A Trailer Equalizer Bar, sometimes called a Pivot Bar or Rocker Bar, is required when the Boat Trailer or Utility Trailer has 2 or more axles. A Tandem Axle Trailer uses 2 Equalizer Bars, and a Triple Axle Trailer uses 4. These EQ Bars evenly distribute the load between 2 or more axles so one axle is never under more stress than it can handle. This set of 2 Equalizer bars is specifically designed for 2 inch wide Slipper Style (or Hook-End) Leaf Springs Only. Most Trailer Leaf Springs are 1 3/4 inches wide, but 2 inch wide springs are seen more often on larger capacity trailers. The Slip-End of the forward most spring rests on the bottom of the 9/16 inch diameter bolt (in the lower of the 2 holes on the Equalizer Bar) and Eye of the Rear Spring Connects to the 9/16 inch diameter trailer bolt at the other end of the equalizer bar (The higher of the 2 bolt holes on the EQ Bar). Each Equalizer Bar is specifically made for 1 type of Trailer Leaf Spring. If you have Double Eye Springs, or Reverse Curve (Open Eye) Springs you would need a different type of EQ Bar. Please Double Check the Measurements below to confirm this Equalizer bar set is a good replacement for the one on your trailer.