diff --git a/frc971/control_loops/drivetrain/BUILD b/frc971/control_loops/drivetrain/BUILD
index 72a32cc..05fc4a3 100644
--- a/frc971/control_loops/drivetrain/BUILD
+++ b/frc971/control_loops/drivetrain/BUILD
@@ -93,6 +93,32 @@
   ],
 )
 
+genrule(
+  name = 'genrule_down_estimator',
+  visibility = ['//visibility:private'],
+  cmd = '$(location //frc971/control_loops/python:down_estimator) $(OUTS)',
+  tools = [
+    '//frc971/control_loops/python:down_estimator',
+  ],
+  outs = [
+    'down_estimator.h',
+    'down_estimator.cc',
+  ],
+)
+
+cc_library(
+  name = 'down_estimator',
+  hdrs = [
+    'down_estimator.h',
+  ],
+  srcs = [
+    'down_estimator.cc',
+  ],
+  deps = [
+    '//frc971/control_loops:state_feedback_loop',
+  ],
+)
+
 cc_library(
   name = 'drivetrain_lib',
   srcs = [
@@ -102,12 +128,14 @@
     'drivetrain.h',
   ],
   deps = [
+    ':down_estimator',
     ':drivetrain_queue',
     ':gear',
     ':polydrivetrain',
     ':ssdrivetrain',
     '//aos/common/controls:control_loop',
     '//frc971/queues:gyro',
+    '//frc971/wpilib:imu_queue',
     '//aos/common/util:log_interval',
     '//aos/common/logging:queue_logging',
     '//aos/common/logging:matrix_logging',
diff --git a/frc971/control_loops/drivetrain/drivetrain.cc b/frc971/control_loops/drivetrain/drivetrain.cc
index b4c04a3..7c4f760 100644
--- a/frc971/control_loops/drivetrain/drivetrain.cc
+++ b/frc971/control_loops/drivetrain/drivetrain.cc
@@ -13,9 +13,11 @@
 #include "frc971/control_loops/drivetrain/drivetrain.q.h"
 #include "frc971/control_loops/drivetrain/polydrivetrain.h"
 #include "frc971/control_loops/drivetrain/ssdrivetrain.h"
+#include "frc971/control_loops/drivetrain/down_estimator.h"
 #include "frc971/control_loops/drivetrain/drivetrain_config.h"
 #include "frc971/queues/gyro.q.h"
 #include "frc971/shifter_hall_effect.h"
+#include "frc971/wpilib/imu.q.h"
 
 using frc971::sensors::gyro_reading;
 
@@ -32,11 +34,13 @@
       kf_(dt_config_.make_kf_drivetrain_loop()),
       dt_openloop_(dt_config_, &kf_),
       dt_closedloop_(dt_config_, &kf_, &integrated_kf_heading_),
+      down_estimator_(MakeDownEstimatorLoop()),
       left_gear_(dt_config_.default_high_gear ? Gear::HIGH : Gear::LOW),
       right_gear_(dt_config_.default_high_gear ? Gear::HIGH : Gear::LOW),
       left_high_requested_(dt_config_.default_high_gear),
       right_high_requested_(dt_config_.default_high_gear) {
   ::aos::controls::HPolytope<0>::Init();
+  down_U_.setZero();
 }
 
 int DrivetrainLoop::ControllerIndexFromGears() {
@@ -111,6 +115,36 @@
     LOG_STRUCT(DEBUG, "state", gear_logging);
   }
 
+  if (::frc971::imu_values.FetchLatest()) {
+    const double rate = -::frc971::imu_values->gyro_y;
+    const double accel_squared = ::frc971::imu_values->accelerometer_x *
+                                     ::frc971::imu_values->accelerometer_x +
+                                 ::frc971::imu_values->accelerometer_y *
+                                     ::frc971::imu_values->accelerometer_y +
+                                 ::frc971::imu_values->accelerometer_z *
+                                     ::frc971::imu_values->accelerometer_z;
+    const double angle = ::std::atan2(-::frc971::imu_values->accelerometer_x,
+                                      -::frc971::imu_values->accelerometer_z) +
+                         0.008;
+    if (accel_squared > 1.03 || accel_squared < 0.97) {
+      LOG(DEBUG, "New IMU value, rejecting reading\n");
+    } else {
+      // -y is our gyro.
+      // -z accel is down
+      // -x accel is the front of the robot pointed down.
+      Eigen::Matrix<double, 1, 1> Y;
+      Y << angle;
+      down_estimator_.Correct(Y);
+    }
+
+    LOG(DEBUG,
+        "New IMU value from ADIS16448, rate is %f, angle %f, fused %f, bias "
+        "%f\n",
+        rate, angle, down_estimator_.X_hat(0, 0), down_estimator_.X_hat(1, 0));
+    down_U_ << rate;
+  }
+  down_estimator_.UpdateObserver(down_U_);
+
   // TODO(austin): Signal the current gear to both loops.
 
   if (gyro_reading.FetchLatest()) {
diff --git a/frc971/control_loops/drivetrain/drivetrain.h b/frc971/control_loops/drivetrain/drivetrain.h
index a9541b7..69e9794 100644
--- a/frc971/control_loops/drivetrain/drivetrain.h
+++ b/frc971/control_loops/drivetrain/drivetrain.h
@@ -48,6 +48,9 @@
   PolyDrivetrain dt_openloop_;
   DrivetrainMotorsSS dt_closedloop_;
 
+  StateFeedbackLoop<2, 1, 1> down_estimator_;
+  Eigen::Matrix<double, 1, 1> down_U_;
+
   // Current gears for each drive side.
   Gear left_gear_;
   Gear right_gear_;