Simulating KnC Rigs

The Suspensions library is full of experiments that test the full vehicle dynamics. However, it can be very difficult to validate models using full vehicle experiments, such as the common Kinematic and Compliance (KnC) test. This kind of test, as the name suggests, details the suspension kinematics (force and movement as the result of regular suspension operation) and the compliance (amount of force and movement in directions that aren’t bump or steering) of a vehicle.

These tests are conducted on KnC rigs, which are large experiment platforms with actuators and measuring equipment, normally designed to test full vehicles. How this is done is dependent on the tests carried out, but in its simplest form it consists of 4 pads that support the vehicle, which can be moved up and down to simulate road input. This tests suspension reaction to known inputs, such as sinusoidal sweeps or measured road inputs.

Video 1: 4 Post Sinusoidal Sweeps to test suspension response to different input frequencies.

Other types of rigs hold the chassis stationary and apply force to the wheel contact patch. This is done by clamping onto the chassis so that it is held stationary and, most commonly, the tyre contact patch is moved on a very grippy surface.

Video 2: Showing a longitudinal compliance test, with the vehicle held stationary and the contact patch moved forward and backwards.

The rig models in the Suspensions library test the vehicle at the experiment level, which, in the addition to the driver and vehicle model, adds a rig model, controller and replaces the road for the road model for the pad road models.

Figure: Experiment with fixed rig model, that holds the chassis, rig and controller with conventional vehicle and driver model

Figure: Experiment with fixed rig model, that holds the chassis, rig and controller with conventional vehicle and driver model

Suspension compliance can be added to a linkage by implementing bushes in each mount. Then to recreate the test seen in the YouTube video above, the same forces are applied to the contact patch or by forcing motion of the wheel hub as shown below.

Video 3: Longitudinal compliance test of the front double wishbone suspension

Written by: David Briant – Senior Project Engineer

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