Dude, stay away from those rolling 1 wheel bike dolleys that let the rear wheel stay on the ground. Other than wearing down the back tire, you will also destroy your constant mesh trans because you don't oil it when the bike is off. Also there will be excess chain wear, and a host of other stuff, sounds like a great idea, but it isn't.
Also don't bother building your own trailer, you can buy a somewhat decent one from harbor freight for a few hundred.
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I had one of these a while back, I needed it for one trip and sold it for more than I paid for it. All I did was add 2 1/2" pieces of plywood on top, and two of the front wheel chocks. I also went the extra step and added some supportes to the frame with some 1x1" steel tube. I also replaced the bearings as they looked fairly cheap, I think it cost me $50 for a decent pair. All in all it was a great trailer and did more than I asked it to. Plus building your own trailer would be just about the same price, a ton of extra trouble, and possibly more expensive if you cant get it to roll straight
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