Front crossmember stress bar

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

>
> Does "stiffness" correlate directly to something like
> "load-bearing capacity" (my non-engineering term
> here)? Just wondering...Anyway, a 22% increase in
> stiffness of the front motor mount is still IMHO
> pretty substantial.

If one piece is 5 times stiffer then it will deflect 1/5 as much under load.
Since the crossmember and stress bar are constrained to deflect the same
amount since they are bolted together at the ends and where the load is
applied the crossmember will carry 5 times the load. So, yes, the ratio of
the stiffness of the two pieces is proportional to the load they will carry.
The "load bearing capacity" is a function of the physical design and the
load applied. i.e. I define "load bearing capacity" as the ultimate load
limit. Hopefully the chassis structure will not come near this limit.

Now that you've made me think of  it and just to expand on my belief that
the cracks are due to non-maintained/misaligned engine mounts; I don't
believe the weight of the powertrain is enough to approach the load bearing
capacity of the front crossmember. And as was pointed out by several people
on the list, engine torque is an UPWARD force on the crossmember. So, that's
not causing what appear to be cracks from a downward load.
So, if the mounts are not correctly aligned, the front mount is carrying a
larger load than it's design load, AND it's transmitting more vibration
since the mount is compressed!
A vibrating load whose median value is lower than the load limit can still
cause cracking and failure due to fatigue, especially if it dwells on
resonant frequencies for any length of time.
Well, just my .02
Dan