In order for vehicle manufacturers to design their vehicles with accurate test data a technique has been developed and used extensibly to measure the moments of inertia (MOI) and centre of gravity (CoG) for a wide range of vehicles. The rig which was designed and built at Cranfield University uses a reverse pendulum methodology. This is in contrast to other methods of measuring inertia which supports the vehicle or object from the ceiling.
Essentially, a vehicle is supported by its chassis and allowed to float on an air bearing and rotate, in a frictionless manner, in three degrees of freedom. The motion of the vehicle is configured to move in one axis of rotation, pitch, roll or yaw. A pair of linear springs is then introduced and simple harmonic motion of the vehicle is obtained by applying small amplitudes. The vehicle oscillates against the springs and the time interval is recorded. From the time period of one oscillation, the inertia for the vehicle and rig with any supporting structures can be calculated. To determine the inertia of the vehicle only, the rig and supporting structure inertia are subtracted.
The inertia is defined about the centre of gravity of the vehicle which is obtained by performing a different test. The vehicle is tilted to small angles up-to 2 degrees and the force required to maintain the vehicle at equilibrium with the angle of rotation is measured at various degrees of tilt. This test is performed in pitch and roll to provide verification of results.
This paper describes the background to the equations derived for calculations, features of the rig which have arisen from over 25 years of testing with this unique methodology and details of the software used.

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