Full definition
The portion of the belt traveling from the driven pulley back to the driver pulley, carrying the lower of the two belt tensions (T2, also called slack-side tension). In a properly designed horizontal drive, the slack side should be on the upper run so that belt sag due to gravity increases the wrap angle on both pulleys, enhancing traction. The tension on the slack side is determined by the Euler friction equation: T1/T2 = e^(μθ), where T1 is tight-side tension, μ is friction coefficient, and θ is wrap angle. Tensioners and idler pulleys are placed on the slack side to avoid adding bending stress to the already highly loaded tight side. If the slack side tension drops too low, belt flutter and vibration occur, accelerating fatigue.