Don't you just hate it when...

[capncarl]

I ran into a couple of boxes that I built with casters the turned over easily. The swivel casters allow the point of contact with the wheel to the floor to be well under the box. This makes it tip over really easy when you run over something on the floor, more so if it is top heavy. Small diameter casters are not as prone to this as a large diameter caster a big swing on the swivel. You might consider putting a larger base plate for the swivels under the box to widen the wheel base.
capncarl

[dbhost]

The casters are 3", and so far, no tippiness, however it DOES stop rather suddenly when I run over a screw or whatnot missed by the broom!