Tuesday, August 28, 2007

On bondage

Folks have asked me about bonding (perhaps with backup rivets) versus just riveting. Elsewhere in this blog, I've mentioned that I am currently pursuing a "riveting only" strategy. The question is, "why"?

In order to be useful, a design must fulfill a purpose, or "market" niche. The purpose need not be monetary -- the market in question may be that of making folks happy, spreading Peace and Love, or winning a competition just for the sheer challenge of the thing. In my case, my purpose is this:
Hypothesis 1. Traditional aircraft monocoque aluminum construction, using thin sheetmetal and large cross-sections, occupies a useful niche between welded space frames and carbon fiber monocoques. It is competitively light weight and rigid while being easy for beginners to build.

There is also another claim, less easily made:

Hypothesis 2. The methods of Hypothesis 1 can be used as the basis for selling parts to homebuilders allowing them to design and build their own configurations.

With that in mind, the question is: should we just use plain rivets, or should we mess with bonding?

The aircraft industry has been using rivet bonded construction for many years. Thus it is instructive to follow their trajectory. The original F-18 A-D had a multi-part metal fin. The newer E-F models have a single piece, bladder molded, heat cured composite fin. Sound familiar? Bikes like the M5 Carbon High Racer and the Velokraft bikes all use this method, as do many upright carbon frames. Clearly, if you have the equipment, this is The Future. In addition, absent bladder molding, people like Garrie Hill, Jim Scozzafava, Tom Traylor and a whole host of others have shown that, if one is willing to do layups, carbon construction rules the roads. Hence, once again, the niche is to find something easier than carbon, but lighter than space frames.

There seem to be conflicting notions out there about how much one needs to prepare a metal surface for bonding. However, among people who rely on it for a living (or whose companies will fail dramatically if their structures come apart in service), the consensus seems to be that it's not easy. Check these out for starters:
In general, it seems to me that proper surface prep for bonding in order to achieve the rated strength of modern epoxies involves at least lots of care, and sometimes toxic and/or caustic chemicals.

Thus my current direction is to just rivet. Every Schmoe can build one and it will very likely stay together. It's easy to see a crappy rivet and, conversely, if a rivet looks pretty, it's probably adequately driven.

Thursday, August 23, 2007

Easy Racers Javelin clone

My current design is a clone of an Easy Racers Javelin: 700c rear, 451 front, LWB. I figure this is an easy way to get started -- later on, I can try more fancy designs with integrated seats. I'm trying not to "overmodel", so here is a sketch of just the parts I need to build a simple prototype of the rear wheel attachment. I would certainly not build such a long structure to the right, but I want to give an idea of how it would fit together:
The dropout is made from 1/4" 6061-T6 plate. The stays are 5/8" diameter, .035" wall 2024-T3 round drawn tubing, flattened at the ends and attached with two #8 MS27039 machine screws:
The stays are attached to the body by (roughly T-shaped) brackets made from 1/8" thick 6061-T6 material:

They are held together at the middle by a spreader piece made from 1/16" 6061-T6:
Removing the stay and spreader piece, we can peek in to see that the 1/8" bracket is built up on each side with a 1/16" thick spacer, such that the stay is flattened on both ends to an inside dimension of 1/4". This ensures that it has a nice radius without cracking:
The approach I show here, with a thicker bracket and a stay flattened onto the bracket, is a lot simpler and more symmetrical than either riveting the stay from the sides or inserting the stays into the structure. Note that I'm relying on the brackets themselves to resist side-to-side forces since the 1/8" plate has significant bending strength (though I haven't done the math on that part to know for sure, mainly because I am not sure what the design lateral loads should be).

Early monostay design

This early monostay design was far simpler than my subsequent ones. Here is the overall view:Removing the right side skins, we can see inside. Each monostay side is made up of a top and bottom channel piece; these come together in the middle and are attached to top and bottom plates. The skins hold the whole thing to the main body of the bike.
Looking back, this design is somewhat appealing. Perhaps I should return to it one day.

Triangulated stays, funky mounting

Another remark about the design discussed earlier in this post:
Mainly, I just want to record the treatment of the stay attachment. The stays are riveted into two straps, made of 1/16" 6061-T6:
A third component, made of the same material, acts as a brace in between the stays and is riveted to the back of the bike structure to cleanly transmit side-to-side forces:

An experiment with tubular monostays

This was a quick experiment with tubular monostays riveted into the structure. I didn't really do any strength calculations on this; I include it here just to record the idea for posterity. Here is an overall view:Zooming into the attachment area and removing the side skin, we have:
After we remove a couple more parts, we can see the tubes nestled in there:
This design assumes that each tube is attached by a line of rivets on each of 3 sides (top, bottom and outboard) to achieve the necessary strength.

Funky design with tubular stays

This is a rather funky-looking design, again in the dual 406 SWB configuration. Note the mid drive attachment, a 1/2" diameter tube that is riveted between two bulkheads in the main structure:Here is the seatstay attachment. All 3 brackets are made from 1/16" 6061-T6 plate:
And here is the chainstay attachment. The main bike structure is extended backwards a bit to avoid making the stays too long, on the theory that the large rectangular cross-section monocoque is more rigid and lighter than the stays, so we use it as far as we can till we get very close to the wheel:
Removing the right hand mounting bracket and skin, we get a clearer view into the structure:

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. :)