trailer hitches

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).

Trailer Hitch Classes
Class GTWR
(pounds)
Ball Receiver Chain
(pounds)
I <2000 1 78 2″ × 58″ bar 2000
II <3500   1¼ × 1¼″ 3500
III <5000 2″ 2″ box 5000
IV <10,000   2″ box  

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.

Hitch Ball Sizes and Typical Capacities
Ball size
(inches)
Bolt diameter Capacity
(pounds)
178 ¾″ 2000
1″ 2000
2 ¾″ 3500
1″ 6000
1¼″ 6000
138 10,000
2 516 1″ 6000
1¼″ 10,000

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.

Resources

William E. Dotterweich.
SAE Trailer Hitch Standard J684c - Its early History and Current Developments.
SAE Document Number 710358

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