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.


Melissa said...

This rocks. Do you give workshops? Are you selling your plans?

Melissa said...

I think your blog is very cute. Do you have any personal photos you could add?

robert said...

As a recumbenter and helicopter pilot (who spends lots of time doing maintenance flights) I certainly appreciate the idea of riveted aluminum construction.

What I think the real draw may be is the looks.

robert said...

My first recumbent was welded out of 2" aluminum box beam, with a 700c rear wheel and 451 front.

I went with a polished aluminum and black look with a black rear wheel disk. It looked real rough up close but from a distance kids would shout: "WOW MAN - COOL BIKE!"

Here is a picture.

Keeping the seat seperate from the frame allows a much simpler design. A design that tapered to a tall mid section would be light, and more importantly allow for that totally cool looking row of increasing diameter lightening holes. The lighter frame would allow you to make it longer, making the ride better and handling easier.

We forget the preeminence of the image factor. If I won the lottery I would spend a lot of money making a "chopper-esque" recumbent with an electric drive hidden in the saddlebags. This would allow the user to look cool and go fast without onlookers being able to tell how much help they got from their little secret.

Phil said...

I love the idea of a riveted Al monocoque 'bent. We seem to hyave forgotten just how strong and reliable riveted aircraft and early monocoque F1 cars were. I'm not sure that commercial SS pop rivets are the best item for the job. You may be setting yourself up for some nasty corrosion and mechanical problems. Could I suggest that Cherry or Avdel structural rivets may be a better long term bet ?