Sunday, June 24, 2007

Headtube assembly

This is my first attempt to record some old-ish (maybe 1.5 years ago) work. Here is a rendering of one of my riveted aluminum recumbent bike designs:

and here it is with the side skins removed for illustration:

This was back when I was thinking of using rivet bonded, rather than just riveted, construction. I planned to use 3M Scotch-Weld DP460 adhesive, and the seat was to be a sandwich of Rohacell structural plastic foam bonded between two very thin layers of aluminum. The only subassembly I actually constructed was the headtube mounting, shown here in CAD:

The parts are primed with Cortec 373 (thanks to Century Corrosion for arranging for a sample to be sent to me) prior to final assembly. The primer provides bonding support and corrosion resistance, and is non-toxic. However, the stuff is a bit hard to apply, and I was not able to get a uniform coat. Here is the final result:

As you might be able to see, the headtube has 4 flat surfaces providing solid surfaces for bonding and fastening into the sheetmetal. Lacking access to a machine shop, I improvised a jig from some hardware store metal and filed it down; it took several nights, and I would never, ever attempt this again, but I was finally done.

I used 3/32" stainless steel POP rivets, and #2 stainless steel machine screws (yes, #2 -- these little guys are small -- but, in this design, all they do is stabilize the bonded joints to let the adhesive do its work). The screws are countersunk from the inside of the tube to provide clearance for the steerer (sorry for the blurry pic):

and this feat is accomplished by my handy reverse (aka back) countersink cutter and pilot (tools I didn't even know existed till I needed them). Here is one more view for completeness:

Lessons learned:
  1. Rivet bonding is tricky business. The adhesive is gooey and sticks to everything, and you have to quickly get all your parts assembled and riveted within the work time of the adhesive or else you have an ugly piece of scrap. It's all a bit stressful.
  2. Did I mention that the adhesive sticks to everything? This stuff is amazing. And it dries hard and tough as nails. I tried some test pieces, ripping them apart with my hands, to get an idea of how strong it is and, subjectively, that stuff is serious.
More broadly, this launched me into some navel-gazing regarding the use of bonding versus just riveting. In favor of bonding, I get more rigid joints and better strength. But the flip side is the stress of having to get the surface preparation just right or else the glue doesn't stick, and the stress of assembling everything on a strict time limit while the glue hardens. And my whole idea is to devise a construction technique that can be built without fuss (or else, why not just do a carbon fiber layup and be done with it?). As a result, my subsequent designs have moved progressively away from bonding towards pure riveting.

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