As I mentioned, I basically stole the design from an existing product, making some modifications as necessary, usually to remedy things I screwed up or miscalculated, or to make up for my lack of a machine shop.
Here's what my version ended up looking like:
The first thing I did was to figure out how to go about bending plywood into the shapes I needed so that the back end would open up to accommodate a wheel when it's in the two-wheeler phase (it just gets a spacer through which the axle slides when it's a trike), like so:
So I found some articles and videos online about making bent plywood furniture, and felt pretty confident that I could handle it.
I went to a specialty lumberyard that's mainly for furniture grade wood, and got a sheet of 1/4" mahogany cross-grain plywood that's specially made for bending applications. I would have preferred birch, just because I like the way it looks, but all they had was mahogany. It was cheaper than I thought--only about 26 bucks.
After drawing some sketches based on my memory of the actual bike I had seen in the store, and trying to scale the pieces off the images from the company's website, I cut out cardboard templates of the frame and fork, and tweaked them until I achieved the geometry that I wanted.
The one problem that plagued me at that point was wheels. I have a friend who builds bike wheels and can get me components at his price, but he couldn't get me the tiny 12" wheels you see on little kids' bikes. So I made my templates based on a plan to use 16" wheels, which are typically used on bigger kids' bikes, and which my friend could get cheap. It would make the bike taller than I wanted, but I wasn't willing to pay the retail price of about 35 bucks a wheel (including tires and tubes) for these homemade hoopties that might not even work. That would have been 210 bucks right out of the gate!
I made a mold out of 3/4" medium density fiberboard (MDF) I had left over from another project. Using a jigsaw, I cut a bunch of identical templates in the shape of the curves that I wanted, and stacked them on top of each other, gluing and nailing them together with a trim nailer, until the mold was thick enough that I could clamp my pieces of plywood onto it. Then I sanded the mold until the curve was nice and smooooth.
Next, I cut my pieces of 1/4" flexible ply into the arc that would form the frame of the bike. I had to cut twelve identical pieces, since I was laminating three of them together to form one side of the two-sided frames for two bikes.
I ended up with an 11/16" thick (because 1/4" ply is slightly thinner than a quarter inch) piece that would form one side of one of the bikes. The frame of the bike I was copying was only 1/2" thick, but I didn't trust myself to make it that thin and still structurally sound. The trade-off for the extra strength and rigidity is, of course, extra weight. But in the end, the bikes don't feel very heavy at all and they're quite sturdy.
I repeated that procedure three more times, and then sanded the bejeezus out of the pieces, using my neighbor's table sander and my random orbit sander.
For the forks, I used 3/4" birch plywood, which is widely available but a bit pricey (usually around $44.00 for one 4'x8' sheet). The spacers that go between the two pieces of the fork, and through which the steering "tube" (just a threaded rod, really) would eventually run, was made of the same material.
|One section of frame, the forks, and the spacers that go between the two sides of the fork|
Around the time I was getting all the woodwork together, I wised up and decided to look for some cheap used bikes that I could scavenge the wheels off of instead of paying for wheels that were not the right size. So I looked on Craigslist, and easily found a couple bikes for ten bucks apiece.
I made a couple adjustments to allow for the new wheel size, and then I started screwing the frame together and attaching the fork to the frame. To attach the two pieces, I just ran a piece of 1/4" threaded rod through the spacer on the top of the fork, the frame, and then the spacer on the bottom of the fork. I put fender washers and locking nuts on the top and bottom of the assembly, and nylon washers between all the wooden parts that were resting on one another. I actually don't have a great picture that shows how that works, but I do have this dramatic shot of what the frame looked like once I bolted it all together. I took the pic with my iPhone at about midnight, in the alley behind my house, where I was working under a halogen work light:
As soon as I got that part done, I slid the wheels on to see how everything fit.
I still hadn't figured out how to make the long rear axle for the trike configuration, but it looked like it would work as a two-wheeler anyway. I also decided at this point to add the sissy bar. The bike I copied had a seat on a plywood seatpost-ish contraption that slid between the two halves of the frame and adjusted to fit the rider. I figured if I just made a little padded seat that screwed directly onto the top of the frame, I could keep the stand-over height a little bit lower for my dinky little kids. The sissy bar was to keep them from sliding down the frame and onto the tire. Plus I thought it looked cool. When the kids get too big for this setup, I'll probably make an adjustable seat kind of like the one on the original.
Once I got the frame dialed in, I had to figure out how to build the rear axle so the machines could run on three wheels. I used more or less the same idea as the bikes I was copying, which was a threaded rod running through a steel tube with the wheels held in place with various nuts and washers. But I had essentially two different sized sets of three wheels, and and it took me many trips to different hardware stores to find the parts I needed. I finally had to order some parts from an amazing industrial hardware outfit that a machinist/engineer/mad inventor friend told me about.
Oh...the wheels. One amazing piece of luck I had was that when I went to REI to get some helmets for the girls, I struck up a conversation with a bike mechanic there, and told him about my project. I asked how much it would cost to buy the wheels that came with the Skuut balance bikes that they sell there. He proceeded to give me three of those wheels, which he happened to have in the shop on a couple of returned bikes. So the spokeless plywood "aero" wheels that you see on one of the trikes are from that exchange.
Putting the axles together was one of the biggest challenges, not so much skill-wise, but just in figuring out how to go about it. The trikes that I copied used sections of larger steel tube as spacers between the wheels and the frame, and in the center, between the two sections of frame. I tried that, but the whole assembly seemed rattly and weak. So I decided that I would use sections of wooden dowel as my spacers. I drilled out my sections of dowel, and made discs out of the birch plywood into which I drilled impressions that would act as seats for the dowels. I screwed the discs to the frame, slipped the dowels into their seats, and slid the steel tube and threaded rod through the dowels. When I bolted the wheels onto the threaded rod, the whole rear end became very solid and tight. And it looked cool.