The inertial dynamometer does not measure torque directly, but instead measures the acceleration of a known mass. To arrive at a power figure, it applies the f = ma equation to equate the force being applied to the drum surface.

The inertial-exclusive chassis dynamometer utilizes a large drum in order to achieve a high mechanical inertia, which acts as a static load. Incidentally, the large size of the drum leads to several more interesting advantages, most notably top end speed and peak power measurement capabilities. The inertia dynamometer actually calculates torque and power independent of a force sensing transducer and power absorption device. While this precludes the need for regular calibration, it does present the user with a number of challenges. Most notably: the inertia dynamometer only calculates Acceleration Torque and Acceleration Power – and cannot measure, nor create the conditions necessary to measure, static torque and static power. Its also important to note that, despite the inertial-exclusive dynamometer’s imposing size, it usually fails to provide a load, or measure of work, to the vehicle’s engine equal that which would be required for it to accelerate its own mass. In essence, driving on an inertial dynamometer is a kin to driving down a hill.