What is a steering knuckle?
A steering knuckle is the cast part at each front wheel that carries the wheel bearing and hub. The suspension holds it at the top and bottom, the tie rod end pushes its steering arm, and it turns about the steering axis when you steer. Knuckles rarely wear out, but a hard curb strike can bend one and throw off the steering.
Also called knuckle, spindle, hub carrier
- Steering knuckle
- Outer tie rod end UK: Track rod end
- Lower control arm
- Strut and coil spring UK: Strut and spring
- Hub and brake rotor
What does a steering knuckle do?
It holds the wheel and turns it. The wheel bearing presses into the knuckle and the hub turns in it, and the brake caliper bolts to it. The lower ball joint holds its bottom; on a MacPherson strut the strut clamps its top, and on double wishbone suspension an upper ball joint does. The line between those two pivots is the steering axis, and the knuckle turns about it when you steer.
The steering arm is part of the knuckle. It points toward the tie rod end, and the angle of the two arms makes the inside wheel turn further than the outside one in a turn: in the app's model, 33.2° and 28.6° at full lock. Because the arm length and those angles are cast in, a bent knuckle changes how the car steers and how the tires wear.
What are the symptoms of a bad steering knuckle?
- The steering wheel off center after an impact. The car goes straight but the steering wheel sits crooked, typically after hitting a curb or a deep pothole hard. Why is my steering wheel off center?
- Alignment that cannot be set. A shop cannot bring the camber or toe back into range with the normal adjustments, which points to a bent knuckle, strut or control arm.
- Uneven tire wear on one side. One front tire wears on its edge much faster than the other because its angles are off. What causes uneven tire wear?
- A pull to one side. The car drifts toward the damaged side, or steers differently left and right.
- Noise from a worn bearing bore. A hum or growl that rises with speed, from the wheel bearing; a bearing that keeps failing early can mean the knuckle's bore is out of shape.
Can you drive with a bent steering knuckle?
Have it checked before you drive far
Only carefully, to have it checked. A bent knuckle still steers, but it can make the car pull, wear a tire quickly and change how the car behaves under hard braking. A cracked knuckle is a different matter: if it breaks, the wheel is no longer held by the steering or the suspension. After a hard impact with a crooked steering wheel or a new pull, have the corner inspected before you drive far.
How do you know if a steering knuckle is bent?
Usually from the alignment. After a curb strike, a shop measures the camber, caster and toe on each side and compares them with the car’s specifications. If a value is far out and the normal adjustments cannot bring it back, the next step is to find which part is bent: the control arm, the strut, the tie rod or the knuckle. Each can be measured or compared with the other side.
A knuckle can be bent with no sign you can see, so it is rarely the first part suspected. Other parts are checked first: a bent tie rod is easier to replace, and a worn ball joint or tie rod end can look like a geometry fault. The knuckle is replaced once those check out and the readings are still wrong.
What does replacing a steering knuckle involve?
- Loosen the axle nut and lug nuts, raise the car onto a jack stand and take the wheel off.
- Remove the brake caliper and its bracket and hang them from the spring, then take off the brake rotor and unplug the ABS sensor.
- Free the tie rod end and the lower ball joint from the knuckle with a separator, and push the axle shaft out of the hub.
- Unbolt the knuckle from the strut, or free the upper ball joint on double wishbone suspension, and lift it away.
- Fit the new knuckle, often with a new wheel bearing and hub already pressed in, and refit everything in reverse.
- Have the wheel alignment set before driving any distance.
Tighten every nut and bolt to the torque spec in the manufacturer's service manual for your car.
It is a larger repair usually done by a shop, and a knuckle is often bought with the bearing and hub installed. The alignment always needs setting afterward.
Questions people ask
Is a steering knuckle the same as a spindle?
Close enough in everyday use. On older cars the spindle was a stub axle the wheel turned on, cast as part of the knuckle. On front-wheel-drive cars the axle runs through the knuckle's hub instead, and spindle is still often used for the whole part.
Can a steering knuckle wear out?
The knuckle itself rarely wears. What wears are the parts it holds: the wheel bearing, the ball joints and the tie rod end. A knuckle is usually replaced because it was bent or cracked, or because a bearing bore was damaged.
Does a steering knuckle affect the alignment?
Yes. Its shape sets the camber and the steering arm's position, so a bent knuckle moves the alignment out of range and can make it impossible to set until the knuckle is replaced.
Related
Symptoms
- Why is my steering wheel off center? A steering wheel that sits off center while the car drives straight usually means the front toe is out of adjustment, or the tie rods were set unevenly at the last alignment.
- What causes uneven tire wear? Uneven tire wear usually means the wheel alignment is off, the tire pressures are wrong or a suspension part is worn, and the pattern points to which.
Parts nearby
- What does a ball joint do? A ball joint is the ball-and-socket joint that connects a suspension arm to the wheel's steering knuckle.
- What is a tie rod end? A tie rod end is the ball joint at the outer end of each tie rod, where the steering connects to the front wheel.
- What does a wheel bearing do? A wheel bearing lets the wheel spin freely while carrying the car's weight and the cornering and braking loads.
- What is a control arm? A control arm is a hinged suspension link between the car's subframe or body and the wheel.
Explained
- What is camber? Camber is the lean of a wheel seen from the front of the car.
- Toe in vs toe out: what's the difference? Toe is the angle of a pair of wheels seen from above. With toe in, their fronts point slightly toward each other; with toe out, slightly apart.
The whole system
- How does rack and pinion steering work? The rack and pinion steering, part by part, with an interactive diagram.
