英和和英

語源

A fifth wheel (sense 1) is a type of trailer hitch which is used to couple, for example, a semi-trailer to a prime mover or tractor unit.

A fifth wheel (sense 1.1) is also a large caravan or travel trailer that is connected to a pickup truck for towing by a hitch similar to the one shown above.

A diagram showing the gearing of a horse-drawn coach seen from above. The fifth wheel (sense 2) is the circular component in the center of each figure.

From the fact that since many vehicles such as carriages, coaches, wagons, prime movers, and trucks generally have four wheels on which they move, this component which resembles a wheel is the fifth.

• (anything superfluous or unnecessary): Sense 3 refers to the fact that a fifth wheel is not needed for a four-wheeled vehicle to operate.

別の表記

• 5th wheel

発音

• (Received Pronunciation) IPA: /ˌfɪfθ ˈʍiːl/, /-ˈwiːl/
• (General American) IPA: /ˌfɪfθ ˈʍil/, /-ˈwil/

名詞

fifth wheel (plural fifth wheels) (chiefly US)

1. (road transport) A type of trailer hitch, which consists of a horseshoe-shaped plate on a multidirectional pivot, with a locking pin to couple with the kingpin of a truck trailer.
   • 1919 August, “Connecting the Trailer in Record Time”, in Waldemar Kaempffert, editor, The Popular Science Monthly, volume 95, number 2, New York, N.Y.: Modern Publishing Company, […], →OCLC, page 28, column 1:

     In most forms of semi-trailers a great deal of time is wasted in connecting the trailer to the tractor or pulling vehicle. Most of the connecting gears are some form of fifth wheel, consisting of two horizontal circular bands of iron, one fitting on top of the other, with grease between and with a bolt or king-pin dropped through holes in the hubs or center bosses of both parts of the fifth wheel.

   • 1934 August 25, “M.D.T. [Merchants Despatch Transportation Company] Develops Trailer Tank for Milk Service”, in Samuel O. Dunn, editor, Railway Age, volume 97, number 8, Philadelphia, Pa.: Simmons-Boardman Publishing, →ISSN, →OCLC, page 235, column 2:

     The semi-trailers are equipped with hydraulic brakes which operate through the king pin of the fifth wheel. The booster-controlled fifth wheel gives full and complete cab control of all braking and coupling operations.

   • 1994 January, “Atmospheric Hazards – Oxygen Deficient Air [FACE 92-17 Fracturing Tank, 2 Fatalities]”, in Worker Deaths in Confined Spaces: A Summary of NIOSH Surveillance and Investigative Findings (DHHS (NIOSH); no. 94-103), Cincinnati, Oh.: National Institute for Occupational Safety and Health, Centers for Disease Control and Prevention, Public Health Service, U.S. Department of Health and Human Services, →OCLC, page 114:

     From the blow-back tank, the fluid is piped into a larger tank commonly known as a fracturing (frac) tank or wheely tank. This is a large, 21,000-gallon tank mounted on wheels and provided with a fifth wheel for towing from jobsite to jobsite by a semi-tractor.

   1. In full, fifth-wheel trailer: a large caravan or travel trailer that is connected to a pickup truck for towing by a hitch similar to the one described in sense 1 located in the center of the truck's bed.
      • 1997, Kim Baker, Sunny Baker, “Towing, Tow Vehicles, and Towed Vehicles”, in The RVer’s Bible: Everything You Need to Know about Choosing, Using, and Enjoying Your RV (A Fireside Book), New York, N.Y.: Simon & Schuster, →ISBN, part II (The Mechanical Side of RVing), page 182:

        Fifth-wheel hitching is a simple affair compared to trailer coupling. Choose a quality hitch and don't overburden an inadequate pickup truck with a big fifth wheel and you're almost in business.

      • 2000, Janet Rosenstock, Dennis Adair, “About the Author”, in The Fire, the Sword and the Devil, Lincoln, Neb.: toExcel, iUniverse, →ISBN, page 382:

        Janet [Rosenstock] and Dennis [Adair] are avid travelers who venture to Europe now and again, and as full-time RVers travel through the United States and Canada. Look for them on the open road; their fifth wheel bears the sign "Read a Joyce Carlow lately?"

2. (road transport, historical) A horizontal wheel or segment of a wheel above the front axle and beneath the body of a carriage, forming an extended support to prevent it from overturning.
   • 1873 April 1, H. F. Porter, “The Relative Merits of Full and Half Fifth-wheels”, in The Hub, volume XV, number 1, New York, N.Y.: Hub Publishing, →OCLC, page 10, column 1:

     Full fifth-wheels have become quite frequent of late, and it may not be out of place to make a few remarks on their merits as compared with half fifth-wheels. With the exception of heavy coaches and rockaways, nearly all vehicles were until lately made with half fifth-wheels; but at present we find nearly all the different kinds of work, such as landaus, clarences, coaches, landaulets, T-carts, and coupés, made with full fifth-wheels.

