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The Society of Automotive Engineers has defined four classes of trailer hitch for automobiles, based on the weight of the trailer and its cargo (the Gross Towed Weight Rating, GTWR). Another class, V, is commercially available, but is not included in the SAE standard, and in any case is made for heavier loads and more power than passenger automobiles or light trucks can manage.
minimum breaking force
|I||<2000||1 7⁄8″||2″ × 5⁄8″ bar||2000|
|II||<3500||1¼ × 1¼″||3500|
|IV||<10,000||2″ box||= GVWR|
The standard applies to all types of hitches, ring and pintle as well as ball and socket, but ball and socket 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.
SAE J2638. Recommendations regarding fifthwheel and goosneck trailers, with Gross Trailer Weights ≤ 30,000 pounds.
William E. Dotterweich.
SAE Trailer Hitch Standard J684c - Its early History and Current Developments.
SAE Document Number 710358
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Last revised: 8 March 2016.