Huntindog wrote:
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The few lines of surface welds induce some heat stress to the beam, which then bends up. This helps to end with a someone flat beam when load is applied and axles put underneath to push up.
It does NOT add any strenght. It only changes the bending line (crooked before weight, somewhat raight when loaded 'dry', somewhat crooked again when additional load is added (full tanks, food, storage, stuff on the rear bumper, stuff on the A frame, etc).
There may ONE very special/rare load condition when this beam is STRAIGHT. For practical terms, it will NEVER be straight. The amount of bending depends ONLY on a) the stiffness of the frame (fixed, and NOT influenced by the pre bending) b) the load and load distribution.
Lifting at the 2 rear corners (the front is already lifted at the corner, the A frame (front jack) lifts the 2 front corners, and all the static bending is/has taken place every time you put your front jack down, or hook up your trailer to the car), will add equal (but in reality less) than the normal static flex. Less, because A the axle is not in the middle of the lifting point and be the load is heavier on the front section ('tonque weight').
So lifting on the rear corners will ADD static bending (in the opposite direction. NO QUESTION.
The AMOUNT of bending could be the range (but opposite direction of the 'empty load' (the weight of the chassis itself, plus weight of the empty 'house'). In my case: 4400 lb weight empty, assumpe 2200 on 'rear' half. Lets call this the 'flat static' (assuming pre bend frame to make it 'flat' with this load). Now adding 120 gal of fluid (black, grey , fresh water) (on earth, roughly 1000 lbs) near the rear bumper. That ADDS 50% to the static bend.
Now we bounce it over a speed bump at 3 g. That is now an additional 2 g or 2000 lb near the rear bumber for a total of 3000 lbs. That is 1.5 TIMES the bending amount of the static 'assume flat with pre bent beam).
Now if we push UP the rear end of the empty trailer, that is 2200 lbs. This will result in roughly 50% LESS bending as the 'bump' with full tanks'.
Ditto, if the trailer is full (here: 7500 lbs, or 3750 on the rear portion), if we lift the rear, the bending is about equal to the amount exerienced during every 3G bump no the road (but only done once every time to jack it up, so say one per vacation, vs xxxx during driving).
Just some perspective on the issue.
