<Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (July 2016) (Learn how and when to remove this template message) </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> This article needs additional citations for verification . Please help improve this article by adding citations to reliable sources . Unsourced material may be challenged and removed . (July 2016) (Learn how and when to remove this template message) </Td> </Tr> <P> 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 . </P> <P> Friction - based braking systems are susceptible to "brake fade" when used extensively for continuous periods, which can be dangerous if braking performance drops below what is required to stop the vehicle--for instance if a truck or bus is descending a long decline . For this reason, such heavy vehicles are frequently fitted with a supplementary system that is not friction - based . </P>

What is a retarder on a semi truck