The invention seeks to provide a transmission system, in particular for a starter-alternator, capable both of damping large variations in torque under starting conditions (drive from the alternator) and of providing effective decoupling under running conditions (alternator driven). In an embodiment, the decoupling located between a central hub and a rim over which a drive belt (not shown) is tensioned, is provided by at least one decoupler element designed to provide optimized stiffness. In addition, the decoupler assembly is associated with at least one deflection limiter having two components: a central plate secured to an outside face of the central hub and coupled to abutments that are regularly distributed around the periphery of an inside face of the rim, the abutments and the plate being shaped so as to come into deforming contact. The resilient assembly provides two stiffness values, whether in the drive direction or in the driven direction. The invention is applicable to starter-alternators, crankshafts, and other power transmission members (compressors, etc.).