[EricH]

If the casters go beyond the FRAME PERIMETER as defined in Section 8.2 of the Manual, then no, it is not legal. It is not one of the exceptions listed in <G30>.

FRAME PERIMETER – the polygon defined by the outer-most set of exterior vertices on the ROBOT (without the BUMPERS attached) that are within the BUMPER ZONE. To determine the FRAME PERIMETER, wrap a piece of string around the ROBOT at the level of the BUMPER ZONE - the string describes this polygon. Note: to permit a simplified definition of the FRAME PERIMETER and encourage a tight, robust connection between the BUMPERS and the FRAME PERIMETER, minor protrusions such as bolt heads, fastener ends, rivets, etc are excluded from the determination of the FRAME PERIMETER.

<G30> ROBOT Volume – During a MATCH, no part of the ROBOT shall extend outside the vertical projection of the FRAME PERIMETER, except as follows:[list of exceptions]

If the GDC isn't allowing bolt heads to extend past the FRAME PERIMETER, except in one very specific case, then I doubt that they'd allow a caster to do the same.

Now, if your lower frame is smaller than your upper frame, and the caster goes beyond that, but not beyond the FRAME PERIMETER, then you'd be OK.