It was pretty cheap to make with most of the cost in bolts and paint.
1 child's 16 inch BMX bike
20mm plywood (800 x 160mm)
22.2mm steel tube
5 x 8mm bolts and nylock nuts
4 x 6mm bolts and nylock nuts
7 x panel washers
1 can undercoat spray paint
1 can topcoat spray paint
8 x 60mm wood screws
Brake cable inner and outer
Drill with 6 and 8mm bits
Step 1: Deconstruct the Bike
Put aside the following:
Headset stem and handlebars
Forks, bearings etc
Step 2: Saw the Frame
Hacksaw off the top tube and seat tube.
Cut through the bottom bracket. Make sure the bearing cups are out of the bottom bracket as these will be harder to saw through than the bracket itself. Clean up the parts for painting later.
Step 3: Make the Deck
Cut out a hole for the rear wheel. I made mine by using the chain stays as a guide.
I wanted the fit to be tight and ending up filing a groove out for the frame to sit in between the wheel and the deck.
Also cut out some supports to bolt the chain stays to.
Step 4: Assemble the Rear Deck
I used 4 x 60mm screws in each side. Counter sink the screws as you don't want them sticking up in the top of the deck.
Once the glue has dried, work out how you can bolt the chain stays onto the deck. Ensure you have good ground clearance
for the back wheel, but not too high to make scooting a major work out.
I ended up changing the position of the frame on the deck to give more ground clearance. The second photo shows how it was before modification.
Step 5: Front Deck
It would have been great to get a plate welded onto the down tube, and just bolted that on. This is one part of the build that needs improvement as there is some flexing in the deck.
This was improved later by screwing a aluminum support under the deck.
Step 6: Extend the Handlebars
The handlebars can be set at whatever height is comfortable.
I sourced some old grips and remounted the brake after painting. The brake cable has been extended.
This design works well enough, but I'm sure it can be improved. Good luck!