diff --git a/simulator/simple_planning_simulator/include/simple_planning_simulator/utils/mechanical_controller.hpp b/simulator/simple_planning_simulator/include/simple_planning_simulator/utils/mechanical_controller.hpp
index 6923305dce286..3456e61a76052 100644
--- a/simulator/simple_planning_simulator/include/simple_planning_simulator/utils/mechanical_controller.hpp
+++ b/simulator/simple_planning_simulator/include/simple_planning_simulator/utils/mechanical_controller.hpp
@@ -129,11 +129,11 @@ class SteeringDynamics
 
 private:
   SteeringDynamicsState state_;
-  double inertia_{0.0};
-  double damping_{0.0};
-  double stiffness_{0.0};
-  double friction_{0.0};
-  double dead_zone_threshold_{0.0};
+  const double inertia_;
+  const double damping_;
+  const double stiffness_;
+  const double friction_;
+  const double dead_zone_threshold_;
 };
 
 struct StepResult
@@ -191,7 +191,7 @@ class MechanicalController
   DelayBuffer delay_buffer_;
   PIDController pid_;
   SteeringDynamics steering_dynamics_;
-  MechanicalParams params_;
+  const MechanicalParams params_;
 
   [[nodiscard]] StepResult run_one_step(
     const double input_angle, const double speed, const double prev_input_angle, const double dt,
diff --git a/simulator/simple_planning_simulator/src/simple_planning_simulator/utils/mechanical_controller.cpp b/simulator/simple_planning_simulator/src/simple_planning_simulator/utils/mechanical_controller.cpp
index 49188ae15215c..25b0935ef952b 100644
--- a/simulator/simple_planning_simulator/src/simple_planning_simulator/utils/mechanical_controller.cpp
+++ b/simulator/simple_planning_simulator/src/simple_planning_simulator/utils/mechanical_controller.cpp
@@ -223,6 +223,7 @@ double MechanicalController::update_runge_kutta(
 {
   const auto dynamics_state = steering_dynamics_.get_state();
 
+  // NOTE: k1, k2, k3, k4 suffix denote the intermediate points of RK4
   const auto k1 =
     run_one_step(input_angle, speed, prev_input_angle, dt, delay_buffer_, pid_, steering_dynamics_);