A drive unit includes a gear set driving a drive pulley that in turn drives a flexible drive member which is attached to a driven member by an attachment assembly that travels in a channel. The attachment assembly is attached to the flexible drive member by a coupler that has teeth that extend through respective windows of the flexible drive member. The coupler is disposed between side walls of a yoke and attached to a bridge wall of the yoke. An outer shoe engages each side wall of the yoke. The attachment assembly includes an optional spring to tension the flexible drive member and the shoes have optional flexible tapered fingers to stabilize the drive pulley. The flexible drive member and the spring are trapped between the coupler and the bridge wall of the yoke. In an alternate arrangement, the flexible drive member is trapped between the coupler and fingers of the shoes that extend behind the flexible drive member and through spaces between adjacent teeth of the coupler and into sockets of the other shoe. The flexible drive member may be a drive chain or a drive belt having spaced windows.