[VACList] Re: Mid-size SUV and tow vehicle dynamics

[jon fitz <fitzjo1@xxxxxxxxx>]

I don't have an opinion about the Durango either way,
but I have been pondering this issue for some time. 
If you hate technical stuff--delete this! :)

If you do a Free Body Diagram of the tow vehicle in
question, it is pretty enlightening.  (A FBD isn't a
political analysis--it's a technique taught to
first-year mechanical engineering students.)  Measure
the distances between the important bits (wheels,
hitch and trailer wheels).  Draw the system out like
simple beams with point supports.  Solve for forces
and moments...et cetera.  If this sounds confusing,
just think about the see-saws that acrobats use in the
circus.  

What becomes pretty obvious is that wheelbase and
*overhang* interact on a conventional ball hitch. 
(Overhang is what I call the distance from the rear
wheel to the hitch.)  If you have a long overhang
relative to wheelbase, the trailer can exert more
influence over the tow vehicle.  You can compensate
this in the vertical direction with a load
distributing hitch.  You CANNOT compensate it in the
horizontal direction with a conventional hitch. 
(Perhaps the Hensley does this, but I don't have one
to evaluate.)  The hitch sticks out like a lever
relative to the rear wheels.  It competes with the
front wheels for control of the vehicle's direction. 
The relative influence of the two is determined by the
ratio of wheelbase to overhang.  In this case, longer
wheelbase helps a lot, although you can get the same
benefit by reducing the overhang.

Semi's are very different because they don't have
overhang.  Wheelbase still influences handling, but
for different reasons.  There are good reasons to
shorten it--such as the ability to maneuver more
easily in tight spaces while backing up.

If you look at potential tow vehicles, I would suggest
looking at both wheelbase and overhang.  You may be
surprised to find that not all long-wheelbase trucks
have favorable overhang ratios.  If the ratios is
worse, I think that a longer wheelbase could still
prove inferior to a shorter one.

Interestingly, you trade performance in the forward
direction against the backward direction when you
change the overhang ratio.  A tow vehicle with short
overhang will need more side-to-side room to back the
trailer through a corner.

Jon in SC
68 Overlander