The Society of Automotive Engineers has defined four classes of trailer hitch, based on the weight of the trailer and its cargo (the Gross Towed Weight Rating, GTWR).
|I||<2000||1 7⁄8″||2″ × 5⁄8″ bar||2000|
|II||<3500||1¼ × 1¼″||3500|
The standard applies to all types of hitches, ring and pintle as well as ball and socket, but the latter is by far the most common type.
The weight a vehicle can tow is not determined by the class of the hitch attached to the vehicle, but by the vehicle's suspension, the horsepower of the engine, its cooling system, and so on. Simply attaching a class IV hitch to a passenger car will not make it capable of towing 9,999 pounds. To determine a vehicle's towing capacity, consult its manual or contact the vehicle's manufacturer.
SAE standard J684 (July 2005). Trailer Couplings, Hitches, and Safety Chains--Automotive Type.
William E. Dotterweich.
SAE Trailer Hitch Standard J684c - Its early History and Current Developments.
SAE Document Number 710358
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Last revised: 2 July 2003.