page_title: Retarder (mechanical engineering)
hierarchical_section_title: Retarder (mechanical engineering)
caption_attribution_description: English: Fluiddynamic torque converter for automated gearboxes of PORSCHE cars. Own photograph. PORSCHE Museum, Stuttgart, Germany, November 2006.
context_page_description: 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.
  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.
context_section_description: 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.
  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.
  Retarders are not restricted to road motor vehicles, but may also be used in railway systems.  The British prototype Advanced Passenger Train (APT) used hydraulic retarders to allow the high-speed train to stop in the same distance as standard lower speed trains, as a pure friction-based system was not viable.
caption: Torque converter, opened, interior similar to a retarder
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