A mobile surface cleaning robot that includes a robot body having a forward drive direction and a drive system supporting the robot body above a floor surface. The drive system includes right and left drive wheels and a caster wheel assembly disposed rearward of the drive wheels. The caster wheel assembly includes a caster wheel supported for vertical movement and a suspension spring biasing the caster wheel toward the floor surface. The robot also includes a cleaning system supported by the robot body forward of the drive wheels and having at least one cleaning element that engages the floor surface. The suspension spring has a spring constant sufficient to elevate a rear end of the robot body above the floor surface to maintain engagement of the at least one cleaning element with the floor surface.