Abstract: An assembly for a bicycle including a steerer tube configured for insertion in a head tube of a bicycle, the steerer tube comprising a lower portion for securement to a bicycle fork and an elongated upper portion having a noncircular cross-section, and a coupler for engaging the steerer tube with a handlebar stem of the bicycle. The coupler further including an expansion plug having a bolt threadably engaged to a lower wedge such that rotation of the bolt provides longitudinal translation of the lower wedge, the lower wedge having a plurality of angled lower wedge sliding surfaces, a headset top cap threadably securable to the bolt, an upper wedge having a plurality of angled upper wedge sliding surfaces and a bolt aperture, a plurality of side wedges, and a flexible fastener positioned around the plurality of side wedges to compress the plurality of side wedges inward towards the bolt.