On some engine designs, the "back" of the belt may drive some accessories. This is typically limited to components requiring less torque or where a large angle of wrap is present. Such accessories will counter-rotate vs. the "normal" pulleys. Some vehicles use two serpentine belts for their system, such as the manual transaxle equipped Ford Taurus SHO, 1995-1999 DOHCNissan Maxima, vehicles using the superchargedGM 3800 engine, and many BMWs.
Advantages
It is more efficient than the older multiple belt system and may consume less space in the engine compartment. 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 is eliminated, and a serpentine belt is also much easier to maintain and replace, since there is no need to remove multiple belts in order to replace one of them, although newer adjustable-length V-belts can be put on without having to remove other belts, and their link design also allows a self-tensioning characteristic to reduce the risk of flipping over. Also since only one movable belt tensioner is required all of the peripheral components can simply be mounted to the engine without the need to swivel.
Disadvantages
The drawback of this single belt is that if the belt breaks or is knocked loose, the vehicle instantly loses multiple critical functions. The water pump, power steering pump, and alternator would cease functioning. The vehicle becomes quickly unusable due to loss of engine cooling. The belt typically gives ample visual warning of impending failure, sometimes even totally shedding several grooves while continuing to function acceptably. Another disadvantage is a lack of redundancy. Whereas a car can continue to run if, say, a power steering belt or A/C compressor belt breaks, the breakage of a serpentine belt means loss of all accessories. Furthermore, the loss of function of a single component causes the failure of the entire accessory drive. Sometimes, this can be overcome with a shorter belt to bypass the failed components. But this is not always possible. Thus, it is possible to be stranded because of, say, a seized power steering pump or A/C compressor clutch. It may also not be possible to restore use of the vehicle without first repairing the bad power steering pump or A/C compressor, which precludes a simple roadside repair.
