This replacement bracket is required if you will be transporting DH bikes with dual crown forks on your Mammoth Mounts rack. The bracket replaces one of the existing standard brackets. The bracket is wider to accommodate the width of dual crown forks such as a FOX 40, RockShox BoXXer, and others. The brand cannot guarantee that bikes without dual crown forks will work in this bracket, it is recommended for use only with dual crown forks.