This is a fairly straight forward woodworking project where a lot of the technical aspects can be done in different ways. I made this trike for my daughter's 1st birthday from scrap wood around the shop.
This Instructable covers all the main details but you can see it come together visually in this Youtube video:
It uses a steel axle rod at the front and back, and the seat is set back closer to the back wheels, so most of the weight won't be on the wood. This means you could use just about any wood, regardless of its strength.
The wheels and axle rod come together and were both bought at my local big box store. They are designed for wheel barrows and come with a bearing already installed.
Step 1: Create the Down Tube
This trike only needs one part for the body, unlike a traditional bike with a top and down tube. I glued 2 pieces of timber together to make up the blank, but you could just as easily by this piece all ready to go from a hardware store.
The size will depend on the size of the child, but my blank was around 470mm long and 40mm wide. The height determines how much curve can be added. You could make this piece straight, with no curve, but I allowed an extra 20mm in height to cut the curve.
Draw the curve onto the blank but don't cut it out yet! Before that you'll need to drill a hole through for a pin that holds the front handle bars in place. Doing this while the piece of wood is square will make life a lot easier.
From there you can cut the shape into the piece and smooth out the curve using a sander.
A lot of my measurements were based off the trike in the background. This is a good way to test the height; sit your child on it, get an idea, then apply to your trike.
Step 2: Headset Assembly
I used 2 thinner pieces of wood to create the headset. This is the component that holds the front forks to the body.
Using a curved object I marked and cut a shape into the pieces. Then I drilled a small hole in the centre and two 25mm holes on either side. These bigger holes are for dowels that will be inserted at the end. The size of this hold will depend on what size dowel you plan on using. I would suggest 19mm (3/4") or bigger.
The centre hole is for a bolt. I used an M5 bolt so I drilled a 5mm hole. The tighter this fit the better.
It's also important that the holes on the 2 pieces of wood line up with each other. A good way to do this is to drill the centre holes of both pieces at once then use the bolt to clamp them together. Now drill the dowel holes and they will be the same distance from the centre.
Step 3: Back Axle
The back axle is made by gluing 2 pieces of wood together around a steel bar.
I started by cutting the bar to length, 300mm. At each end I drilled a hole for a split pin to hold the wheels in place.
Then I took 2 pieces of wood (I cut mine down from 1 piece) and used a router to cut a groove, half the thickness of the rod. You could use a straight bit but I used a round nose bit so the result was a tube through the 2 pieces. Ease up on the depth; you want the bar to be as snug as possible without impeding the glue up. It would be better for the bar to have more room than less.
Once the 2 pieces of wood are glued together cut the board to length. find this length by making a cut, attaching the wheels, repeating until the wheels fit the assembly with as little rod extending past as possible. You'll need enough room to slide a split pin into the rod on the outside of each wheel, but try not have the rod extend too far. You can also use washers to space the wheels.
Step 4: Handlebars
I made the handlebars by cutting a rectangular board down and then using a big router bit to round over the corner. If this is done right you'll end up with tubes at the end of board, with a rectangle in the middle. This allows for somewhere to attach the handlebars to the forks.
Cut the board into a rectangular shape and, using a spade bit or forstner bit, drill out holes for the board to fit over the forks. The spacing of these 2 holes is important. Once you've drilled these holes you cut the shape around them. When you drill the holes break through onto the other side of the board, we'll use this centre hole later.
The forks are cut down form dowel stock. Spend some time selecting the straightest dowel you can find. How high you want the dowels to extend above the body is up to your preference.
The forks are held in place using small dowels. The spade bit or forstner bit leaves a centre hole. When the handlebars are place over the forks, follow that centre hole back through with something like an 8mm drill bit. This will self centre the hole in the big dowel.
Make sure to drill holes for the front axle rod.
We'll glue this up later.
Step 5: The Seat
The seat is set into the body using 2 dowels. There are 2 in place to stop the seat from rotating. I made the seat removable so when my child's legs get longer I can make another seat, but you could just glue it in place.
Start by drilling holes into the body. This step is made a lot easier with a drill press but you can do it with a hand drill and some patience. You'll need a spade bit for this because they have a long point at the tip.
Once you've drilled the 2 holes, as plumb as possible, insert the 2 dowels and mark where they need to be cut.
Now place the seat (just a piece of wood cut to shape) over the dowels and mark where they are located. Then you can drill into the seat to attach it to the dowels.
Step 6: Glue Up and Oil
Lastly glue the pieces together. The only extra bit of work you'll need to do is pin the forks into the headset with a couple dowels.
Pay attention to the back axle join. This is where the majority of the forces will be applied, especially when the trike turns. This join is the most likely to fail so before you throw your little one onto it make sure the glue has properly cured.
I finished the whole trike with a food safe oil.
The headset fits over the body and an M5 bolt is slid in. This needs to be done before the wheel goes on. Once you've tightened this down, but still allowing the handlebars to rotate, you can install the front wheel in place.