Ultra-Fab Products Trailair Pin Box, Medium Style model M19, The beauty and reliability of the Trailair air ride system is in its simplicity. The air ride pin box acts as a buffer, arresting the transfer of road impact between the tow vehicle and the coach to produce smooth, stabilized towing performance. Generally, a truck and a trailer connected by a ridged hitch system (basically any standard fifth wheel or ball connection) will tend to fight each other as you travel at highway speeds. The faster you go and the worse the road conditions, the more the ride quality suffers. Not only are you dealing with road forces from traveling down the road, but also additional forces are created as the two vehicles fight each other causing a inch diving board inch effect. The Trailair coupler's unique scissor action allows the air spring to absorb these forces and dissipate them and force the shock absorber to handle the rebound effect. The result is an improved ride for the truck and its occupants, the coach and its contents. Improves the overall ride while protecting the fifth wheel trailer. Improves handling and enhances towing performance. Model M19 has a 21,000 lb. capacity and replaces Lippert 1116 pin boxes.