Hello. I don't know what the exact minimum could be, but I believe the standard separation from centre rail to centre rail is 6" (15cm). So you would go from O60 to O72 to O84. These all have a 12"(30cm) difference in diameter, or a 6"(15cm) difference in radius. You could probably get away with a little less at the larger radii, but it's always better to be safe than sorry.
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