This replacement bolt on 4 hole rear spring hanger for boat trailers is made out of hot dipped galvanized steel. This hanger will work on a double eye spring, on a single axle trailer. This hanger is skinnier than the front hanger to allow for the spring shackles to attach on the outside of the hanger. You would not be using this hanger on a tandem axle trailer, only a single axle trailer. The leaf spring shackles would mount to the outside of this hanger.