Well, I happened to be one of those idiots that when my primary field of
interest had the bottom drop out (wanted to be an aeronautical engineer, but
then all those jobs dried up and died) I decided to get my mitts into
everything (Drafting, CAD, Electronics, Mechanics, Lasers, Economics,
Political Science, Desktop Publishing, Computers<where I ended up> and such).
I my drafting/engineering courses we did some work with finding the center
of gravity (CG) of vehicles. I'll use "basic" CG logic cause that's all I
ever learned. If any of you engineers out there wanna give the advanced
version, go ahead. Just an eyeball guess, but I would place the CG of a Dak
somewhere near the back of the cab or front of the bed. With this in mind,
think about the weight of the shell. All weight past the CG of the Dak goes
proportionately increasing onto the rear axle until you reach verticle center
over the axle and then it continues at 100% and slightly increasing with
distance from the axle rearward.
These are force amplification factors. If you opened your tailgate and put
your 80lbs of sand on it, to the rear axle it would actually weigh a bit more
than 80lbs because you drop the springs and shift the CG rearward forcing the
axle to act as a fulcrum, lifting the front of the truck and proportionately
adding weight to the rear.
All this being said, a shell does distribute it's weight more evenly, so
you are correct there. I disagree with your appraisal of 80lbs of sand over
the axle and a 150lb shell being "lighter" because it's weight is distributed
over the box. Since the shell is all past the CG of the Dak, the minimum
weight cancellation is 50% at the very front of the shell (at CG the weight is
split over both axles: thus 50%). This weight increases dramatically with
angle (vector) to axle. Why use 150lbs? It takes 3 people to comfortably
lift the shell on my truck, it's mostly glass. The installers used a
forklift. It's not light. Again, just rough guessing it, I would say that
that shell puts about 100-130lbs of dead weight directly onto the rear axle
which is more than the 80lbs of sand. You can flush this logic if you want,
but I'm just trying to keep that Newton fellow happy. ^_^
I can't disagree with your 300lb MIL though... but it would have to be one
tough rocker, and driving the Dak would be a bitch with that constant shifting
(rocking) weight load.
Shaun H.
"Just don't rock too far back! It's hard to steer with the front wheels off
the ground!"
----original message----
A shell isn't heavy enough and it's weight is evenly distributed over the
box, not the rear wheels. Now, if mother-in-law weighs 300 lbs, simply put
her rocker between the rear wheels and place her in it.
Rob Agnew
ragnew@islandnet.com
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