After 2 years of use, the hitch, the connecting posts and the bearings have been changed https://www.instructables.com/id/4-wheel-bike-trailer-version-2/ which makes the trailer MUCH better!
A 4 wheel cart, or trailer for behind the bike is more complicated to build than a simple 2 wheel model, which can be rigid. The front wheel assembly needs the ability to turn, the hitch has to be able to tilt up and down, and suspension should be used to allow the front wheels to follow the unevenness of the road independently from the ones in the back.
I built the trailer in less than a week, including painting.
The costs were about E 55 ($ 80)
Needed: 2 hand trucks with inflatable wheels
Some plywood, preferably water resistant
About 3 m of wood , like 40x70 mm or 10 ft of 2x4�
2 (broken) bike seats, for the springs
Nuts and bolts, and lots of washers
Steel central heating pipe 22mm, 2 elbow compression fittings and one tee
4 teflon furniture gliders
Tools: Drill, pipe bender, wrenches, hacksaw, saw.
Step 1: Back Assembly (rear Unit)
1 Take the wheels A and the 2 connecting brackets B apart from the hand truck
2 Bolt these brackets on a piece of plywood C, which needs 4 holes for connection to the front unit
Step 2: The Front Unit
Two suspension bearers of wood F are resting on both sides of the axle, fastened by brackets G. In order to obtain independent suspension, the main board H is resting on 4 bike seat springs I, which rest on the suspension bearers.
4 bolts J stick down from the main board trough each spring and trough wide holes in the suspension bearers, to guide the springs and keep them in place. A washer and locknut at the end prevents each bolt from coming loose. A hole at the front K in both suspension bearers is made for the tilting part of the boom L, which will connect to the rest with 2 compression elbows M.
The rotating board N will be rigidly attached to the back assembly and will rotate on the main board. It rests on 4 teflon furniture gliders P and rotates on a bolt O and locknut in the centre, with washers in between to keep spacing.
4 bolts sticking out on top of the rotating board are to receive the connecting beams from the back assembly.
Step 3: Drawing of Front Unit
O: Rotating bolt, washers for spacing
N: Rotating board, with 4 bolts sticking up for connection to back unit
P: Teflon furniture glider
H: Main board, with:
J: 4 bolts sticking down, to contain springs. Locknuts and washers at the ends
I: Springs from bike seat
F: Suspension bearer, only one is drawn. Rests on axle in middle, not drawn
K: Hole for horizontal part of hitch - 22.5 mm
Step 4: The Hitch
The hitch has proved to be too long, amplifying vibrations.
The new hitch can be connected by the same compression fittings. Compression fittings are a very good way of making a strong, but removable connection.
Step 5: Receiver
It allows pivoting up and down, allows turning and allows some left and right tilting.
The updated receiver (for the new hitch), is located near the left side of the rear axle of the bike