Introduction

In automotive suspension

Suspension (vehicle)

Suspension is the term given to the system of springs, shock absorbers and linkages that connects a vehicle to its wheels. Suspension systems serve a dual purpose — contributing to the car's roadholding/handling and braking for good active safety and driving pleasure, and keeping vehicle occupants...

, a steering knuckle is that part which contains the wheel hub or spindle, and attaches to the suspension components. It is variously called a steering knuckle, spindle

Spindle (automobile)

In an automobile, the spindle is a part of the suspension system that carries the hub for the wheel and attaches to the upper and lower control arms. The spindle is referred to as an Upright in UK-built vehicles and in areas dominated by these cars like Australia, New Zealand etc.-Design:There are...

, upright or hub, as well.

The wheel and tire assembly attach to the hub or spindle of the knuckle where the tire/wheel rotates while being held in a stable plane of motion by the knuckle/suspension assembly.

In the attached photograph of a double-wishbone suspension, the knuckle is shown attached to the upper control arm at the top and the lower control arm at the bottom. The wheel assembly is shown attached to the knuckle at its center point. Note the arm of the knuckle that sticks out, to which the steering mechanism attaches to turn the knuckle and wheel assembly.

Types

Steering knuckles come in all shapes and sizes. Their designs differ to fit all sorts of applications and suspension types. However, they can be divided into two main types. One comes with a hub and the other comes with a spindle.

Applications

In a non-drive suspension, as shown in the first photo, the knuckle usually has a spindle onto which the brake drum or brake rotor attaches. (In this picture, the central spindle upon which the wheel assembly rides cannot be seen.) The wheel/tire assembly then attaches to the supplied lug studs, and the whole assembly

Wheel hub assembly

Wheel hub assembly , also referred to as hub assembly, wheel hub unit, wheel hub bearing, etc., is an automotive part used in most cars, passenger vehicles and light and heavy trucks....

 rotates freely on the shaft of the spindle.

In a drive suspension, the knucle has no spindle, but rather has a hub into which is affixed the bearings and shaft of the drive mechanism. The end of the drive mechanism would then have the necessary mounting studs for the wheel/tire and/or brake assembly. Therefore, the wheel assembly would rotate as the drive shaft (or half-shaft

Half shaft

The term half shaft is another name for a front-wheel drive shaft, which is also called the constant velocity axle or CV axle. There is one half shaft per wheel. Some may further to this part as a CV shaft....

) dictates. It would not turn freely by itself, but only if the shaft was disengaged from the transaxle

Transaxle

In the automotive field, a transaxle is a major mechanical component that combines the functionality of the transmission, the differential, and associated components of the driven axle into one integrated assembly....

 or differential.

A driven suspension as described may also be steerable. This is often called a drive/steer arrangement.