JRscooby wrote:
But the design expects force applied upward from the suspension and tongue. That force increases and decreases, but the location does not change.
Nope!
The bad thing about gravity: it works everywhere.
The force is NOT induced just at the tongue and suspension. There is mass (force) induced into the frame at any point along the length of the house (as long as there is gravity).
Likely not linear (more mass close to axles, since water tank, fridge, etc are often placed there). But not always (and more than likely, the manufacturer uses the SAME FRAME for the various floor plans, sometimes with fridge in the rear, or in my case the heavy (when full) black water tank all the way at the rear of the chassis.
Now that (lets say 200 kg) weight WILL put a lot of dynamic force onto the chassis all the way at its end. Just for kicks using this number (published in a paper on speed bumps): "In one case, driving 32 MPH over a standard 1.5 foot long/roughly 4 inch high speed bump produced a maximum g-force of about 2.2.(gravity included). (Measurements were taken from inside the vehicle).".
So 200 kg mass with 2.2 g (rounded to 22 g/m2 acceleration) will give us 4400 N force (or about 880 lbs weight). That is the force JUST FROM THE FULL BLACK WATER TANK going over speed bump.
With that said: there is flex (any force even the smallest will flex a beam (unless it is infinitely stiff).
There is a lot of DYNAMIC force causing flex.
There is static flex.
There is DIFFERENT flex from model to model (assumption: manufacturer uses the same frame for various floor plans (and gives a hoot about the more or less flex).
Putting a jack at the corners CAN actually reduce the flex (compensating the static flex from the 'house' weight).
But that is all theoretical since I don't have the frames bending stiffness (and not even the frame's beam dimension to calculate the stiffness).
