Picture of Make a Mini Recumbent from a Kid Bike
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Step 1: CAD system Layout

Picture of CAD system Layout
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.

Step 2: Layout donor bike on CAD setup

Picture of Layout donor bike on CAD setup
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.

Step 3: Layout the Boom once cut from the Donor Bike

Picture of Layout the Boom once cut from the Donor Bike
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.

Step 4: Drill the hole that makes the Mitre

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

Step 5: Cut and File the Miter for the Joint

Picture of Cut and File the Miter for the Joint
12 inch bent dry 2.JPG
File and shape the wings.  The wings add joint strength and they help spread the pedal force from the boom to the frame.

Step 6: Use CAD to check chainlines, etc

Picture of 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

Picture of Strap and clamp the Joint for Welding
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.

Step 8: Tack weld the joint in a few spots

Picture of Tack weld the joint in a few spots
12 inch bent chain side.JPG
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.


Step 9: Complete the weld and plan the chain-lines

Picture of Complete the weld and plan the chain-lines
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.

Step 10: Make and install the chain and guard

Picture of Make and install the chain and guard
12 inch bent chain front.JPG
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 http://www.instructables.com/id/Using-a-Bike-Chain-Tool/

Step 11: Add the wide bars and temporary seat

Picture of Add the wide bars and temporary seat
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." 

Step 12: Long seat CAD layout

Picture of Long seat CAD layout
12 inch bent Seat Concept.JPG
12 inch bent Seat wood lay-up.JPG
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.

Step 13: Construct the long seat

Picture of Construct the long seat
12 inch bent Seat wood lay-up.JPG
12 inch bent seat apart.JPG
12 inch bent seat together.JPG
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).

Step 14: Test Ride the long Seat

Picture of Test Ride the long Seat
It worked, but it felt a bit wobbly so the next step shows a new seat brace added.

Step 15: Add Seat Brace and Adjustable Seat-back

Picture of Add Seat Brace and Adjustable Seat-back
12 inch bent seat back slant.JPG
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.

Step 16: Enjoy, share, impress your neighbors

Picture of Enjoy, share, impress your neighbors
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.
gravityisweak12 months ago

This is great! Could you add some huge curved handlebars to this? The steering looks less than comfortable.

Yup, chopper style handle bars I think.
Woodenbikes (author)  gravityisweak12 months ago
These little bars are a stretch down to reach. The good thing is they pass under the seat so you have a lot of steering input ability of low speed balance.
With big curved bars you want to maintain steering clearance so they need to be big enough to pass around the chosen (and possibly carved down) seat. A longer forward stem reach could help.
also double check to c if u have the same metals as well.
Cdn Sapper3 years ago
Amazing ible!
Love it! This will be my new project...
MaxineLaRue3 years ago
OooH! I want to see the video too!
This is super-cool.
rockgod13 years ago
Nothing quite like a grown man riding a small girls pink bike, lol.
snotty3 years ago
Nice ride!

But your video has disappeared and it's what really made this instructable. Any chance you could repost the video?
B4SEC4MP4 years ago
This is awesome! Might I suggest adding a wheelie bar!?!? That would take this bike to the next level. The only thing cooler than riding this bike down the street would be riding this bike down the street popping a wheelie!!!
husamwadi4 years ago
hey if you dont want those monkey lights.....heh heh heh :)
husamwadi4 years ago
this is cool, but man! I hope the tires don't pop!

-hey check out this mini recumbent i made, "POW, POW" uurghh, never mind...

Lol, just kidding, but this thing is really tight!

keep up the good work!
Cardboard Aided Design...nice
Woodenbikes (author)  mattthomas9920034 years ago
'Keepin' it real' since 2002.
Questor4 years ago
I notice a tire change.
minor mishap, perchance?
Woodenbikes (author)  Questor4 years ago
Dumpster bike problem... old rubber... tire bead wire was exposed and was puncturing inner tube. (Tire possibly not meant for plus-sized kids like me.) I replaced the tire with another dumpster tire from a high-end baby stroller.
toad4 years ago
Great ride, maybe bolting a rear deraleur somewhere around the headset to take up the slack in the chain. Some thing like a long cage mountain bike one.
Woodenbikes (author)  toad4 years ago
That would handle the chain, but with a rear coaster (back-pedal) brake, one would need to back pedal a whole revolution (winding up chain from the lower run and sagging it into the upper run) before the brake would engage.
caarntedd4 years ago
This looks super dangerous, and super fun. Five stars.
mjursic4 years ago
Love it! Now let's figure out a way to get a decent ratio on it so you're not pedaling like the dickens.
Daftehh4 years ago
my goodness look at it go
avicena4 years ago
when any body think it a junk, it can reused by some way...I like it!!...
The concept alone is great, but the CAD pushed it over into brilliance.

Wonderful work!
I know who I'll be voting for in the contest!
jovino4 years ago
HAHAHAH!!!! Brilliant!
macrumpton4 years ago
Very cute!
It looks like it could use a longer boom for your leg length.
Matin4 years ago
What a Cute and Fun bike!
I am keeping my eye out for small kid bike
How about no weld wood version
BtheBike4 years ago
SA-WEET !! I've been trying to mock up a concept for a cargo bakfiets style bike with a regular and kid bike . this give me a few ideas . great work Wood
berserk4 years ago
Very nice! Would not want to commute on it, but I have not had a bike make me smile like that in a while! :-)
I've been building recumbents since '86 and your first effort looks good. You did make one junior mistake... the bottom bracket threads are in reverse and will eventually unwind themselves loose.
Woodenbikes (author)  PurplePeople4 years ago
The BB shell is the large one piece crank type with press in thread-less bearing cups. (no threads there). The chain drive remains on the right hand (traditional) side of the bike. Nothing is reversed from traditional, so I do not expect anything out of the ordinary.
Yeah... I see that now. My bad. Maybe we'll see you at the races.
ilpug4 years ago
Of all the crazy stuff... I love it!
Alecw354 years ago
maybe you could add a 2 ring chainset to the pink bikes bb. then run front chain to smaller ring. big ring to rear wheel. make the gearing a little bit higher.
tho I geuss its the smaller diameter threadless bb. so wont easily take cheap mtb one piece cranks
Woodenbikes (author)  Alecw354 years ago
One could easily put the larger Mountain bike cranks and chainheel setup in the front BB for a higher gear. However, I'm building this bike to share with little kids so I wanted to use the short cranks to accomodate small and large riders and I wanted to use the small chainring to keep the top speed down the to 10-12 MPH for safety at Maker Faire. (It's crowded with excuberent people riding 8 of my bikes at a time.)
Great concept and fun but the mechanical advantage is probably less than desired
wobbler4 years ago
Utter madness! Somewhere in you there is Coco the Clown desperate to get out. I love it!
lemonie4 years ago