An autonomous mobile surface treating apparatus having a chassis, a drive mechanism mounted to the said chassis by a suspension, and a substantially rigid shell movably mounted to the chassis. The suspension includes a resilient member interposed between the drive mechanism and the chassis so that when the shell is pushed toward the supporting surface the resilient member to permit the drive mechanism to move and the shell and/or the chassis to contact the supporting surface. The shell is supported by a plurality of elongated elastic supports received within plurality of elongated openings in the chassis. Preferably, a passive portion of a collision detection sensor is attached to a central portion of the shell. A non-skid lower edge member movably attached to the shell to adjust a clearance between the non-skid lower edge member and the supporting surface. A vacant volume that defines a module receiving area adapted to removeably receive a surface treatment module.