crossmember update (more pics..)

[treed2@u.washington.edu (T. Reed)]

> I dont see how those bolted items will help strengthen the crossmember. I
> also dont think the long plates will be strong in a lateral load. They
> will flex and in turn, put more stresses where the crossmember is bolted
> to the bar. think of where its bolted at the plates as a lever from the
> point from the strut tower. The front end will flex and in turn, transfer
> the load to the point where its bolted up at the tall pieces. Looks good
> tho.

Shawn,

That was my impression at first also, then I thought about it a little
more carefully and did some research -

First of all, the force vector acting on the crossmember points straight
down. So the long plates will be in tension, not compression, torsion,
bending or shear. They're thick steel and should be strong enough to hold
at /least/ 500 pounds. They do flex a small amount from side to side but
because the bar is designed to combat crossmember flexing in the vertical
direction, reducing horizontal flex was not a priority.

Even so, the brackets flex much less than they would appear to just by
looking at them. I tried bending one over my knee as hard as I could and
could not get it to deflect more than 1/16". I would not describe the
material they are made out of as sheet metal but rather "plate steel".
It's heavy duty stuff and the brackets weigh more than the actual bar.

The most stress will be at the center of the side brackets where the
horizontal and vertical legs meet. Here, the outer radius will be in
tension and the inner radius will be in compression. Look at the
pictures, though.. the bar is bolted at the very front end of the vertical
"legs" of the brackets. The "lever arm" is really only about 5 inches
long at most.

Your point about the lever was my biggest concern until I actually took
the bumper bolts out and discovered that they do not thread in to a nut
welded in to the sheetmetal "frame" like the front motor mount or a-arm
bolts. They thread in to the bumper supports.. So the weight of the
bumper is counteracting the lever action of the crossmember bar. Not to
mention the bumper bolts are of the high strength variety anyway, and are
quite large (M12x1.5). I installed longer bolts to make sure the
additional thickness of the brackets and spacers would not reduce the
number of threads reached by the bumper bolts.

Lastly, there is no way that the bar is going to place additional stresses
on the crossmember. Remember that it ties in to the crossmember in three
places, not just the two sides. Even if the frame flexing caused the
distance between the driver and passenger side bumper bolts to increase by
half an inch, the bar can serve only to strengthen the crossmember
laterally because it is tied solidly to the center of the crossmember.
If I grab the vertical leg on either side and push and pull on it rapidly,
the whole car shakes. It is very solid.

Anyway, I appreciate your skepticism - there's nothing worse than being
overly confident in a design and having it fail..

-Toby