A 4WD dynamometer absorbs power from both axles of the vehicle. However, to simulate the road correctly, the dynamometer must also be capable of maintaining the same roll speeds on the front and rear, even while the vehicle axles are applying different tractive forces.
Real Time's 4WD dynamometers employ two different methods to achieve these goals. In our IM240 4WD dynamometers, the front and rear rolls are belted together. In our other 4WD dynamometers, speed synchronization is achieved by two mechanically independent AC motors, one for each axle. In the latter case, the ARTIC computer calculates the correct loading independently for the two motors, based on the commanded road load and inertia.