With a mecanum drive robot, you
will need some sort of a suspension system to make sure every mecanum wheel is in contact with the floor at all times. Mecanum is far less forgiving to uneven carpeted fields that other types of robot drivetrains, and unless your robot has a suspension system it will fly off at weird and "unpredictable" angles on uneven floors, which are quite common at competition venues.
There are several ways to accomplish this. The first is that you can have each mecanum wheel pivot on one end, with the other end supported by some sort of rubber piece. (See
this picture from Team 488. Team 40 also used pieces of rubber hose, although I can't quite find a picture of their mecanum module at the moment.) Your other option is to split the frame in half, a la Team 190 in 2005, and have the two halves of the frame pivot in the middle. (See below picture)
CAD drawing of Team 190's 2005 lower chassis.