Why pallet is not smooth enough ?

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I want to attach a pallet to a robot. However, it is not smooth enough. Here you can find the https://youtu.be/wU0WEoQxzTo how it works. Any idea how could I improve the smoothness.

<?xml version="1.0"?>
<robot name="jackal" xmlns:xacro="http://www.ros.org/wiki/xacro">
  <xacro:property name="PI"                        value="3.1415926535897931" />
  <xacro:property name="wheelbase"                 value="0.262" />
  <xacro:property name="track"                     value="0.37559" />
  <xacro:property name="wheel_vertical_offset"     value="0.0345" />
  <xacro:property name="footprint_vertical_offset" value="-0.00" />
  <xacro:property name="wheel_radius"              value="0.065" />
  <xacro:property name="wheel_width"               value="0.040" />
  <xacro:property name="chassis_length"            value="0.420" />
  <xacro:property name="chassis_width"             value="0.310" />
  <xacro:property name="chassis_height"            value="0.184" />
  <xacro:property name="dummy_inertia"             value="1e-09"/>
  <xacro:property name="mount_spacing"             value="0.120" />
  <xacro:arg name="gpu" default="true"/>
  <xacro:property name="gpu" value="$(arg gpu)" />

  <material name="dark_grey"><color rgba=" 0.2 0.2 0.2 1.0" /></material>
  <material name="light_grey"><color rgba="0.4 0.4 0.4 1.0" /></material>
  <material name="yellow"><color rgba="0.8 0.8 0.0 1.0" /></material>
  <material name="black"><color rgba="0.15 0.15 0.15 1.0" /></material>

  <xacro:macro name="wheel" params="prefix *joint_pose">
    <link name="${prefix}_wheel_link">
      <visual>
        <origin xyz="0 0 0" rpy="${PI/2} 0 0"/>
        <geometry>
          <mesh filename="package://antboy_gazebo/meshes/jackal-wheel.stl"/>
        </geometry>
        <material name="black" />
      </visual>
      <collision>
        <origin xyz="0 0 0" rpy="${PI/2} 0 0"/>
        <geometry>
          <cylinder radius="${wheel_radius}" length="${wheel_width}"/>
        </geometry>
      </collision>
      <inertial>
        <origin xyz="0 0 0" rpy="0 0 0"/>
        <mass value="0.477"/>
        <inertia
          ixx="0.0013" ixy="0" ixz="0"
          iyy="0.0024" iyz="0"
          izz="0.0013"/>
      </inertial>
    </link>
    <gazebo reference="${prefix}_wheel_link">
      <material>Gazebo/DarkGrey</material>
    </gazebo>
    <joint name="${prefix}_wheel" type="continuous">
      <parent link="chassis_link"/>
      <child link="${prefix}_wheel_link" />
      <xacro:insert_block name="joint_pose" />
      <axis xyz="0 1 0" />
    </joint>
    <transmission name="${prefix}_wheel_trans">
      <type>transmission_interface/SimpleTransmission</type>
      <joint name="${prefix}_wheel">
        <hardwareInterface>hardware_interface/VelocityJointInterface</hardwareInterface>
      </joint>
      <actuator name="${prefix}_actuator">
        <mechanicalReduction>1</mechanicalReduction>
      </actuator>
    </transmission>
  </xacro:macro>

  <xacro:wheel prefix="front_left">
    <origin xyz="${wheelbase/2} ${track/2} ${wheel_vertical_offset}" rpy="0 0 0" />
  </xacro:wheel>
  <xacro:wheel prefix="front_right">
    <origin xyz="${wheelbase/2} ${-track/2} ${wheel_vertical_offset}" rpy="0 0 0" />
  </xacro:wheel>
  <xacro:wheel prefix="rear_left">
    <origin xyz="${-wheelbase/2} ${track/2} ${wheel_vertical_offset}" rpy="0 0 0" />
  </xacro:wheel>
  <xacro:wheel prefix="rear_right">
    <origin xyz="${-wheelbase/2} ${-track/2} ${wheel_vertical_offset}" rpy="0 0 0" />
  </xacro:wheel>

  <link name="base_link"></link>

  <joint name="base_link_joint" type="fixed">
    <origin xyz="0 0 0" rpy="0 0 0" />
    <parent link="base_link"/>
    <child link="chassis_link" />
  </joint>

  <link name="chassis_link">
    <visual>
      <origin xyz="0 0 ${footprint_vertical_offset}" rpy="0 0 0"/>
      <geometry>
        <mesh filename="package://antboy_gazebo/meshes/ant_boy.stl" scale="0.001 0.001 0.001"/>
      </geometry>
      <material name="yellow" />
    </visual>
    <collision>
      <origin xyz="0 0 ${chassis_height/2}"/>
      <geometry>
        <box size="${chassis_length} ${chassis_width} ${chassis_height}"/>
      </geometry>
    </collision>
    <inertial>
      <origin xyz="0.012  0.002 0.067" rpy="0 ...