I am building stands for my surround speakers. Very simple, cross member base, spindle support with a table top. My question is, how far should the support legs extend from the spindle to ensure good stability? The top will be about 36″ from the floor and the speakers weigh less than 5# each.
There is no single answer to your question. Being a old audiophile myself I have seen many a speaker stand some good design some not so good. Will these stands be used on carpet or wood(or other hard surface) floors? Speakers tend to be less stable on padded carpet than on solid surfaces requiring either wider bases or cones or better yet both. I also suggest you weight the base somehow. Your design does not allow for this easily but drilling out the spindles lower section and adding some shotgun BB's would be better than nothing. I suggest either a prototype to test its stability or making the base oversized and gradually trimming it back until you feel it is appropriate.
The rule of the thumb we used for designing structures in the exhibit/display industry was the width of the base should be at least 25% of the height. In other words, an 8' tall structure should have at least a 2' wide base. Of course that doesn't take weight distribution or center of gravity into account, but it's a start.
Start with an 18" bandsawn disc with feet 2-stuck taped to it and a rough approximation of the rest. Move the feet in or reduce the size of the disc 'till it feels sketchy. +1 with @esch for adding weight down low.
I'm not sure a disk would not be a good replacement for his base design. The disk would have equal radius at all points resistanting tipping but his simple cross member base would have significantly less stability if it were to tip along the axis bisecting 2 of the cross members. He could probably use a square to simulate his base more accurately.
The disc is just an easy test base to figure out the final size...never said anything about replacing his base. Taped-on feet in any configuration desired can be tested for stability this way.
MJ I understood what you were saying to use the disc as a test platform. My point was that the round disk would not represent an accurate model of his intended base design. Obviously a circle would have an equal distance from the centerpoint to any point along its circumference whereas his planned crossmember base would have varying distances from the center along the hypotenuse of the triangle formed by adjacent legs of his base. The lowest value would be at the midpoint of this line or at a 45° to one of the legs. This is the point where his stand would be most unstable but can't be duplicated by a circle. This where a square could work better if the corners were treated as his crossmembers. He would still need to cut the square smaller for each test to accurately duplicate the real world conditions.
That's why the feet stuck on the bottom with tape are needed. You are not testing for the disc, just for the feet in each spacing variation. A square with feet stuck on would work as well. A flat square would require a perfectly flat floor.
Another idea on adding weight to the base. If your cross members were at least 1" thick and over 2"tall, route a cavity in them with a ½" upspiral bit to a depth of at least 1½" fill this with BB's you can hold them in place with epoxy or if you are so inclined glue a wood cap in place.
Thanks for the input. I am really just looking for a starting point and the 25% minimum suggestion makes sense to me. The top will be 8" x 10", so I think the base needs to be at least that footprint, so I'll start with 12". I always do prototypes of any new design, unless it is a one-off. The only variable in this one is the length of the legs. I think far more stability will be gained by extending the legs than any weight that is added. I want this to be a fairly delicate looking assembly. The legs will be 1/2" in thickness (horizontal) and about 1-1/2" in height (vertical). I'll start with a length of 9" and see how that works. The spindles will be 1" square, with a 1/4" wide by 5/8" deep dado along one surface. That will be a runner for the wire. A 1/4" strip will be inserted into the dado to hide the wire. The base will be a cross member, connected to the spindle with dados to provide stability. I'll send pics and results of what the leg length works out to be. Thanks again for the input guys!
Also, there was a question about the surface. It is a hard surface, but porcelain tile, so not necessarily flat. The ends of the legs will have very small threaded feet attached to enable leveling. Without this, almost any reasonable leg length is bound to be unstable.