An ancillary/serpentine belt (also known as a multi-vee or multi-rib belt) is a single, continuous belt used to drive multiple peripheral devices in an automotive engine, such as an alternator, power steering pump, water pump, A/C compressor, air pump, etc. The belt may also be guided by an idler pulley and/or a belt tensioner (which may be spring loaded, hydraulic, or manual). The serpentine belt was invented by Jim Vance while working for the Gates Rubber Company and first used by Ford Motor Company for the 1979 Mustang.
It is more efficient than the older multiple belt system and may consume less underhood space. By using a single, wider belt instead of multiple, thinner belts, the belt may be put under increased tension without stretching. Higher tension reduces slip, which increases belt life and mechanical efficiency. Reduced slip can allow the use of lower-ratio pulleys; this reduces the load on the engine, increasing fuel economy and available power. The tendency for V-belts to "flip over" in the pulley groove (at high RPM) is eliminated. A serpentine belt also is much easier to maintain and replace, since there is no need to remove multiple belts in order to replace one of them.
On some engine designs, the "back" (smooth side) of the belt may drive some accessories, this is typically limited to components requiring less power to turn or where there's plenty of wrap. Such accessories will counter-rotate vs. the "normal" pulleys.
The drawback of this single belt is that if the belt breaks, the vehicle loses all of its peripheral devices; however, the belt typically gives ample visual warning of impending failure, sometimes even totally shedding several grooves (ribs) while still continuing to function normally. Some vehicles use two serpentine belts for their system, such as the Manual Transaxle equipped Ford Taurus SHO, 1995–1999 DOHC Nissan Maxima, vehicles using the Supercharged GM 3800 engine, and many BMWs.
