasmuch as
it is the single largest cause of failure in metals, estimated to
comprise approximately 90% of all metallic failures."
So yes, exceeding the yeild strength is one way to bend a metal, but
certainly not the only way. It's not that simple, that's my whole
point with this. Do you think the crossmember is breaking due to being
pushed past it's yeild strength? I don't, because if that was the case
drilling holes would do no good and the entire crossmember would break
in half before you even noticed the cracks. The crossmember is
cracking due to fatigue.

>that said, it would take a much sharper 'V' in the bar to generate
enough side load under
> tension (as the 'V' tries to flatten, spreading the ends apart), to make
> those steel plates bend. the steel span will likely bend first, and even
> then, it is sufficiently long enough not to bend beyond its yield point
> (i.e. it will flex, but return to normal once the stress is relieved).

I don't think the span from the motor mount to the bracket would bend
first, but it certainly is a possibility. As for how sharp the V needs
to be, that's only a matter of uninformed opinion until a closer
examination has been done. I agree that it is very unlikely for enough
side load to occur to bend, but again, it's fatigue that really
matters here.

> even with the curve, there is still a couple of inches of 1/4" steel plate
> that is in-line with the direction of force. this part of the bar alone is
> many times stronger than the rest of the bar.

Good, now I know how thick the plate is. It didn't look that thick in
the pictures. But if you look at the geometry of that plate, the
further down that V attaches the larger the moment you are applying to
that plate becomes. This is what I was considering when I said that
looked like the weakest part of the bar to me.

> you're missing an inportant
> point - if you end up putting so much stress on the bar that it ends up
> bending, in turn placing stress on the cross member, that much stress would
> have had to cause the factory piece to severely fail in the first place.

Yes, but they can continue to fatigue together.

> that is the only way enough movement can occur to bend the bar (which is
> more flexible due to its narrower diameter). if all of this was to occur,
> the bar (bent as it may be) would be the only thing providing any kind of
> support for the front motor mount, and it would be placing no pre-load on
> the cross member at that point, because that member had to fail to allow the
> movement in the first place. it would be a flimsy cracked piece of junk. in
> that situation, i'll take bent metal over cracked metal any day.

This is assuming the bar alone could do the job of the crossmember, I
would be surprised if that was true.
 
> 
> it could be argued that everything could use some form of improvement. the
> issue with this bar is that in order to keep it (and its installation)
> simple, compromises must be made. sure you could go nuts and make a
> triangulated mini-subframe down there, but it would be much larger, would
> interfere with the rest of the engine bay, and would likely be way more
> expensive to produce. it could even be made without the curve, but you would
> have to re-route your harness/power steering hoses, which is not what eric
> was going for - he wanted a drop-in installation. in my opinion, this is the
> best/easiest thing out there right now.
> 
> Al

I would agree that it's the best thing out there right now. And making
it a drop in installation is a great plan. Of course compromises must
be made, that is the way of the world. What I'm trying to say is it
would be nice to know that this bar isn't going to fail in 5 years. I
don't know that it will, but I also don't know that it won't. It looks
pretty sturdy to me, but looks can be deceiving. If the bar needs
improvement, then I bet all of us here could work together to make
some small changes that will insure the bar will not fail for the
forseeable future and to make sure that the bar stays a simple bolt on
product. Again, that's only IF it needs improvement.
Nate