Probably the best wheel to use is wheel chair hubs with a solid mount point on either side.I have used 16" and 20" wheels like this and could use any size. I have used a fork end on either side of the trailer that actually uses a front wheel with stock parts. The advantage of these setups is a lower center of gravity for example using a regular 26" wheel the trailer bottom can be 10 inches or lower by mounting the wheel higher on either side of the trailer.



This trailer is about 20 pounds and will haul 200 pounds as it is 4130 cro moly steel and very strong. with an 8" floor it is also very stable and the axle is on just the sides, the wheels are just 20" front wheels and parts are not special.

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LennyHarp of Lenny's Bikes & Things
http://lennyh.com
The true value of a man is not judged by what he has, but rather by what he can do without.
"We are what we repeatedly do. Excellence, then, is not an action, but a habit." -- Aristotle