Jason,
I love your design. It looks like a really great trailer.
My only concern would be with the axle width. If you had to change direction quickly, to avert an incident or just taking a bend too quickly, that the bike's weight my provide enough leverage to topple the trailer over.
Obviously your road test will sort this out but it may be worth having someone looking out of the back window or following to see if either of the wheels pick up off the ground around bends.
But at the moment, I'm still in. How much are they?
