I wish I could explain this in a sentence or two but I never can sorry for the long winded answer and I hope it makes sense complicated topic structures designs takes years to be good at…..
Before we can understand structure we need to understand loads and weight distribution, three different things but very dependant on one another. If you understand them your designs will do much better.
Ideally in the new design R&D phase, which the aftermarket XP900 still is, loads are developed initially by empirical(existing from other models, past experience) data to develop prototype test units, in some cases several to test inconsistencies. In all industries, the first delivered units are field tested, re-doing loads & redesigns can occur years later. I have been involved and am currently doing cost redesigns of structure 10-30 years old. We all make mistakes especially when there is pressure to get a product to market based on competition or other red tape like SAE.
In the case of the front arms, load comes into the tire patch based on caster, camber, toe alignment. I like to think of load as water flow through a hose that does not like kinks or bends, and at every juncture there has to be structure to make a smooth loads transfer. I mean through the arms, chassis, from the arms to bolts, bushing’s, etc. With that said, the lower arms see the highest loads since they are closets to the tire patch, that is why PoPo put twice as many chassis lugs there than top that are too thin. So if one were to strain gage the upper/lower lugs of the same geometry and found that upper sees half the loads, the lowers are just as susceptible to failure.
Basically loads have several components at any joint or juncture, up/down(vertical), forward/rear(horizontal), in/out(side), and resultant loads or forces. It really does not matter how your orientate the loads axis the resultant load will be the same. Structure can then be designed to take the loads out or not depending on the type of load @ the joint(i.e.: chassis lugs) are seeing(tension(how cracks occur), compression, shear, bearing, fatigue, etc…..
Different shape structure takes out different loads. Flat plate you will see in DF’s design takes it out along or in the same plane as the width/length directions, not that well the thickness direction in bending. If the plate were square it takes it out equally in width and length. Now you can look at DF’s rectangular plate and see what it’s doing to add strength in what direction. Tube is good or better at taking out vertical, horizontal, side loads, best along its length.
If you increase the distance the load is from the center of gravity of certain structure or car by running widening kits it multiples the load(bending moment) by that distance(i.e. + 6) and acts on all structure to a degree.
We usually pad up loads and structure for a margin of safety, once proven by field testing a weight reduction effort reduces the impact to structure.
I leave you guys with that to think about while I'll try and get a hold of XMF today then we can take a closer look at the structure.