My trailer was originally designed and built to be towed behind a Mountain Bike on 26 inch diameter wheels. Like you, my trailer hitch started out on the seat post tube, and this position was chosen because my regular commute to work, and to town included off road gravel ballast covered farm tracks, a bridal way for horses, and about 800 metres through the forest on varying surfaces (alas, I no longer live there ) The seat post hitch seemed better suited to the terrain that I was cycling over, as a chain stay mounted side hitch would possibly have hit; rocks, logs, fallen branches, vegetation, and other objects etc.

As an Agricultural engineer, I also knew how difficult, time and material consuming getting a side mounted hitch set up correctly for the trailer to track perfectly central would be.

When I sold my old MTB, and bought the hybrid, wheel size was a major consideration for me and the city streets I would be using it upon, and so I chose a bike on 700c wheels, with absolutely no thought at all about using my trailer at the time. Because of the drawbar length the trailer's parking prop stand tube contacted the bike's mudguards which made using the trailer impossible. So rather than extending the drawbar (20mm O/D X 1.6 mm wall thickness mild steel tube) and increasing the leverage that would have entailed, I decided to mount the hitch ball off the top off the wheel stays, and stand it back so the trailer cleared the bike's rear wheel, and to level the trailer using a 16mm dia solid bar. This means that no stress loading has been imparted upon the seat post in its extended position for my long legs, or it's single 8mm clamping bolt .

The 10mm spring release quick-attach ball joint has proven to be very reliable in use: I have towed up to 100 kgs through it covering about 2000 miles since it was fitted 2 years ago, and it has saved me from injury during a recent accident: I got rear ended in the trailer when I was stationary at the lights. The ball joint was popped off, and my cheap padlock shackle was sheared. One of the rear horizontal cross frame tubes on the trailer was bent (and still needs replacing), and the sheet metal pull back shroud on the ball joint was slightly damaged, but I was unhurt and my Bike undamaged. Two minutes work with a pair of pliers has repaired the ball joint shroud and everything is now working correctly and securely.