-increase total weight capacity
-lower the center of gravity
-improve the hitch system
-make it compatible with Rubbermaid's 18 gallon bins
Here is a picture of the old trailer...
I decided that I could improve the weight distribution by allowing space for three bins on the trailer bed. If I was hauling the same four bins only one would be on top and the center of gravity would be lowered. This makes for a bed length of four feet.
Lowering the bed below the wheel hubs necessitates using a large wheel. Most bicycle trailers use 20" wheels or smaller. Mine is equipped with 26" wheels and could accommodate 700c wheels. Smaller wheels may be a bit stronger, however, when I consider the amount of weight and abuse that a pair of 26" mountain bike wheels are designed to endure, I have no trouble trusting them to haul loads of 200-300lbs on short trips around town. Additionally, larger wheels roll over bumps more gently, which is an important advantage. While it's tempting to think that using a smaller wheel would lower the center of gravity, the goal is to lower the center of gravity in relation to the wheel hubs. This is what makes for a more stable load less prone to tipping.
Finally, I chose to run the tow bar up the center of the trailer and mount it to a ball-joint on my rack directly above my bikes rear wheel. Many trailer hitches attach to the rear triangle but this makes it more difficult to use with a kick-stand, which I consider indispensable. This center mounting design is relatively simple and is common on cargo trailers. My only concern was how it would affect the handling of the bike, especially when the bike wants to lean. After three weeks of practice I can say that leaning is not a good idea while carrying a heavy load. As you lean, the front of the trailer dips downward and applies more weight into the lean; my experience is that this forces me to steer dramatically in the direction of the lean before I can stabilize the bike (at high speeds this would surely be disastrous but at slow speeds it is merely comical). The video below demonstrates how the trailer responds to the leaning of the bike. To adjust one needs to take curves slowly and remain as upright as possible. The advantage on curves is that your turning radius will not be hindered by your tow bar contacting your rear wheel.
You can see the trailer for yourself at the Salem Saturday Market between 9am and 3pm. Our coffee booth is near the corner of Marion and Winter Streets.