cart_rigid_suspension consists of four non-steerable wheels attached to a chassis. Very simple.
cart_front_steer consists of two non-steerable rear wheels and two steerable front wheels attached to a chassis. The front wheel steer and spin degrees of freedom are implemented with a single universal joint.
cart_soft_suspension extends cart_front_steer by adding an additional prismatic joint between the chassis and the wheels with stiffness, damping, and spring reference position parameters. This model requires sdformat pull request 102 and gazebo pull request 1117.
The models are written as parameterized erb models. This allows easy modification of the chassis size, wheel size, suspension travel, inertial properties, suspension properties, etc.