What Is Fifth Wheel in Trucking?
Also called: 5th Wheel, Fifth Wheel Coupling, Kingpin Coupler, Sliding Fifth Wheel
A fifth wheel is the coupling plate on the back of a truck tractor that connects to a semitrailer's kingpin. It holds the trailer to the tractor and lets the two turn against each other. Federal rules say it must lock. The two halves cannot separate unless a driver releases it by hand.
Last reviewed September 23, 2026 · Facts checked October 21, 2026
Key Points
- Every fifth wheel must have a locking mechanism. It must stop the upper and lower halves from separating unless a positive manual release is used. The release may be reached from the cab (49 CFR 393.70(b)(2))
- If a fifth wheel is built to separate easily, its locking devices must lock automatically on coupling (393.70(b)(2))
- The lower half must be secured to the frame with proper brackets or plates and tightened bolts. It must have a device that keeps it from shifting (393.70(b)(1)(i))
- The upper half must be fastened to the vehicle with at least the same security as the lower half (393.70(b)(1)(ii))
- The fifth wheel must sit where the weight of both vehicles is spread properly over their axles. It must not interfere with steering or braking (393.70(b)(3))
- In a straight line on a level paved surface, the trailer must track within 3 inches of the tractor's path (393.70(a))
- A vehicle fails the periodic inspection if horizontal movement between the upper and lower halves is over 1/2 inch, or the operating handle is not closed and locked (49 CFR part 396, Appendix A)
- It also fails if the kingpin is not properly engaged or light shows through between the halves (Appendix A)
What is a fifth wheel on a semi?
It is the flat, horseshoe-shaped coupling plate mounted on the frame behind a truck tractor's cab. A semitrailer has a kingpin under its front. The kingpin slides into the fifth wheel and locks there. This joint holds the trailer to the tractor. It carries part of the trailer's weight. It lets the two pivot when the truck turns.
How does a fifth wheel work?
As the tractor backs under the trailer, the kingpin slides into the throat of the fifth wheel. The jaws close around it. A locking mechanism holds the jaws shut. To uncouple, the driver pulls a release handle. The rule says the release may be placed so it can be worked from the cab (49 CFR 393.70(b)(2)). Some fifth wheels slide fore and aft on rails so the driver can move weight between the axles.
What do the federal rules say about fifth wheels?
49 CFR 393.70(b) covers mounting, locking and location. The lower half must be bolted to the frame with proper brackets or plates and must not shift. The upper half must be fastened at least as securely. Every fifth wheel needs a locking mechanism that stops the halves from separating unless a positive manual release is used. It must sit where weight is spread over the axles and steering and braking are not harmed. Drivers must also report on coupling devices in the daily inspection report (49 CFR 396.11).
What makes a fifth wheel fail an inspection?
The annual inspection standards in 49 CFR part 396, Appendix A list the defects. One is more than 1/2 inch of horizontal movement between the upper and lower halves. Others are an operating handle not in the closed and locked position, a kingpin not properly engaged, and light showing through between the upper and lower coupler. Cracks in the fifth wheel plate also fail it. A sliding fifth wheel fails with missing or ineffective latching fasteners, a missing fore or aft stop, or more than 3/8 inch of movement between the slider bracket and base.
Is a trucking fifth wheel the same as an RV fifth wheel?
They use the same idea. An RV or camper fifth wheel is a smaller plate mounted in a pickup bed. The federal rules on this page are the Part 393 rules for commercial motor vehicles. If you tow a personal RV, check your State's towing and license rules.
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Frequently Asked Questions
What is a fifth wheel in trucking?+
The coupling plate on the back of a truck tractor. It connects to a semitrailer's kingpin and locks it in place (49 CFR 393.70(b)).
What is a kingpin?+
The pin under the front of a semitrailer that locks into the fifth wheel. The fifth wheel must sit where the kingpin and the tractor's rear axles spread weight properly (49 CFR 393.70(b)(3)).
Can a fifth wheel fail?+
Yes. It fails a periodic inspection for more than 1/2 inch of movement between the halves. A handle not closed and locked, a kingpin not engaged, or light showing between the halves also fail it (49 CFR part 396, Appendix A).
Must a fifth wheel lock?+
Yes. Every fifth wheel must have a locking mechanism that stops the halves from separating unless a positive manual release is used (49 CFR 393.70(b)(2)).
What is a sliding fifth wheel?+
A fifth wheel that moves fore and aft on a slider so the driver can shift weight between axles. The inspection standard checks its latching fasteners, stops and slider movement (49 CFR part 396, Appendix A).
Do drivers inspect the fifth wheel?+
Drivers report on coupling devices in the daily vehicle inspection report (49 CFR 396.11).
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