[bmitchell82 (Brendan)]

Just for those who love the science and the facts behind the shield of Arm Chair Experts S (ACES)
I know the Ixx (second moment of inertia) or rigidity formula follows

Sum of ((b*d^3)/12)+(A*h^2)) for rectangular sections

for a circular section

(Pi*(do^4)-(di^4))/64 where do is Diameter Outer and di Diameter inner

Unit is generally in the form of XX*10^6 mm^4

there is a lot more going into this than saying hey yeah i have a good value of stiffness. I should do a simple space gass load run down for the "average" pier

As has already been said thickness of the material doesn't really matter for "stiffness" but i beg to disagree, the amount of material away from the NA (neutral axis) hence the thickness is paramount for stiffness in a rectangular section (Look at a I beam for instance, thick top and bottom flanges thin web the web resists shear and gives the section depth (stiffness) and the flanges give moment resistance for compression/tension coupled with the resistance of buckling of the web), also for a circular section it does with the formula subtracting the inner from the outer diameters hence the thickness of the material. For the circular section the geometry leads to good torque (twisting) control as the shear flows around the section.

Just out of pure interest a little while back i did do some calculations on a 200x5 SHS at approximately 900mm in height it required 50kgs of horizontal force at the top of the section to produce approximately 5 micron of movement. Now i don't know about you but that is a bucket load of force for something that tracks with the earth and isn't flying around all over the shop. You are more likely to get larger errors within your mount.

Yes a larger diameter is stiffer, but unless your mounting 16"+ scopes with PME's that are in the 100+kg bracket and the like 90% of the time its over kill *100.