Wheel substitution dynamometers. A major problem faced by engineers carrying out NVH testing on a chassis dynamometer is that tyre noise can dominate sound measurements. One answer is to absorb the power of each drive wheel with an individual dynamometer. If the wheel hub is modified the vehicle can still sit on its tyres, giving approximately the correct damping effect, while the tyre contact and windage noise is eliminated. Alternatively, the wheels may be removed and the individual wheel dynamometers used to support the vehicle. The dynamometers should be four-quadrant machines, to simulate both driving and coasting/braking conditions. Hydrostatic, a.c. and d.c. machines have been used for this application.