Just finished making the disk sander from http:benchnotes.comI added a little touch and used double sided carpet tape so I could use regular size sheets of sandpaper, just lay the tape on the disk pulled the backing off, lay the sandpaper, trim off excess from disk, works very well.
Think skateboard trucks and wheels - with a kickplate to hide them and 1/16 inch shorter than the distance between the bottom of the wheels and the top of the truck. Non-marring, good weight bearing support, readily available in most towns (or in your kid's room) roll smoothly because of composition of the wheels and because they use bearings ...
BUT - be very aware that the door bottom must accurately parallel the floor throughout it's range of motion or the wheels with either try to lift the door - screwing up the hinges OR not make contact with the floor unless the door sags - also not good.
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