Tread drive

<?xml version="1.0"?>

<sdf version="1.4">
    <model name="track_drive">
        <static>false</static>
        <pose>0 0 0 0 0 0</pose>

        <include>
            <uri>model://wheel</uri>
            <pose>-0.7 0 0.145 0 0 0</pose>
            <name>front_wheel</name>
        </include>

        <include>
            <uri>model://wheel</uri>
            <pose>0.7 0 0.145 0 0 0</pose> <!-- 0.125 for the wheel, 0.02 for the link -->
            <name>rear_wheel</name>
        </include>

        <include>
            <uri>model://chain_straight</uri>
            <pose>0 0 0.02 0 0 0</pose>
            <name>chain_straight_bottom</name>
        </include>
<!--
        <include>
            <uri>model://chain_straight</uri>
            <pose>0 0 0.29 0 0 0</pose>
            <name>chain_straight_top</name>
        </include>

        <joint name="slip_rear_top" type="prismatic">
            <pose>0 0 0 0 0 0</pose>
            <child>chain_straight_top::chain_link_01::link_a</child>
            <parent>rear_wheel::wheel</parent>
            <axis>
                <dynamics>
                    <damping>0.1</damping>
                    <friction>1.5</friction>
                </dynamics>
                <limit>
                    <lower>-0.01</lower>
                    <upper>0.01</upper>
                </limit>
                <xyz>1 0 0</xyz>
            </axis>
        </joint>

        <joint name="slip_front_top" type="prismatic">
            <pose>0 0 0 0 0 0</pose>
            <child>chain_straight_top::chain_link_21::link_b</child>
            <parent>front_wheel::wheel</parent>
            <axis>
                <dynamics>
                    <damping>0.1</damping>
                    <friction>1.5</friction>
                </dynamics>
                <limit>
                    <lower>-0.01</lower>
                    <upper>0.01</upper>
                </limit>
                <xyz>1 0 0</xyz>
            </axis>
        </joint>
-->
        <joint name="slip_rear_bottom" type="prismatic">
            <pose>0 0 0 0 0 0</pose>
            <child>chain_straight_bottom::chain_link_01::link_a</child>
            <parent>rear_wheel::wheel</parent>
            <axis>
                <dynamics>
                    <damping>0.1</damping>
                    <friction>1.5</friction>
                </dynamics>
                <limit>
                    <lower>-0.02</lower>
                    <upper>0.02</upper>
                </limit>
                <xyz>1 0 0</xyz>
            </axis>
        </joint>

        <joint name="slip_front_bottom" type="prismatic">
            <pose>0 0 0 0 0 0</pose>
            <child>chain_straight_bottom::chain_link_21::link_b</child>
            <parent>front_wheel::wheel</parent>
            <axis>
                <dynamics>
                    <damping>0.1</damping>
                    <friction>1.5</friction>
                </dynamics>
                <limit>
                    <lower>-0.02</lower>
                    <upper>0.02</upper>
                </limit>
                <xyz>1 0 0</xyz>
            </axis>
        </joint>

        <joint name="skid_rear_bottom" type="prismatic">
            <pose>0 0 0 0 0 0</pose>
            <child>chain_straight_bottom::chain_link_01::link_a</child>
            <parent>rear_wheel::wheel</parent>
            <axis>
                <limit>
                    <lower>0</lower>
                    <upper>0</upper>
                </limit>
                <xyz>0 0 1</xyz>
            </axis>
        </joint>

        <joint name="skid_front_bottom" type="prismatic">
            <pose>0 0 0 0 0 0</pose>
            <child>chain_straight_bottom::chain_link_21::link_b</child>
            <parent>front_wheel::wheel</parent>
            <axis>
                <limit>
                    <lower>0</lower>
                    <upper>0</upper>
                </limit>
                <xyz>0 0 1</xyz>
            </axis>
        </joint>

    </model>
</sdf>