   • 1875, “The Proper Care of Carriages”, in The Coach-makers’ Illustrated Hand-book, […], 2nd edition, Philadelphia, Pa.: I[saac] D[elaney] Ware, […], →OCLC, part V (Miscellaneous), page 350:

     For greasing the axles and fifth wheel, use castor-oil. It is not necessary to put on a great deal. [...] The fifth wheel should be looked after, and not be allowed to become entirely dry.

   • 1912 February 7, “How to Save the Most Money on a Spreader [advertisement]”, in The Breeder’s Gazette, volume LXI, number 6, Chicago, Ill.: Sanders Publishing Company, →OCLC, page 367, columns 2–3:

     The Great Western has a big 15-inch indestructible malleable fifth wheel that weighs 40 pounds. This is attached with malleable braces to two big, heavy, solid oak bolsters. It is arranged so that if one wheel drops into a hole or rut, there's no strain or binding on the frame. The Great Western malleable fifth wheel and heavy oak bolsters are set back two feet under the box so the load is evenly balanced on all four wheels.

3. (idiomatic, informal) Anything superfluous or unnecessary.

   Synonyms: spare tool, third wheel

   I felt like a fifth wheel when both of them started giggling and making out during dinner.

   • [1818 December, “A Grammar of the English Language, in a Series of Letters. […] By W[illia]m Cobbett. New York, 1818. 1 vol. 12mo. pp. 184. 75 cts. [book review]”, in Oliver Oldschool [pseudonym; John Elihu Hall], editor, The Port Folio, volume VI, number VI (4th series), Philadelphia, Pa.: Published by Harrison Hall, […]; London: John Souter, […], →OCLC, pages 464–465:

     A subjunctive mode should no more exist in the English grammar, than a fifth wheel be given to a wagon.]

   • [1839, [James Ewell Heath], Whigs and Democrats; or Love of No Politics. A Comedy in Three Acts, Richmond, Va.: T[homas] W. White, →OCLC, act I, scene i, page 12:

     Why I'm of no more use in my own house, than a fifth wheel would be to a wagon.]

   • [1902 August 21, Henry James, chapter XVII, in The Wings of the Dove, volume II, New York, N.Y.: Charles Scribner’s Sons, →OCLC, book sixth, page 6:

     Compressed and concentrated, confined to a single sharp pang or two, but none the less in wait for him there on the Euston platform and lifting its head as that of a snake in the garden, was the disconcerting sense that "respect," in their game, seemed somehow—he scarce knew what to call it—a fifth wheel to the coach.]

   • 1914, John H[ill] Brinton, “After Shiloh with Halleck”, in Personal Memoirs of John H. Brinton: Major and Surgeon U.S.V. 1861–1865, New York, N.Y.: The Neale Publishing Company, →OCLC, page 162:

     He said his name was Sheridan, Captain Sheridan, and that he was a sort of headquarters quartermaster, to look after the staff comforts. He did not seem to have a very exalted opinion of his duties, rather regarding himself as a fifth wheel.

   • 1935 February 23, Viña Delmar, “Wild as a Hawk”, in Fulton Oursler, editor, Liberty, volume 12, number 8, New York, N.Y.: Liberty Publishing, →OCLC, part 2 (A Kiss, with Astonishment), page 30, column 2:

     After the first minute of conversation with her I stopped feeling like a fifth wheel. We got on fine together. Of course I don't know whether I was a fifth wheel or not. There was no way I could discover just what the line-up was.

   • 1971, Charles C. Denova, “Establishing Training Activities”, in Establishing a Training Function: A Guide for Management, Englewood Cliffs, N.J.: Educational Technology Publications, →ISBN, page 17:

     The most common excuse is that they [line managers] do not have enough time to train every employee and perform their other supervisory duties. In an environment such as this, a new hire often feels like a fifth wheel and begins to develop frustration and fear.

   • 2015, Kate James, chapter 13, in The Truth about Hope (Harlequin Heartwarming), Don Mills, Ont.: Harlequin Enterprises, →ISBN, page 192:

     Hope suddenly felt like a fifth wheel. She was happy for Noah and Gillian, but seeing them together just made her feel lonelier.

   1. (idiomatic) An unwanted person accompanying two couples on a double date.

      Coordinate term: third wheel

Notes

Further reading

• caravan (towed trailer) on Wikipedia.Wikipedia
• fifth-wheel coupling on Wikipedia.Wikipedia
• fifth wheel (disambiguation) on Wikipedia.Wikipedia
• “fifth-wheel, n.” under “fifth, adj. and n.”, in OED Online ⁠, Oxford: Oxford University Press, 1896.
• “fifth wheel, n.”, in Lexico, Dictionary.com; Oxford University Press, 2019–2022.