Steering Systems

The steering and suspension systems of a car are not only important for safety reasons but also enhances the comfort level of the vehicle. The two systems are directly related to each other, which is why they are always referred to together.

First lets look at the steering system. There are two basic types, standard mechanical steering and rack and pinion steering. The standard mechanical steering can be either power assisted or non-power. Rack & pinion is almost always power assisted although there are rare cases where it is not.

Standard mechanical steering

This system uses a series of links and arms to insure both wheels turn in the same direction at the same time. It hasn't changed much in all the years it's been used and is quite simple. Basically this is how it works, the steering wheel is connected to the steering box through the steering column. The steering box turns the rotation of the steering wheel 90 degrees and, in the case of power steering, uses high-pressure fluid to help actuate the steering. The steering box has an arm attached to the output shaft called the pitman arm. This connects the steering box to the steering gear. The pitman arm is connected to one end of the center or drag link. In the other end of the center link is an idler arm. Between the idler and pitman arms, the center link is supported in the proper position to keep the left and right wheels working together. The inner tie rod ends are attached to either end of the center link and provides pivot points for the steering gear. From there it goes to the outer tie rod ends through an adjustment sleeve. This sleeve joins the inner and outer tie rod ends together and allows for adjustment when the front wheels are aligned. The outer tie rod ends are connected to the steering knuckle that actually turns the front  wheels. The steering knuckle has an upper and lower ball joint that it pivots on and creates the geometry of the steering axis.

Rack and Pinion

As you can see, it's pretty simple. It is just a simple mechanical connection from the steering wheel to the front wheels. The weaknesses of the system are at the pivot points. The pivots are ball and socket joints that do wear out over time and will require replacement. Loose steering parts will make a car difficult to handle and will cause the front tires to wear out prematurely. That's why it's important to have the steering checked at least once a year. A great time to do it is when you're in for an oil change. Rack and pinion steering is somewhat different. Basically it combines the steering box and center link into one unit. The steering wheel, through the steering column, is directly connected to the rack. Inside the rack is a pinion assembly that moves a toothed piston to move the steering gear. One end of the inner tie rod ends is connected to either end of this piston and the other end is connected to directly to the outer tie rod end. The inner tie rod end is actually threaded into the outer tie rod end and can be rotated to make adjustments during a wheel alignment.

The advantage of rack & pinion steering is that it is more precise than the mechanical system. By reducing the number of parts and pivot points, it can more accurately control wheel direction and is more responsive. The down side of a rack & pinion steering system is that they are prone to leakage requiring replacement of the rack assembly.

Rack & pinion steering is almost always used with a MacPherson suspension system. The bottom of the steering knuckle still pivots on a lower ball joint, but the top of the knuckle is connected to the MacPherson strut. In this system the outer tie rod end is connected to an arm on the strut housing itself. The MacPherson strut assembly replaces the upper control arm, front shock absorber and ball joint, increasing handling and responsiveness. It controls ride much the same way as a standard hydraulic shock absorber. It also keeps the front end aligned and eliminates, in some cases, the need for caster and camber adjustments.

Electric power steering (EPS or EPAS)

Designed to use an electric motor to reduce effort by providing steering assist to the driver of a vehicle. Sensors detect the motion and torque of the steering column, and a computer module applies assistive torque via an electric motor coupled directly to either the steering gear or steering column. This allows varying amounts of assistance to be applied depending on driving conditions. The system allows engineers to tailor steering-gear response to variable-rate and variable-damping suspension systems achieving an ideal blend of ride, handling, and steering for each vehicle. Most EPS systems have variable assist, which allows for more assistance as the speed of a vehicle decreases and less assistance from the system during high-speed situations. In the event of component failure, a mechanical linkage such as a rack and pinion serves as a back-up in a manner similar to that of hydraulic systems. Electric power steering should not be confused with drive-by-wire or steer-by-wire systems which use electric motors for steering, but without any mechanical linkage to the steering wheel.

Electric systems have a slight advantage in fuel efficiency because there is no belt-driven hydraulic pump constantly running, whether assistance is required or not, and this is a major reason for their introduction. Another major advantage is the elimination of a belt-driven engine accessory, and several high-pressure hydraulic hoses between the hydraulic pump, mounted on the engine, and the steering gear, mounted on the chassis. This greatly simplifies manufacturing and maintenance. By incorporating electronic stability control electric power steering systems can instantly vary torque assist levels to aid the driver in evasive manoeuvres.

Reviews in the automotive press often comment that certain steering systems with electric assist do not have a satisfactory amount of "road feel". Road feel refers to the relationship between the force needed to steer the vehicle and the force that the driver exerts on the steering wheel. Road feel gives the driver the subjective perception that they are engaged in steering the vehicle. The amount of road feel is controlled by the computer module that operates the electric power steering system. In theory, the software should be able to adjust the amount of road feel to satisfy drivers. In practice, it has been difficult to reconcile various design constraints while producing a more pronounced road feel. The same argument has been applied to hydraulic power steering as well.