I have a Jeep with off road bumpers and the same mounting problems with
the rear as the OP. The 8 bolts go through holes that are on either side
of where the main frame rail joins up to the rear cross member. Both
frame and cross member are relatively thin at .104" The bolt holes are
1.25" on center vertically, 8" horizontally (spaced evenly on either
side of the main rail joint. This is NOT a solid item to bolt to for
straight pull, really flimsy for the torque loading of the motorbike rack.

I used some 1/4"x2" backer plates with radiused ends to match the curve
of the cross member in an attempt to get some of the load transferred
out to the edges of the cross member. That still didn't look good enough
so I ran more 1/4"x2" strap forward on both top and bottom, then drilled
all the way through the frame to bolt the sandwich together with 2 ea 7"
grade 5 bolts per side. Spacing on the through bolts was set to allow
standard tow hooks to be added on either top or bottom.

BTW: bumpers were 2"x5" rectangular tube, tapered ends, sealed to allow
use as air tanks. Rear bumper has intergal (through the tube) 2"
reciever hitch, front has built in winch mount. People have told me they
look like factory or aftermarket parts.

Wayne Cook wrote:

On Fri, 16 Sep 2005 05:02:00 GMT, RoyJ wrote:


I actually was working with some demo programs on Matlab, ran this one
as a test. I figured support on both ends (doubles the load), length is
not a factor (but degree of twist before failure is definately related
to length!) I'll generate the full table to finish testing but the
first pass said that the flat bar was NOT very good in pure torsion. Nor
is it very good in bending mode in the flat side axis.


I agree that it's not very good in those directions compared to the
weight of the bar. However is it good enough for the job is the real
question. Admittedly I'm just going by experience and gut feel but I'm
no where near as worried about it as I was other parts of hitch. There
was a lot weaker portions in this hitch when he started. It's much
better now but I admit that there's still weak points. I still feel
that the bolts or what they attach to on the frame will fail before
the flat bar.


Wayne Cook wrote:

On Tue, 13 Sep 2005 14:23:42 -0400, "wallster"
wrote:



"Wayne Cook" wrote in message
m...


On Tue, 13 Sep 2005 03:15:06 GMT, RoyJ wrote:



The 1"x4" steel bar has about the same polar section modulus (failure by
torsion) as a piece of 2"x4"x .120. It should bend at about 1500 pounds
at the point where the bike mounts. A couple of good chuck holes should
bend it.


Did you figure in the fact that it was held on both ends.


Another thing about the above I should point out now that I've got
more time is that if you look and any of the class 5 hitches out there
now days and you'll see something like 3" x .120" square tube or even
2 1/2" x .120 round tube for the torque member (and that's on the
better built ones I've seen many have smaller tube). I've not got a
table handy but I'd bet that we're in the same ball park strength wise
here.



Wayne, here's how the 1"x4" steel bar is mounted. Roy, i think it's strong
enough for my application, but i DO appreciate your input.
http://www.wallys-speed-shop.com/mounting_bolts.JPG
Also, I welded two 4'x6" 1/4" gussets to the L bracket.
http://www.wallys-speed-shop.com/new_gussets.JPG
here's the finished hitch... i 'm not taking it off anymore!!! (i smacked my
head on the jeep, left a nice cut!)
http://www.wallys-speed-shop.com/painted_gussets.jpg



That looks much better. Of course in any system there's always a
weak link. The trick is for the weak link to be strong enough to do
the job. You've now moved the weak link up to the bolts and frame in
my opinion. Most likely strong enough but keep a eye on the frame tabs
that the bumper is bolted to for a while just to make sure that they
don't start to deform.

A proper receiver hitch would be mounted under the frame for a
distance of about 12" apart for the bolts. This longer leverage helps
compared to the closer spaced holes in the bumper.

I hate to say it at this stage but just for the record I would of
probably of made a extension of the receiver and run it up under the
Jeep for about 12" farther forward than the bumper. Then put something
like a piece of tube or channel across the frame at that point and
tied the extension to the cross piece. This would make for a better
leverage and remove most of the torque on the bumper itself turning it
into more of a downward force.

A easier fix for you at this stage if any sign of problems crop up
would be to weld some brackets under the bumper in such a way that
they go up along the bottom of the frame for a ways and bolt them.

Wayne Cook
Shamrock, TX
http://members.dslextreme.com/users/waynecook/index.htm


Wayne Cook
Shamrock, TX
http://members.dslextreme.com/users/waynecook/index.htm