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Power Steering System

TRENDS IN VEHICLE PLATFORMS DOCUMENT THE NEED FOR COMPREHENSIVE CHASSIS AND SUSPENSION SERVICES

Edited from an article by Gary Goms, ImportCar Magazine

The rationale for complete chassis service isn’t so difficult to understand if you take a few minutes to analyze the trends in vehicle design. First, the conventional rear-wheel-drive vehicle platform is rapidly vanishing from the market. High-end imports and light trucks may continue to use this platform, but, for the most part, the trend is toward building front-wheel, four-wheel, and all-wheel-drive (AWD) systems. Of those three systems, the all-wheel-drive platform is becoming the most popular, especially in wintry states that experience a lot of winter snowfall.

The major difference between a four-wheel and an all-wheel-drive platform is that an all-wheel drive operates with all four wheels engaged all of the time. In most cases, the conventional all-wheel drive is a front-wheel-drive platform that typically uses a viscous fluid coupling or differential gearing to drive the rear axles. To eliminate axle hop during a turn, the viscous coupling or differential absorbs the differences in rotating speeds between the front and rear axles.

Moreover, most all-wheel-drive vehicles are designed with a fully independent suspension. The handling and road feel of the fully independent suspension differs markedly from vehicles equipped with the old "solid" or "live" rear axles. When the solid rear axle (technically called a Hotchkiss design) is coupled with an independent front suspension, the handling is generally inferior to that of a fully independent suspension. Unlike the Hotchkiss design, the front and rear axles of fully independent suspension are very compatible in such areas as rebound control, suspension geometry, and sway control and, thus, offer superior handling and ride control.

COIL VS. LEAF SPRING SUSPENSION
In the "old-school" thought process, the front and rear axles became separate service entities because they had entirely different service needs. To illustrate, the front usually is an independent short-long control arm (SLA) design suspended by a coil spring with an externally mounted shock absorber to dampen spring rebound. In other cases, a MacPherson strut may be used, which combines the shock absorber with the spring. In direct contrast, the rear axle is usually mounted on leaf springs with shocks mounted fore and aft to dampen axle torque wind-up and spring rebound. The problem with combining a front-mounted coil spring with a rear-mounted leaf spring is that each reacts very differently to the same bump in the road.

In the front, the shock absorber is completely responsible for dampening spring rebound. In the rear, the friction between the multiple spring leaves will also dampen spring rebound. Depending upon design, the shock absorber almost becomes a redundancy in the leaf spring’s ability to dampen spring rebound. These differences between the inherent characteristics of the coil and leaf spring give each axle different ride, handling and rebound control properties.

CHASSIS ANALYSIS
The most prominent example of observational powers is spotting the well-used trailer hitch. Vehicles that are used for towing light trailers and campers are usually very much in need of chassis and suspension services because, in many cases, these vehicles also see a certain amount of off-road use. The rear springs, for example, may begin to sag, which affects front wheel alignment and reduces ground clearance.

Wheel alignment is affected because, when the rear of the vehicle is loaded, the suspension height at the front increases. The toe angle, which is very critical to handling and tire wear, may drastically change when the vehicle is loaded at the rear. The result may be a tendency for the vehicle to dodge, especially in crosswinds. The other tendency is for the vehicle to rapidly wear the tires on the inside or outside edges, which is more of an indication that the rear springs are overloaded than it is a need for a conventional wheel alignment.

In most cases, a simple spring replacement or the addition of helper springs, air bag levelers or air-type shock absorbers will eliminate these drastic changes in suspension height. The air bag or air shock absorbers will give the driver the option of leveling the vehicle by adding air pressure to the air-type leveling device.

STEERING WEAR ANALYSIS
The addition of rack-and-pinion steering to many light truck and sport-utility vehicles has eliminated many weaknesses inherent in the old parallelogram-type linkage steering systems. In most cases, the steering rack should be checked for torn boots, leaking seals, and worn inner and outer tie rod ends. Worn inner tie rod ends, for example, make a knocking noise on rough road surfaces. Worn ends can be detected while the vehicle is suspended on a lift by quickly turning each wheel in and out by hand.

As for testing tie rod ends, I still prefer squeezing the tie rod end with a common pair of water pump pliers. Although some experts claim that testing a tie rod end in this manner is fraudulent, I believe it provides the most accurate way to determine the actual amount of wear in the joint. Since most tie rod ends are spring-loaded to compensate for wear in the ball and socket, performing the old "dry" check by having a helper turn the front wheels while the tech visually checks for wear can be misleading. The dry check seldom indicates the cumulative effect of wear in the individual tie rod ends. Four slightly worn ends, for example, might allow as much as 1/8-inch toe change.

In the linkage-type systems, I test idler arms by jacking up the right-hand front wheel and observing how much toe change is taking place when I turn the wheel in and out by hand. Worn idler arms are a big contributor to steering wander and right-hand front tire wear, especially when crowned road surfaces tend to disproportionately load the right front wheel.

SERVICING THE AWD VEHICLE
Getting back to the fully independent, all-wheel-drive configuration, it’s extremely important to abandon the old philosophy of servicing the front and rear suspension systems separately. In the fully independent, AWD configuration, the front and rear axles operate as a unit and should be serviced accordingly.

For example, all four wheels should be aligned at the same time. In the AWD design, attempting to align the front suspension as a separate unit will often add to, rather than reduce, the steering or tire wear complaint because the front and rear axles drive as a unit. The reason is because the concept of drive thrust angle is slightly different on the AWD design. If the rear wheels aren’t tracking with the front wheels, the drive thrust may tend to push the vehicle in a slightly different direction if the rear axle becomes the main driving force.

As with all four-wheel and all-wheel designs, tires must be of the same tread and casing design in order to provide the same rolling circumference. Rolling circumference is the exact number of inches the tire travels in one revolution. When the rolling circumference of the tires is different, the drivetrain is stressed and, in the case of the AWD, the road feel and handling may become markedly inferior.

Last, it’s always important to remember that the shock absorbers and/or strut assemblies should be serviced in sets of four on the AWD design. Again, when an independent suspension is coupled with all-wheel drive, the shock absorber rebound rates must match each other in order to provide optimal ride, handling and steering control qualities.

Given that the SUV design has an inherently high center of gravity, and tends to roll over very easily compared to the average front-wheel-drive passenger or sports sedan, good handling is, in fact, a life or death matter for the customer and the shop supplying the complete chassis and suspension service.

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