| abstract | - A retarder is a device used to augment or replace some of the functions of primary friction-based braking systems of (usually) heavy vehicles. Friction-based braking systems are susceptible to 'fade' when used extensively, which can become dangerous if the braking performance drops in what is required to stop the vehicle – for instance if a truck or bus is descending a long incline. For this reason, such heavy vehicles are frequently fitted with a supplementary system that is not friction-based. Retarders are not restricted to road vehicles, but may also be used also in railway systems. The British prototype Advanced Passenger Train used hydraulic retarders to allow the high-speed train to stop in the same distance as standard lower speed trains, as a purely friction-based system was not viable. Retarders serve to slow vehicles down, or maintain a steady speed on inclines. They are usually not capable of bringing vehicles to a standstill, as their effectiveness diminishes at low speeds. They are usually used to slow vehicles down, with the final braking being carried out by a friction brake. As the friction brake does not then need to be used so much, particularly at higher speeds, the service lifetime of friction brakes is enhanced.
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