Originally Posted by
rdenney
While those students are working up their free body diagrams, have them consider the overturning moment. If the vector representing gravitational pull on the camera setup falls outside the triangle represented by the tips in the ground, the whole thing will overturn. For nature photographers with a small and light tripod, but with a monster doubled 600mm f/I-can't-afford-it lens mounted out of balance, this could be an issue, particularly on a slope. With four legs, that vector has farther to go--a square fills up a circle better than a triangle because it has more and shorter chords.
Given that leg tips will seat into the ground, I think it's fair to say that not all tips will be equally loaded when there are four of them, because of the difficulty in getting four points to be in one plane (assuming the ground represents a plane, which is rarely does). But the more heavily loaded leg (and, as you say, there will always be one) will just seat a little farther. The problem you mention is most likely with hard tips on a hard surface. Even then, resting it on four legs and allowing the wobbly leg to settle down before locking it in place will take up that slack, and allow the flexibility of the system to distribute the load. Most of us would increase that flexibility (as well as adding some damping) by using rubber tips in that situation.
All systems are flexible, especially long, slender structures.
I think I could see how it would work to solve a narrow but very real range of problems, such as mounting a view camera on its side, to the side of the head, which a given situation may require. With four legs instead of three, keeping the overturning moment under control would be easier.
If one is going to argue something on engineering terms, one should consider how the design may have accommodated the problem mentioned, or at least how it responds to the requirements of the design. That's actually required by the canon of ethics for engineers (in every state I'm aware of) before publicly challenging an engineer's work, isn't it?
(Bob ought to send me one as a gift for coming to his aid, heh, heh. Oops--that would be a conflict of interest--doggone that canon stuff.)
Rick "a registered professional engineer in four states" Denney
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