A pulley assembly for a flexible motor vehicle transmission, comprising a supporting member rotatable about an axis, a pulley adapted to cooperate with a belt and connected to the supporting member to be rotatable about said axis with respect to the supporting member, elastic spiral means interposed between the supporting element and the pulley and comprising a sliding portion elastically loaded in radial direction against a sliding wall carried by one of either the supporting members or the pulley for tangential force drive coupling pulley to the supporting member. In particular, the elastic spiral means comprise a supporting element and sliding portion comprising at least one runner carried by the supporting element and sliding along the sliding walls.