I had seen a previous Instructable that relied on a single swivel caster to allow the bike trailer to turn from side to side and move up and down vertically, but then suggested a "pipe and sleeve" sort of coupler to allow trailer rotation (tilt). Although that proposed Instructable would work as suggested, it seemed to me that, with the investment in a second swivel caster mounted on a fixed frame at a 90 degree angle to the first swivel caster, you could very easily attain flexibility in all 3 planes of motion: (1) allowing horizontal turning side to side; (2) allowing the trailer tongue to move vertically up and down; and (3) allowing the trailer to "rotate" - to "tip" from side to side.
The proposed plan allows this movement in the following ways: (1) one caster wheel is trimmed flush on one side to allow it to be bolted horizontally to a point on the bicycle, and then the caster wheel frame is again reattached to the wheel, allowing the caster to turn horizontally on the bicycle mounting point; (2) the caster swivel on this first caster also allows the trailer tongue to move up and down vertically; and (3) the caster swivel on the second caster allows the trailer tongue to "rotate/tip" from side to side during travel independent of the vertical angle of the bicycle.
The parts you will need for this plan are simple:
(1) Two swivel casters of sufficient size - I used 2" swivel casters that are available at local hardware or home improvement stores for about $4 apiece.
(2) A frame to attach these casters at right angles to each other. I purchased a heavy duty right angle galvanized construction connector (i.e., Simpson), readily available from local lumber yards or construction supply stores, also for less than $4.
(3) Sufficient short bolts and locking nuts to attach the caster to this frame. I used eight 1/2" long 1/4" bolts and nylon-insert lock nuts to bolt the casters to the galvanized construction connector.
(4) Appropriate hardware to attach the coupler both to your bicycle and to the trailer frame. I fabricated an extended mount on my bicycle (see step 2) that is shown as the red square tubing in the picture illustrating this step. The trailer tongue is the black section of square tubing also shown in the same picture. I used the caster wheels as described in the following steps to attach the trailer coupler to both the mounting point on the bicycle and to the bicycle trailer.
(5) Though purely optional, I wanted to be able to quickly attach and detach the bicycle trailer from the bike; and therefore replaced one of the two caster wheel bolts with a similar diameter and length locking hitch pin that slips through the caster wheel bolt holes and the caster wheel bushing (see ring on top of this locking hitch pin inserted through the caster on the right side of the accompanying photo). This is also a hardware store item and should cost less than $4.
Step 1: Determining or Building an Attachment Point on Your Bicycle Frame.
In order to move the pivot point between the bicycle and the trailer to a point where it would allow the trailer to turn from side to side as far as possible, I attached a 3/4" X 3/4" length of square channel steel tubing that fastens behind the main chainwheel of the bicycle, and is firmly clamped with several coaster brake frame clamps along the left chainstay to the left rear dropout (hub), and then extends past the rear dropout past the rear tire of the bike. I painted this piece of tubing the same color as the bicycle and it remains on the bike at all times.
At the very end of this piece of added tubing, before it was installed, I drilled and attached one of the two caster wheels. This caster wheel was cut flat on one side with a Dremel tool cutting wheel to facilitate its attachment to the tubing, and was drilled though vertically to allow it to be bolted to the tubing. The caster wheel frame is then attached over the caster wheel and turns around the caster wheel (much in the manner it was originally designed) and allows the trailer to turn left and right freely.
Step 2: Building Swivel Attachment.
Step 3: Attach Trailer Swivel to Bicycle Trailer.
So when I attached this second caster to the handle of my bike trailer, I used the wheel frame of the caster wheel to straddle the tongue of my bicycle trailer, cutting the caster wheel with a Dremel tool cutoff wheel to fill the gap between the width of the trailer tongue and the bolt hole in the caster frame for the caster wheel. Although I filled this gap tightly with the cut caster wheel and it was probably sufficiently "friction fit", I did not want to take a chance with the wheel slipping off the trailer tongue so I also drilled this second caster wheel and bolted it to the trailer tongue (you can see the bolt hole in some of the pictures of the caster wheels - I drilled a hole through the wheel in the diameter of the bolt, but then use a larger drill bit to countersink the bolt head into the wheel).
Step 4: Attach Trailer to Coupler.
To that frame, I added a large plastic tub with a locking latch lid to shelter what I am carrying in my bike trailer from the elements. In order to attach the plastic tub to the frame, I used two short sections of 1" X 3" dimension lumber that fit into the bottom of the plastic tub perpendicular to the trailer frame, attached the boards in place with Liquid Nails glue and through the bottom of the plastic tub with short wood screws, and then attached the plastic tub to the frame using 3/4" conduit clamps that bolt through through those short pieces of wood, the bottom of the plastic tub and the "ears" of the conduit clamps using 1/4" X 1 1/4" carriage bolts and nylon-insert lock nuts. I used two short pieces of board in the bottom of the plastic tub and two conduit clamps to create two attachment points for the plastic tub.
I also reinforced the lid of the plastic tub using a piece of 1" x 3" dimension lumber that spans the top of the plastic tub under its lid. I glued this board in place at its ends using the Liquid Nails and a short wood screw at each end, but also used 4 small eye screws that go through the side walls of the plastic tub and into the ends of this board. These eye screws will provide anchor points for bungee cords if I were to strap items to the lid of the trailer that is reinforced by the extra 1" X 3" board attached immediately underneath the lid.