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A chassis dynamometer, sometimes referred to as a rolling road, measures power delivered to the surface of the "drive roller" by the drive wheels. The vehicle is often parked on the roller or rollers, which the car then turns, and the output measured thereby.
Modern roller-type chassis dyno systems use the "Salvisberg roller", which improves traction and repeatability, as compared to the use of smooth or knurled drive rollers. Chassis dynamometers can be fixed or portable, and can do much more than display RPM, horsepower, and torque. With modern electronics and quick reacting, low inertia dyno systems, it is now possible to tune to best power and the smoothest runs in real time.
Other types of chassis dynamometers are available that eliminate the potential for wheel slippage on old style drive rollers, attaching directly to the vehicle hubs for direct torque measurement from the axle.
Motor vehicle emissions development and homologation chassis dynamometer test systems often integrate emissions sampling, measurement, engine speed and load control, data acquisition, and safety monitoring into a complete test cell system.
The Graham-Desaguliers Dynamometer was invented by George Graham and mentioned in the writings of John Desagulier in 1719. Desaguliers modified the first dynamometers, and so the instrument became known as the Graham-Desaguliers dynamometer.
The eddy current dynamometer was invented by Martin and Anthony Winther around 1931, but at that time, DC Motor/generator dynamometers had been in use for many years. A company founded by the Winthers brothers, Dynamatic Corporation, manufactured dynamometers in Kenosha, Wisconsin until 2002. Dynamatic was part of Eaton Corporation from 1946 to 1995. In 2002,  Dyne Systems of Jackson, Wisconsin acquired the Dynamatic dynamometer product line. Starting in 1938, Heenan & Froude manufactured eddy current dynamometers for many years under license from Dynamatic and Eaton.