This one, made from scrap lumber I had laying around, has thinner plywood and a frame of 2x6s. The mid brace keeps the 2x6s from rolling, but honestly due to the stiffness of the dolly vs. camper, the load really piles up on the corners above the wheels. So far it's shown no flex issues. It's not terribly heavy (but not super light either).
It's kinda like holding up a grand piano on two floor joists, which my 900 lb piano has done to several homes.
The plywood is on the bottom. I didn't want to load the plywood, since it's weak in that direction, but used it as a shear member to keep the frame from going parallelogrammy (that's a new word!). It's handy to put lightweight items in the dolly when the camper is out playing.
I wish the wheels were a little larger. It can be hard to push it over bumps in our driveway and garage. Deb and I are a couple of small weaklings. We just went through that exercise yesterday. We belay it from a good fence post anchor using a climbing rope and prussik knots, so it won't take off on us down the alley!
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