CHASSIS-PARTS GLOSSARY
Steering Knuckle
Also called: knuckle, upright, spindle, stub axle, knuckle arm (for the steering arm on the knuckle). On rear axles the equivalent non-steering part is usually called a hub carrier.
What it is
A steering knuckle (also called an upright or spindle) is the forged or cast part at each front wheel that carries the wheel hub and bearing and turns with the wheel when you steer. It is where suspension and steering meet: the control arms attach to it through ball joints (or, on older solid axles, a kingpin), the tie rod end connects to its steering arm, and the brake caliper usually bolts to it. When the tie rod pushes the knuckle's steering arm, the whole knuckle pivots around the steering axis and the wheel turns.
How it works
The steering pivot
The upper and lower ball joints, or a strut top mount and a lower ball joint on MacPherson-strut suspension, or a kingpin on a solid axle, form the steering axis. The knuckle rotates around this axis when the wheel is steered.
The steering arm
A lever on the knuckle, forged in or bolted on, receives the tie rod end or drag link. Its length and angle set the steering geometry, including the Ackermann effect that turns the inner wheel more sharply in a corner.
The load path
The knuckle carries wheel loads, braking torque and cornering forces from the hub into the suspension arms, which is why it is a heavy forging or casting rather than a light bracket.
Construction
Knuckles are made of forged steel, cast iron or, on lighter vehicles, cast or forged aluminium. They have a bore or flange for the hub and wheel bearing, tapered holes or clamps for the ball-joint studs, a steering arm with a tapered hole for the tie rod end, and mounting points for the brake caliper and often an ABS sensor.
Common failure symptoms
- Alignment cannot be set
- After a kerb strike, a bent knuckle or steering arm leaves camber or toe outside the specification even with new tie rods and arms.
- Pull or uneven tyre wear after an impact
- A bent knuckle changes the wheel angles permanently.
- Stud keeps working loose
- A damaged or worn tapered hole will not hold a ball-joint or tie-rod-end stud tight.
- Wheel-bearing noise
- Usually the bearing itself; the knuckle is replaced only if the bearing bore is damaged.
Frequently asked
Is a steering knuckle the same as a spindle?
The terms are often used interchangeably. Strictly, the spindle is the stub the wheel bearing rides on; on many rear-drive trucks it is forged as part of the knuckle, so the whole part is called a spindle.
What connects to the steering knuckle?
The control-arm ball joints (or a strut and a lower ball joint), the tie rod end or drag link at the steering arm, the wheel hub and bearing, the brake caliper and often an ABS sensor.
Can a bent steering knuckle be repaired?
No, it should be replaced. A knuckle is a safety-critical forging or casting, and straightening or welding can hide cracks.
How do I know if a steering knuckle is damaged?
After an impact, look for an alignment that cannot be brought into specification, an off-centre steering wheel, or a pull that remains after new tie rods and arms are fitted. A workshop can measure the knuckle against the specification.
Do I need a wheel alignment after replacing a steering knuckle?
Yes. A new knuckle changes camber and toe, so the alignment must be checked and set.
Example OE numbers
- MN-101741 Mitsubishi
- 545000C40B Nissan
- 43308-09080 Toyota
- 51220-STK-A01 Honda
- 21067-GA050 Subaru
- 0K201-34-550D Kia
External references
Independent technical sources (in English) for further reading. SLOOPZONE is not affiliated with these publishers.
- Steering knuckle Wikipedia
- Kingpin (automotive part) Wikipedia
- MOT inspection manual, 5. Axles, wheels, tyres and suspension GOV.UK