In one aspect, a decoupler assembly is provided for use between a shaft and an endless drive member that is used to drive the shaft. The decoupler assembly includes a pulley, a hub and an isolator spring that is preferably a coiled torsion spring. The two ends of the spring are engageable, at least indirectly, with the pulley and the hub for the transfer of torque therebetween. At least one of the ends of the spring engages an engagement structure (on either the pulley or the hub) that includes a helical axial shoulder and a driver wall. The spring transfers torque in one direction through the driver wall (e.g. when the pulley overruns the hub), but the spring end is not fixedly connected to the driver wall. When the hub overruns the pulley, there is relative rotation between the spring and whichever of the hub and pulley it is not fixedly connected to. Accordingly, there is relative rotation between the spring end and the helical axial shoulder and the driver wall. This causes the spring end to separate from the driver wall and ride up the helical axial shoulder. This causes the spring to compress axially. The spring coils have a selected amount of spacing so that the spring can be compressed by a selected amount axially. This sets the amount of relative rotation (and the amount of overrun) that is available between the pulley and the hub in the situation when the hub overruns the pulley.