On Legs

Most modules are designed with four legs, one in each corner. A few designs use two legs on one end and rely on the legs on the next module to support the other end. These modules, without the addition of optional legs, do not stand on their own. There is even an innovative design with one leg per module that relies on the entire assembly of modules to provide stability. Obviously these modules do not stand on their own.

So how many legs should a module have?

Why three of course. Oh, and they should not be at the ends either.

Why three legs? For the same reason a tripod uses three legs. Three legs always touch the ground. How many times have you gone to eat and ended up at a table that wobbled? Tripods never wobble they merely tilt. Put a module on an uneven floor; on a module with a very sturdy frame, it will wobble just like a table; on a flexible frame, which is the vast majority of modules, the frame will flex to compensate for small variances and only wobble for large variances. Avoiding flex is the first step in maintaining solid and reliable modules.

Many, if not most, modules have some sort of arrangement to adjust the length of the legs an inch or so to level the module. This fixes the problem, right? I don’t think so. I think that while they help, the modules still flex and that the levelers just take up the slack left over once the module has flexed. I contend that the flexing will increase because of the strains placed on the frame over time.

Why shouldn’t the legs be at the ends? For the same reason that L-Girder frames put the legs at 1/5 and 4/5 of the length of the frame. Sag is a major factor on frames. If the legs are at the outer edges of the module, the entire weight of the frame and all items on the top of the frame are between the supports. Sag is inevitable. If the legs are at 1/5 and 4/5, 2/5 of the weight is outside the supports and 3/5 inside the supports. The weight outside the supports reduces the weight between the supports (by acting as a lever) and the weight inside the supports reduce the weight outside the supports the same way.

The statement on three legs, not at the ends, is somewhat tongue-in-cheek because the number and placement of the legs has a lot to do with the physical dimensions of the frame and the height off of the floor. If you have a narrow module you may need to cantilever the legs or add outriggers to add stability especially if you have a large floor to rail-height distance. If a module is very wide you may need four legs to avoid tipping the module due to the torque that can be applied if the distance from the single leg (pivot point) to the edge of the module is great. Likewise if a module is very short then placing the legs at 1/5 and 4/5 could place the legs too close together for stability.

None the less, for many modules three legs placed at 1/5 and 4/5 would be ideal.

While I’m on the subject of legs, two other areas are worthy of investigation.

We tend to build heavy frames and then build the legs as light as we can get away with to reduce the overall transport weight of a unit. It would make more sense to build heavy legs and make the frame as light as possible. This would lower the center of gravity and make the whole unit more stable.

If we make the legs adjustable we tend to make the smaller section the bottom. By reversing this and putting the smaller member at the top, we also lower the center of gravity.

I am going to give variable height legs their own entry since there are lots of possibilities specific to height adjustment.

One thought on “On Legs

  1. ChrisA

    Telescoping legs would be a boon, provided the friction mechanism at each joint was sufficient to carry it’s share of the load without any downward creep over time. You could eliminate the ubiquitous carriage bolt and tee nut and provide infinitely variable adjustment between its extremes. For instance a 4-segment, 18″ tube could be set between 18″ and 60″+ from floor to railhead. Aluminium tubing would be the ideal material, however some good work could be done using rigid PVC or ABS pipe with matching OD and ID’s. Simply create some relief spilts in the end of each section (about 4″ long) with a hacksaw and use a metal band clamp to squeeze the segments together around the inside tube. What this does NOT offer is the cross-braced stabilty so necessary at higher setup elevations.

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