The 42-inch/70-inch Bumper Plates/Medicine Balls Storage Shelf is compatible with the Mass Storage System and features two 40 mm durable steel rails. Allows ample amounts of space to store Bumper Plates, Cast Iron Plates, or Medicine Balls. The shelf features two 40mm durable steel rails that are spaced 7-inches apart. This single storage add-on will help in customizing your base model 2-Tier or 3-Tier Storage Systems into an extended mass gym storage unit with additional uprights, sold separately.