I had one... for a month. It was a home-built unit. Built to withstand a class 5 hurricane, I think, or maybe it was to move other RV's. Either way it was heavy and would have extended our 33' coach another 15 or so feet before adding the car so I re-sold it. The weight of the car behind the axle (depending on how far back) will offset the weight of the bike. I scrapped the entire idea (and left the bike home) and got a convertible (Mini cooper Roadster) to tow behind 4-down. I miss my bike sometimes but I don't miss the trouble that I would have had trying to get the trailer to work for me. I think I'd only want to do this if I had a DP and not try it with a gasser.
If I do try to do something with my motorcycle in the future I'll likely go with one of the lifts as then I can still tow 4-down.
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Thom and Diane Boles
2010 Winnebago Vista 32K