  The force transmitted from the front tires and tie rods to the steering rack
of a vehicle, called the rack force, significantly influences the torque
experienced by a driver at the steering wheel. As a result, estimates of rack
force are used in a wide variety of advanced driver assist systems. Existing
methods for producing rack force estimates are either susceptible to steering
system disturbances or are only applicable for driving on roads with low
frequency profile variations such as road slopes. In this paper we present a
model that can produce disturbance-free rack force estimates for driving on
roads with high frequency profile variations, such as road cleats and potholes,
in addition to roads with low frequency profile variations. We validate the
estimation accuracy of our model by presenting results from two driving
experiments that were performed on test tracks with known low and high
frequency road profile variations. We further demonstrate the merits of our
model relative to the existing models by comparing the various estimates to
rack force measurements obtained using a sensor mounted in the test vehicle.
