Thanks Tack, yes I did think about it, I have the Standard for trailers downloaded.
The C of G at the moment is just where the forward taper starts so with my design, the axle pivot point is to the rear by about 200mm giving about a 3kilo load on the tow point. Dimensions are similar to a Bandicoot trailer.
Today just located bike springs/shocks to the size I wanted. Off a Kwaka Z250 or similar. Not to strong, and not to weak. Now to design and build the mount points. As with everything on the trailer, its over engineered. The mounts on the Z250 look positivelyweak compared to mine.
The pivot point and bushes for the trailing arm are 4 tonne bushes, mounted into heavy spring hangers. I dont think any thing is going to break in a hurry.