Kingpin on a truck: the trailer kingpin and the steering kingpin

Ask a driver about the kingpin and they think of coupling to a trailer. Ask a technician and they may think of the front axle. Both are right. This guide covers both parts, what they do, how they wear and how to check them.

The two kingpins

Tap each part for what it does and what to check.

The trailer kingpin

The trailer kingpin is the part the whole coupling depends on. When a tractor backs under a trailer, the kingpin slides into the throat of the fifth wheel and the locking jaws close around it. The trailer's upper coupler plate sits on the fifth wheel's top plate, carrying part of the trailer's weight.

Size

Most semitrailers in North America use a 2 inch kingpin, built to a common industry standard so any tractor can pull any trailer. Heavy haul trailers often use a larger 3 1/2 inch kingpin for greater strength, and the tractor's fifth wheel must match.

Position

The distance from the trailer's front to the kingpin, called the kingpin setback, affects how the trailer sits on the tractor and how the weight is spread. It also matters for state limits on the distance from the kingpin to the rearmost trailer axle. Our kingpin setting and fifth wheel position calculator works out that distance and checks it against the California limit.

Wear and damage

Every time a trailer is coupled and pulled, the jaws rub on the kingpin. Over time the shank wears smaller, which creates slack in the coupling: clunking when starting and stopping. Kingpins can also be bent by a hard coupling, a high hitch or an accident.

Trailer shops check kingpins with a wear gauge that measures the shank diameter, and they inspect the plate for cracks and bends. Kingpin replacement involves welding into the trailer's structure and must be done by a qualified shop to the trailer maker's specification.

Coupling checks

At every coupling, confirm the jaws are closed around the kingpin shank and not the head, that there is no gap between the upper coupler and the fifth wheel, and that a tug test holds. Our fifth wheel truck guide covers coupling step by step.

Steering kingpins

On the front axle, each steering knuckle turns on a kingpin. The knuckle carries the wheel, hub and brake, so the kingpin carries the weight of the front of the truck and every steering force.

Wear

Kingpins and their bushings wear, especially if they are not greased often enough. Worn kingpins let the wheel tilt and move, which causes:

• Steering wander and poor return to center.
• Clunking over bumps or during turns.
• Uneven and rapid steer tire wear.
• Alignment that will not stay set.

Greasing

Steering kingpins usually have grease fittings at the top and bottom of each knuckle. Grease them at every chassis lube interval, with the type of grease the axle maker specifies. On some trucks, the front axle should be raised when greasing so grease reaches the loaded side of the bushings; follow the maker's instructions.

Inspection and replacement

A technician checks kingpin play by raising the front axle and measuring movement at the wheel with a dial indicator, comparing it with the axle maker's limit. Replacing kingpins and bushings is a shop job, often followed by an alignment.

Kingpins on different trucks

• Day cabs in regional work couple and uncouple trailers many times a week, so fifth wheel and trailer kingpin wear add up quickly. See our guide to the day cab truck.
• Yard trucks couple trailers dozens of times a day, putting heavy wear on kingpins in yard service. See the yard truck guide.
• Heavy haul tractors pull trailers with larger kingpins and heavier fifth wheels; our heavy duty truck guide covers truck classes and types.

Electrical connections at the coupling

Coupling is not complete until the trailer's electrical cable and air lines are connected. Lighting and ABS faults on the trailer are often caused by corroded connectors at the coupling. Our truck battery guide covers the electrical basics.

Kingpin wear and fifth wheel wear go together

On the coupling side, the trailer kingpin and the tractor's fifth wheel wear against each other. A worn kingpin in a new fifth wheel, or a new kingpin in a worn fifth wheel, can still leave slack in the coupling. When drivers notice clunking, both parts should be checked: the kingpin with a gauge, and the fifth wheel's lock and jaws by a technician. Owner-operators who pull many different trailers, such as power-only drivers, often see a range of kingpin conditions; reporting a badly worn kingpin to the trailer owner helps everyone.

Steering kingpins and tire wear

Worn steering kingpins change the angles of the front wheels, which shows up as irregular wear on the steer tires: feathered edges, cupping or wear on one shoulder. Replacing steer tires without fixing the kingpins means the new tires wear the same way. If steer tires are wearing unevenly, have the front end checked, including kingpins, tie rod ends and alignment, before buying new tires.

Kingpin checks in your routine

Fold kingpin checks into the work you already do. At every coupling, look at the trailer kingpin and the plate around it with a flashlight before backing under. At every chassis lube, grease the steering kingpins and watch for grease purging from the top and bottom of each knuckle. At every PM service, ask the technician to check steering kingpin play and note it in the records, so wear can be tracked over time instead of found all at once. Small, regular checks cost almost nothing and catch wear long before it becomes a safety problem or a failed inspection.

Where we fit

We are a dispatch desk for 26 ft box trucks and larger, including tractors pulling vans, reefers, flatbeds and heavy haul trailers. Learn more about our dispatch service.