Step 1: CAD system Layout
You can also see the Center of Gravity (approximately the bellybutton) is about 1/3 of the wheelbase distance ahead of the rear wheel contact point. This good weight distribution leads to nice handling.
Step 2: Layout donor bike on CAD setup
Step 3: Layout the Boom once cut from the Donor Bike
The boom is positioned on head tube to see where to drill miter hole. The drill with attached hole saw** bit is positioned to convey the idea of mitering.
* Headtube is the pink frame's front frame tube through which the bike's fork steering tube passes.
** Hole saw bit is the 1.5 inch diameter white toothed cylindrical saw in the drill chuck.
Step 4: Drill the hole that makes the Mitre
Step 5: Cut and File the Miter for the Joint
Step 6: Use CAD to check chainlines, etc
Layout the boom and frame on CAD to look at Chain-lines (shown here with Yellow battens) Also look for leg extension and possible heel to wheel interference. e.g. sometimes my shoes get scuffed by the front tire while riding some bikes I make, but not this one.
Keep adjusting (cutting or filing) the miter to keep it snug against the head-tube when the boom is positioned where you want it.
Step 7: Strap and clamp the Joint for Welding
Mitered boom is dry-fit onto the pink head tube and strapped in place with a big clamp or cargo strap.
I also clamped down the wings with a C clamp to get good tight metal to metal contact.
Re-check the alignment before welding.
Step 8: Tack weld the joint in a few spots
I use a MIG [Metal Inert Gas] flux core wire feeding welder. It's pretty easy. Just pretend you are like Martha Stewart with plasma heated molten metal hot-glue.
Re-check the alignment to see if the boom is centered and angled right and not twisted.
Then it's time for thorough welding.
Step 9: Complete the weld and plan the chain-lines
The tight side chain line (top section of chain that pulls the rear wheel cog) takes routing precedence and gets first crack at the straightest path possible. The slack section of chain on the bottom return route gets second pick of the routes and may have to wind around obstacles.
Step 10: Make and install the chain and guard
Digression on chain line management:
On some recumbents you see the chain looping over and under one or more pulleys. This adds chain drag and noise. The more severe the chain bends around a pulley, the more force the chain will apply to the pulley axle (more noise and vibration and lost work.) This is not a big deal on the slack side of the chain (returning to the cranks). But it is a big deal on the tension side of the chain (driving the rear wheel).
Bottom chain run is enclosed in a 5/8" diameter piece of old plastic (polyethylene) drip irrigation tube to help route it around the fork blade.
Long chain is made by connecting a couple of chains using a chain tool http://www.instructables.com/id/Using-a-Bike-Chain-Tool/
Step 11: Add the wide bars and temporary seat
Hybrid of the littlest pink princess with the big bad mountain bike down-tube welded to its head-tube looking like a little pink wheeled narwhal. "Aye, It makes me want to get all scrimshaw on ye."
Step 12: Long seat CAD layout
My cardboard assistant is reclining on part of a salvaged IKEA wood chair.
Step 13: Construct the long seat
Pencil lines outside the mounting bolt holes show where I plan to saw (with a Sawzall).
Step 14: Test Ride the long Seat
Step 15: Add Seat Brace and Adjustable Seat-back
Adjustable seat back (upright for long legged riders, scooted forward for short legged riders)
Adjustable seat back is made from 1/2" plywood screwed to 2 parallel hardwood rails. (so it can be slid to different positions while cradling the seat back support between the rails.
Step 16: Enjoy, share, impress your neighbors
Come test out the bike at SF Bay Area Maker Faire 2011.
My next project is a similar bike but avoiding welding by using woodenbikes technique by making the boom/seat combination out of wood bolted to the little bike.
Technique will be similar to the one used in this link.