Parts of a Truck Axle – Steer Axle and Drive Axle Labeled

All 12 parts on the diagram

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Front steer axle 6

Axle beam The front axle itself, a forged steel I-beam. It carries weight but does not drive.
Kingpin The pin each front wheel swings on to steer. It passes through the knuckle and the beam.
Steering knuckle It swings on the kingpin. The spindle and the steering arm are part of it.
Spindle The stub the front wheel turns on. The wheel bearings and the hub slide over it.
Steering arm A lever fixed to the knuckle. Pushing or pulling its end turns the wheel.
Tie rod It joins the two steering arms. Both front wheels then turn together.

Rear drive axle 6

Axle housing The casing of the drive axle. It holds the gears and shafts and carries the weight.
Differential The gear set in the middle. It lets the two wheels turn at different speeds.
Axle shaft It carries the drive from the differential to the wheel. On this axle it carries no weight.
Wheel hub The wheel bolts to it. It turns on two bearings on the end of the housing.
Wheel bearings Two bearings carry each hub. They sit on the axle housing's end.
Brake drum It turns with the wheel. The brake shoes press on its inside to slow the truck.

How this list was compiled: Every name checked on 2026-10-01 and kept only where two independent sources use it. (1) US Army TM 9-8000, Principles of Automotive Vehicles (1985), chapters 29, 30, 33 and 34, https://archive.org/details/tm-9-8000-principles-of-automotive-vehicles-1985 . (2) Wikipedia, text only: Axle, https://en.wikipedia.org/wiki/Axle ; Beam axle; Kingpin (automotive part); Steering knuckle; Drag link; Drum brake. Every name on the page is in both. The manual calls the beam a dead front axle of I-section and Wikipedia a beam axle; the page says axle beam and takes both. Licence: own work

The drawing: Drawn for this site on 2026-10-01 from plain geometry; no maker's product, nothing traced. Licence: own work

How it works

A truck has two kinds of axle. The front one steers and only carries weight. The rear one drives the wheels. Each has its own parts, so each has its own view.

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The steer axle swings its wheels on kingpins

The axle beam stays fixed. At each end a steering knuckle swings on a kingpin, and the wheel turns on the knuckle's spindle. A steering arm hangs from each knuckle, and the tie rod joins the two arms so both wheels turn together.

The drive axle is a casing with shafts inside

The differential sits in the round middle of the axle housing. An axle shaft runs from it to each wheel hub. On a heavy truck the hub turns on two wheel bearings on the end of the housing, so the housing carries the load and the shaft only drives.

A dead axle only carries weight. A live axle also drives its wheels.
The axle shaft bolts to the outside of the hub by a flange on its end.

Questions and answers

What are the parts of a truck axle?

A front steer axle has an axle beam, two kingpins, two steering knuckles with spindles, steering arms and a tie rod. A rear drive axle has an axle housing, a differential, two axle shafts, wheel hubs, wheel bearings and brake drums.

What is the difference between a steer axle and a drive axle?

A steer axle carries weight and lets the front wheels swing to steer, but it does not turn them. A drive axle has a differential and axle shafts inside its housing, and they turn the wheels.

What is a full-floating axle?

A drive axle whose housing carries all the weight. Each hub turns on two bearings on the end of the housing, and the axle shaft only turns the hub. Most heavy trucks use this kind.

What does a kingpin do on a truck axle?

It is the pivot for steering. One kingpin stands at each end of the front axle beam and passes through the steering knuckle, so the knuckle and its wheel can swing left and right.