diff --git a/frc971/control_loops/python/angular_system.py b/frc971/control_loops/python/angular_system.py
index 36da41a..bd4447b 100755
--- a/frc971/control_loops/python/angular_system.py
+++ b/frc971/control_loops/python/angular_system.py
@@ -22,6 +22,13 @@
                  kalman_q_voltage,
                  kalman_r_position,
                  dt=0.00505):
+        """Constructs an AngularSystemParams object.
+
+        Args:
+          motor: Motor object with the motor constants.
+          G: float, Gear ratio.  Less than 1 means output moves slower than the
+              input.
+        """
         self.name = name
         self.motor = motor
         self.G = G
diff --git a/frc971/control_loops/python/control_loop.py b/frc971/control_loops/python/control_loop.py
index 919968e..f124852 100644
--- a/frc971/control_loops/python/control_loop.py
+++ b/frc971/control_loops/python/control_loop.py
@@ -685,3 +685,7 @@
                    (12.0 - self.resistance * self.free_current))
         # Torque constant
         self.Kt = self.stall_torque / self.stall_current
+        # Motor inertia in kg m^2
+        # Diameter of 1.9", weight of: 100 grams
+        # TODO(austin): Get a number from Scott Westbrook for the mass
+        self.motor_inertia = 0.1 * ((0.95 * 0.0254) ** 2.0)