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.

Saturday, June 23, 2007

Latest riveted aluminum recumbent bike part

This is the first of what I hope will be a series of posts describing my work to come up with a simple technique for building recumbent bicycles out of riveted aluminum. Some of these posts will work backwards in time since I have a backlog of un-blogged material (parts and CAD designs), but I'll start with the latest thing I've been working on.

Here is a hypothetical chunk of a riveted aluminum recumbent, held against my Volae Expedition to give you an idea of where it would fit. The boxy part is supposed to be the "body" of the recumbent, forming part of the seat. The tubes sticking out are the seatstays attached to the body:

This work started with a simple sketch on dead trees. Yes, I've done the CAD thing for a while (using SolidWorks), but I spent literally hundreds of hours futzing around with minor 3D modeling details, so my current strategy is, "don't touch the computer":

The first step is to build join the "seatstays" into a flat assembly with the proper spacing. Let's pretend that this is it:

The tubes are 1/2" diameter aluminum from the hardware store (I didn't want to wait for an online order). The brackets in between are .025" 2024-T3 Alclad. By assembling these on a flat jig, I can ensure that the distance between the tubes is exactly 2". I used 3/32" stainless steel POP rivets.

Next, I built the "box" with a slot for the seat tube assembly to be inserted:

This box is also made on a flat jig -- there are no fancy 3D fixtures needed to hold everything in alignment. The two channel sections that form the frame, and the skin doublers that bridge the gap between the channels, are made from .025" 2024-T3 Alclad, while the side skins are .016". (Picky rivet geeks will notice a couple of edge distance boo-boos.)

The next step is to mate these two parts. Presumably, this would happen when the builder completes the body and chainstay and seatstay assemblies. The three assemblies would be clamped to one another and to the dropouts, with an accurately dished wheel, and some 2x4s would be used to align everything like this example. The seatstay tubes can now be match drilled through the pilot holes in the side skins to fix the alignment:

The next step is to add some shear ties to the assembly, maintaining the continuity of the body around the slot and transmitting shear resulting from side-to-side forces to the body:

If you look at the following picture, you'll notice, as I did, that the structure is missing an extra shear tie between the stays to maintain proper continuity of the monocoque. Filed under "note to self":

The last step is to fabricate a cover for the other side. This will presumably be the surface to which the seat (perhaps made of plywood) would be attached. Here it is match drilled, primed and ready to rivet:

(Yes, a couple more edge distance boo-boos.) The finished product looks like this:

The craftsmanship on some of these parts leaves a little to be desired. You'll notice, in addition to the edge distance mistakes, some scratching where I accidentally pushed the drill too far through the structure, and a few parts that were made a bit skewed. I don't believe this is fundamental to the technique -- rather, I think that I, with a young family and a demanding job, am just being a bit hasty.

An inevitable question is, how strong is this stuff? Well, a 5.5" deep by 2" wide beam made from two pieces of .025" channel (top and bottom) and .016" side skins is stiffer and stronger, in the vertical direction, than a 2" diameter, .049" wall thickness CrMo round tube -- and half the weight. With no welding, gluing or composite layups. Which is why I would really like to see this construction technique scale up to a full vehicle.