Wednesday, January 12, 2011

Project Trike: Technical Stuff

This is an addendum to the saga I wrote here about building these trike/balance bike contraptions for my kids.  I wanted to include the technical details here for anyone who is interested, without causing regular folks to glaze over.

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.

Then I glued them up, three pieces at a time, with regular wood glue, and clamped the hell out of them to the mold.

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.

Now I just had to repeat the procedure for the other trike.  But the wheels for the other trike were the ones I had scavenged off of used bikes.  They were quite primitive, with the most basic ball-bearing system ever.  That meant that I couldn't just slip them over a threaded rod (like a skewer on a grownup bike) and bolt them on.  For each side, I had to thread a bolt about four inches onto the rod, then thread the bearing cup on, then the wheel, then the bearing cup on the other side, and then another bolt; all the while making sure not to spill any ball bearings.  What a freaking hassle!  I almost regreted going the cheap route on the wheels.

Next, it was time for the paint.  My artist friend used a paint called "gouache," which she described as kind of like watercolor, but in a tube.  As you can see, she did a tremendous job, and my wife and I, and eventually the kids, were thrilled with it.  She lives in the Bay Area and did the work while she was visiting her family down here in SoCal.  If you are interested in contacting her, please email me and I'll put you in touch with her.  Anyway, after she got the painting done, I sprayed it with about five coats of polyurethane.

The last details I needed to take care of were the seat and the handlebar grips.  For the seat, I just cut a miniature version of a bike saddle out of plywood and screwed it to the top of the frame, shimming up the rear end of it to make it somewhere near level.  Then I cut out hunks of the yellow naugahyde garment bag I had picked up at the thrift store for two bucks and screwed it into the edges of the plywood, using a rolled up piece of old dog blanket for the padding.  I thought that I would try to tuck all the edges of the naugahyde under the plywood and hide the screws to make it look all neat.  But who was I kidding?  I can't even wrap a gift without it looking like the work of an unusually clumsy five year old.  So instead, I left the screws visible and allowed some extra fabric to hang down and then cut it into fringe.  The trikes had veered so far from the aesthetic of the sleek, somewhat austere original I was copying that a fringey cowboy saddle fit right in.  Finally, I slid on the bright blue grips that I had bought at the bike shop where all the hipsters outfit their fixies.  Et Voila:

The very last thing I did was to install footpegs right behind the front forks.  When we took the trikes out on their maiden voyage, the kids didn't know what to do with their feet when they were coasting, or when I was pushing them.  So I simply cut notches into 7/8" dowels so that they would seat on the undersides of the frame, and screwed them on.  Now the girls can put their feet up and relax while they cruise.  


  1. THIS is why people like me, hire people like u or go and buy it already made..omg... Does wifey know the investment she has marrying u??? lol

  2. Very cool man! I would have never guessed they were going to be trikes.

  3. That is VERY cool. I could never make anything like that. If I tried, I might end up in the ER either from a physical injury from something I've messed up, or from spraining my brain! How much did the whole project cost, and you should see if it's doable for you to build and sell these things. At the right price, I'd buy one!

  4. Somewhere, in the distance, Tim Allen does his "Home Improvement" man grunt. Hats off to you, sir.

  5. Pretty rad. SAHDs are just like SAHMs, apparently. Only they get all crafty and ocd using a jigsaw and table sander instead of knitting needles and a sewing machine.

  6. I have to express my disappointment in the fact that it didn't turn out to be a baby flinger.

    But as someone who has almost cut his own hands off just using a saw and driver to replace deck boards, I have to say super bravo.

    The picture of the clamps on the bent piece of wood is awesome. And you SMOOVED it all out and everything.

    Badass, Betadad.

  7. Just curious: do you plan to add pedals when the girls are older and it's a safer proposition, or would that be impossible with the current construction?

    And do Cobra and Butterbean have a specific bike, or do they alternate?

  8. Thanks for the nice comments, all!

    @Denny--materials cost around 100 bucks for both bikes. If I paid myself a decent wage for the labor, they would have cost like a grand. Also, if I tried to sell them, I'd be sued for ripping off the design.

    @Frank--Oh, believe me. Babies have been FLUNG from these things. I'm sure there is much more flinging ahead.

    @Paul--I was just thinking about adding pedals. But I didn't really make any provisions for them in their construction. I think it would be cool, but very hard to pull off. Maybe I'll build them some metal bikes when they get to that point. I better start learning to weld!

  9. recently i ran into a problem which is popular of many of us ! what i need to do and how to go on living, I can not understand ((I have stopped smiling at ALL!!!! :( yes!!,i have bad looking teeth because of heredity ... why I? Teeth is the first thing you see when chat anybody,or doing smth like that, I found a solution in putting lumineers ! and i can say it has guaranteed 100% result,now i know i promise you!

  10. Bravo on these epic posts (and the building of these trikes)! And congrats on the mention on Daddytypes - I chuckle when all the dad blogs I read cross pollinate each other.

    I saw these bikes for the first time on the totcycle blog. I LOVED it, but just couldn't justify the price at the time. It was all around the same time I had come across Sweet Juniper's popcycle post, which led me to get myself a bike and Bobike seat for my daughter. I also lusted after those cargo bikes like Bakfiets, but they're just not that feasible in Chicago. I got stares from just having my daughter sit up front with me on the Bobike seat (which she ADORED), but that might've been also because of my x-ray brain helmet. I'm slowly justifying purchasing this trike for my son and am now thinking about how we're going to haul my daughter around this summer now that she'll be 4. We bought her a crappy training wheels set from Target, but that's just for going around the block. If anyone from the company takes issue with your "infringement", just let them know that your post is about the best advertisement ever for their bike.


  11. That is awesome. I'd love to be able to build something (read: anything) even remotely as cool as these trikes. Sadly, my handyman skills are limited to hanging pictures and changing lightbulbs. As a VERY brand new father of triplets, I have a much greater appreciation for work like this. It may seem somewhat of a simple (albeit time-involved) project for you, but take it from someone who'd rather buy the "Floor model" to avoid the intimidation of "Some Assembly Required": You should be very proud of your work. I have 3 "some-assembly-required" cribs en route, and I'm aiming to have them assembled before the kiddos get to come home!




Don't hold back.


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