Make a Mini Recumbent From a Kid Bike

CAD (Cardboard Aided Design) layout of 12 inch wheeled small recumbent. Yellow lines show the chain line.

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.

Donor bike frame (black) positioned showing where Bottom Bracket (BB) will be near cardboard foot. Only the black BB and downtube will be used in the frame as a boom and BB.  We can also use the longer handlebars, later.

Prepare to drill the Head-tube* sized whole that will be the miter (sophisticated, snug joint where two objects join at an angle).

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.

With miter holes sawn, I cut away some of the black boom tube while leaving some reinforcing "wings" for welding to the head tube.

File and shape the wings.  The wings add joint strength and they help spread the pedal force from the boom to the frame.

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.

Scrape or sand the paint off of the area to be welded.
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.

Make a few small tack welds to hold the pieces together. 
 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.

 

Now to grapple with the chain line on the slack side. (It rubs against the fork blade. And if I lift it, it rubs against the chain-stay.) 
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.

I like an efficient quiet straight chain run especially on the tight power run of chain (the top length of chain pulled by the cranks and chain wheel). 

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 https://www.instructables.com/id/Using-a-Bike-Chain-Tool/

Time for Test-Ride-Fun!.

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." 

OK, It rides well for me, but to accommodate my smaller fans at Maker Faire, I'm building a long seat so shorter legged kids can also ride it by simply sitting closer to the pedals.

My cardboard assistant is reclining on part of a salvaged IKEA wood chair.

A salvaged office chair plywood seat pan has its cover peeled back to expose the foam padding.
Pencil lines outside the mounting bolt holes show where I plan to saw (with a Sawzall).

It worked, but it felt a bit wobbly so the next step shows a new seat brace added.

Seat brace bolted to frame and screwed to seat structure, prevents wobble.

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.

My neighbors enjoy the ride.  This bike turns on a dime.
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.
https://www.instructables.com/id/Wooden-Wedge-Bike/