[MrForbes]

Driving over the bump looks easiest with a robot that has an open area in the middle of the drive train (as viewed from the side). That implies that 4wd or 8wd might be the way to go. 6wd seems to add a lot of complication to try to overcome the falling problem as the robot crests the bump.

And if your center wheels can lift up to follow the bump, the can probably also lift up in normal driving? so you really have a long wheelbase 4wd robot anyways.