[Max Lobovsky]

Eeek! lost my post...

I don't think sheer strength is the material property you need. You will need either the Young's Modulus (aka Modulus of Elasticity) or the Yield or Ultimate Tensile Strength. This, in combination with the proper moment of inertia and description of the load on the beam will let you calculate the stiffness or strength of the beam. There are plenty of good websites out there (I can't seem to find my favorite one and the bookmark isn't with me)

If you haven't done this stuff before and can't get anyone to help you, it's a bad idea to try to pick your extrusion based on what you come up with. It would be better to look at similar parts on other FIRST robots and make your selection based on that.