A retarder is a device used to augment or replace some of the functions of primary friction-based braking systems, usually on heavy vehicles. Retarders serve to slow vehicles, or maintain a steady speed while traveling down a hill, and help prevent the vehicle from "running away" by accelerating down the hill. They are not usually capable of bringing vehicles to a standstill, as their effectiveness diminishes as vehicle speed lowers. They are usually used as an additional "assistance" to slow vehicles, with the final braking done by a conventional friction braking system. As the friction brake will be used less, particularly at higher speeds, their service life is increased, and since in those vehicles the brakes are air-actuated helps to conserve air pressure too.\n\nWhat is a retarder on a semi truck? OK. Let's think. My stream of consciousness:
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The relevant information to answer the above question is: A retarder is a device used to augment or replace some of the functions of primary friction-based braking systems, usually on heavy vehicles. The answer is a device used to augment or replace some of the functions of primary friction - based braking systems.
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