Thursday, August 23, 2007

Full monocoque monostay design

Perhaps the most complex of my riveted aluminum recumbent designs is this one, the culmination of a whole bunch of experimenting with parts and CAD work. The whole structure you see here (not including the wheels) weighs 5.02 pounds.

Here are two SolidWorks renderings and one snapshot from the regular line view.

The way I build these assemblies, I generally construct a "part" that contains just pure geometry, containing most of the important driving dimensions and shapes for the rest of the work. The hope (rarely achieved in practice but true for some things) is that a change to the geometry causes the parts to automatically rearrange and resize themselves:
The design is a dual 406 SWB. The seat is a sheet of plywood, while the remainder is riveted .025" 2024-T3 Alclad sheet with fittings made of 6061-T6. Zoomed in on the front, with the side skin removed, we have:
The headtube mounting assembly is previously documented in this post so I won't belabor it further. The adjustable bottom bracket assembly is made of 2x2", 1/8" wall 6061-T6 tubing and 1/8" 6061-T6 plate. This was a trial design to see how far I could go without any welding at all; everything is held together using machine screws. Here is a detail of this area:
I suspect the middle part of the rail could be machined (or simply drilled with lightening holes) to save a bunch of weight. Here's an example from a different design to illustrate:

The main emphasis of this design was to investigate how I could "use the monocoque, Luke": my goal was to build as much of the primary structure as possible using the monocoque, eschewing tubular stays. First, a view of the overall structure with the monostay assembly and only the skeleton of the rest of the structure, to give some perspective:
When the rest of the skeleton is removed, we can see that parts of the monostay assembly extend into, and intersect, the main structure:
Removing one side skin and the two doublers on the dropout end, we can peek into the structure:
Removing the two long channel sections that come out towards the viewer, we have:
and removing one more part, we have this:
which should explain pretty well how things come together. The one part we just removed is what I call the "stay fork"; this one:
This proved to be a very difficult part to fabricate: in some parts near the "knee" of each leg, where it got pretty thin, the material kept cracking. Which led me to one important realization: parts are easier to draw than to make. One of the dangers of 3D parametric CAD such as SolidWorks is that one can go to town drawing complicated things that all auto-update whenever a dimension is changed, and have lovely geometries, but which are essentially unbuildable. :)

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