Beginning of URDF specified toy car.

simplecar.urdf

<?xml version="1.0"?>
<robot name="simplecar">
    <!-- Base Frame of Car -->
    <link name="base_link">
        <visual>
            <geometry>
                <box size="0.5 0.3 0.1"/>
            </geometry>
        </visual>
    </link>

    <!-- Left Back Wheel -->
    <link name="left_back_wheel">
        <visual>
            <geometry>
                <cylinder length="0.05" radius="0.1"/>
            </geometry>
            <origin rpy="1.57075 1.57075 0"/>
        </visual>
    </link>
    <joint name="base_to_left_back_wheel" type="continuous">
        <parent link="base_link"/>
        <child link="left_back_wheel"/>
        <axis xyz="0 1 0"/>
        <origin xyz="-0.2 0.175 0"/>
    </joint>
    <!-- Right Back Wheel -->
    <link name="right_back_wheel">
        <visual>
            <geometry>
                <cylinder length="0.05" radius="0.1"/>
            </geometry>
            <origin rpy="-1.57075 -1.57075 0"/>
        </visual>
    </link>
    <joint name="base_to_right_back_wheel" type="continuous">
        <parent link="base_link"/>
        <child link="right_back_wheel"/>
        <axis xyz="0 1 0"/>
        <origin xyz="-0.2 -0.175 0"/>
    </joint>
</robot>