Placed the base_footprint frame before the base_link frame.

urdf/pioneer3.gazebo

@@ -7,6 +7,10 @@
     
     <xacro:iri_skid_steering_gazebo name="${name}" config="${sim_config}"/>
 
+    <gazebo reference="base_footprint">
+      <material>Gazebo/Red</material>
+    </gazebo>
+
     <gazebo reference="base_link">
       <material>Gazebo/Red</material>
     </gazebo>

urdf/pioneer3.xacro

@@ -7,6 +7,14 @@
 
   <xacro:macro name="pioneer3" params="name resolution sim_config">
 
+    <link name="base_footprint"/>
+
+    <joint name="base_footprint_to_base_link_joint" type="fixed">
+      <origin xyz="0 0 0" rpy="0 0 0"/>
+      <parent link="base_footprint"/>
+      <child link="base_link"/>
+    </joint>
+
     <link name="base_link">
       <collision>
         <origin xyz="0 0 0.19" rpy="0 0 0"/>
@@ -30,14 +38,6 @@
       </inertial>
     </link>
     
-    <link name="base_footprint"/>
-
-    <joint name="base_link_to_base_footprint_joint" type="fixed">
-      <origin xyz="0 0 0" rpy="0 0 0"/>
-      <parent link="base_link"/>
-      <child link="base_footprint"/>
-    </joint>
-
     <link name="top_plate">
       <visual>
         <origin xyz="0 0 0" rpy="0 0 0"/>

urdf/pioneer3_wheel.gazebo

@@ -11,19 +11,7 @@
     </gazebo>
     
     <gazebo reference="${name}_wheel">
-
-      <material value="Gazebo/Black"/>
-
-      <!--
-      <kp>1000000.0</kp>
-      <kd>100.0</kd>
-      <mu1>1.0</mu1>
-      <mu2>1.0</mu2>
-      <fdir1>1 0 0</fdir1>
-      <maxVel>1.0</maxVel>
-      <minDepth>0.00</minDepth>
-      -->
-
+      <material>Gazebo/Black</material>
     </gazebo>
     
   </xacro:macro>