C.R. LAURENCE 0TCA CRL Overhead Track Carriage Assembly for OT Series Sliding and Bi-Fold Doors - Nylon. This CRL Carriage Assembly for our OT Series Sliding and Bi-Fold Doors has an extruded aluminum body with stainless steel hanging bolt and axles, bronze bushings, and nylon rollers with ball bearings. Minimum order is one each..