just imported a simple differential drive with 2 motor controlled big wheels at the front and 2 small caster wheels in the rear.

Have found 2 issues which have me stumped.

1.) After increasing the weight of the motor, the robot slowly slides across the floor when still.

2.) The rear caster wheels always turn perpendicular to the direction of movement. When the robot stops moving the rear wheels turn to align with the front wheels.

below is the code for one of the caster wheels

<link name="rrw">
  <visual>
    <geometry>
      <cylinder length="0.03175" radius="0.0635"/>
    </geometry>
     <material name="black">
       <color rgba="0 0 0 1"/>
     </material>
  </visual>    
  <collision>
      <geometry>
        <cylinder length="0.03175" radius="0.0635"/>
      </geometry>
  </collision>

  <inertial>
    <mass value="1"/>
     <inertia ixx="5.1458e-5" iyy="5.1458e-5" izz="6.125e-5" ixy="0" ixz="0" iyz="0"/>
   </inertial>
</link>