Thanks for your concern - I hope you decide teach us the other forces you have in mind. I supposed we were all here to share and learn.
I'm not certain which forces Wolfman was thinking of, but his statement is correct. Dynamic forces such as acceleration and static forces such as leverage all play a role in the loads placed on the arms.
Yes, you can apply a weight, evenly distributed and come up with a maximum static load that the arms can overcome "at the pins". But a bucket or pallet load is not applied at the pins. The centre of gravity is at some point beyond the pins and that additional leverage means additional force required to lift the same weight. Even if the CoG is centered on the pins, the forces applied on the arms varies though the lift range, as the travel path is a curve and because the angle of the cylinder to the arm changes as the arms raise.
Leverage has another effect too. Consider a simple balance of a single, horizontal straight lever, centered on a fulcrum. To balance a 500# load at 1g on one end of the lever requires a downward force of 500# at 1g on the opposite end. But the fulcrum does not see 500# -- it sees a downward force of 1000# plus the weight of the lever itself.
Of course, in the case of a loader, we are not considering a single lever, but rather two arms. As the centre of your beam deflects due to the applied load between them, twisting forces are applied to the arms. Weight evenly distributed across the full width of a bucket applies minimal twisting force compared to a concentrated load applied at a point equidistant between the arms. (This may have been Wolfman's concern here -- same weight, but concentrated rather than distributed)
And that's just the static side of the equations. A tractor isn't of much use just sitting there doing arm curls. We want the tractor to move that weight from one place to another which introduces dynamic loads. When you hit a bump on the travel path, the load is no longer against 1g -- the vertical acceleration increases as the front tires hit the bump, reduces as the front tire passes over the bump and increases again when they get to the bottom of the bump. The arms, the towers, etc. etc. down to that single little bolt holding the front axle to the tractor all have to withstand a multiple of the static load.
The trap that some have fallen into is to take the measured maximum lift at the pins and consider this to be the capacity of the loader bucket (or pallet forks). That's more of an issue for somebody like me, that is building a loader from scratch. But we have seen many a thread to the effect of "Kubota says this can lift 750 lbs, but I couldn't lift a 500# pallet off the truck bed until I cranked up the pressure with shims".
In your case, Kubota's engineers already did the math for that bucket, full of dirt, on those loader arms. You are simply altering the attachment point in a manner that does not substantially change the distance between the bucket and pins. As long as you stay within the design criteria, I don't foresee any problems. EG: do your test using weight distributed across the bottom of the bucket, or at least use 2 chains, one near each arm and each lifting 1/2 the weight load. But you will need to be mindful of this and derate the lift capacity accordingly if and when you build pallet forks.
