Shear loads are applied in a manner that is literally trying to cut the material in half (think scissors) and is most often used in highly stressed areas. As a rule of thumb, shear loads are considered to be 60 percent of the material's tensile strength. For example, if a part has a tensile strength of 10,000 pounds, then the shear strength of the material would be 6,000 pounds.
Axial load refers to a heim joint and is the load applied along the bearing bore axis (where the bolt goes through) commonly seen when using the spherical part as a ball joint.
What size material should you use for a particular application? I can't tell you, it is too hard to say specifically what size material/part to use. After a quick call to my buddy, Jack at hm-engineering.com, we netted some spherical bearing (heim joint) information, and the rest of the info was found out on the webbernet. Let's take a look at some commonly used parts and material.
I know... What about the right size material to use on my frame? The best I can say is to look at what the vehicle came with stock and think similar. If the frame was 2x5-inches x.120-inch C-channel, then it is reasonable to think that you could get away with 2x3-inch x.120-inch boxed tubing. The frame flexed from the factory, you can expect it to flex once you modify it. Trying to eliminate any frame flexing is impossible without a roll cage, just do your homework and keep stress risers to a minimum.
Overbuilding is safe, but where do you draw the line? A solid frame, 2-inch heim joints, a rearend from a trash truck?-Max Fish