HEY: All you PMF RSK guys look here!!!!

[Tom S]

The way I see this putting the spring mount up front means the axle is now being located at the end of the frame away from a cross member. It also means that the degree of drop in the hanger up front forms a point to put leverage to want to twist or bow the ends of the frame.

 

[zilla68]

all my pmf stuff showed up 2 days ago, looks really nice, I dropped it off at powder this morning and should be done by Monday.
Got my new toyo mt's mounted on my newly polished welds that I had, boy did the polisher do a great job, they never looked that good, even new

 

[backwoodsboy]

So, what does moving the shackle to the rear have to do with stresses on the front spring mount and crossbar?

A single point of rotation (fixed spring hanger) creates a moment about that mounting location which is not present with two points of rotation (shackle).
I'll admit, I haven't drawn out any free body diagrams of this scenario in a few years, but if my memory serves me correctly there is any increased stress in the form of a moment at the fixed location.
I'm happy to debate this...it's definitely possible that I am mistaken.

 

[old man dave]

A single point of rotation (fixed spring hanger) creates a moment about that mounting location which is not present with two points of rotation (shackle).
I'll admit, I haven't drawn out any free body diagrams of this scenario in a few years, but if my memory serves me correctly there is any increased stress in the form of a moment at the fixed location.
I'm happy to debate this...it's definitely possible that I am mistaken.

The question would be is the increase of the now pivoting moment relatively substantial compared to the normal holding up the front of the truck, not if there is an increase of "stress" at the front pivot point.

Say, if there is 1000 pounds of truck weight at the front spring mount, is the increase 20 pounds or 500 pounds of force in addition to truck weight at this point with a RSK?

I wouldn't care about 20 pounds but 500 pounds would be a big increase, assuming everything in the same vertical plane.