Auto Service World
Feature   February 1, 2006   by Jim Anderton

The Other Suspension Service

How suspension components are mounted is vital to both handling and NVH. They're simple, underperformed service items

Compared to may light vehicle technologies, suspension has gotten a free ride. Emission controls, safety regulation and consumer appetites for more features and performance have driven high technology into most chassis, body and power train systems in modern vehicles. If you could take a Studebaker or Packard mechanic from the 1950’s under a modern SUV, however, he’d feel right at home working on the suspension. The basic principles have stayed the same for decades, but now new geometries and materials are threatening to bring modern vehicle suspensions into the computer age. Will modern technicians adapt?

Every technician is aware of the role of vehicle suspensions: chassis support, handling and ride comfort. Consumer awareness is essentially zero, with few owners approaching service providers without the flag of excessive tire wear or audible clunks and bangs from the chassis. With the vast majority of passenger cars using strut-type front suspensions and simple independent rear setups, sources of wear and noise have been limited to lower ball joints, struts, strut mounts and bushings. They’re still prime factors, but there are more components, and more active control of suspension damping.

On the mechanical side, the number of links, arms and bushings are increasing, as is the engineering behind these apparently simple components. Take bushings, for example. Conventional bushings are rubber doughnuts that are loaded in shear, axially or in torsion, sometimes a combination of all three. The hardness (engineers talk about “durometer”, actually a measuring instrument) of the rubber determines the amount of bushing deflection and the amount of noise and vibration that’s transmitted to the chassis. Stiff “performance” polyurethane bushings, for example are notorious for noise and harshness, but offer more precise suspension tunability and sharper turn-in. Consumers however, want both comfort and handling, forcing manufacturers to use newer technologies that have different stiffness depending on the direction of loading. The Buick LaCrosse, for example, uses stiff jounce and soft rebound polyurethane bumpers and lower control arms with different rates laterally and fore/aft. Even the subframe mounts have asymmetric stiffness to locate the suspension firmly without the steering vagueness common to large luxury cars and SUV’s. The components look the same as conventional parts, but as these vehicles move out of warranty, it remains to be seen if the aftermarket can supply molded rubber and plastic components with the special OEM properties at a reasonable cost. Will white box suppliers mold cheap rubbers into shapes that fit, but don’t perform? It’s already happening, and with high-tech OEM parts, the possibility that cheap replacements will result in excessive noise and vibration is greater with every model year.

The components are increasing in sophistication, and in number, too. Multi-link suspensions, once restricted to luxury imports, are finding their way under mainstream brands. The value proposition for consumers is a compliant ride with sporty handling, but for engineers it’s about “decoupling” steering and suspension functions and handling each as separately a possible. Take conventional leaf spring rear suspension, for example. It’s a 19th century technology; cheap, strong and easy to produce, it’s still the choice for pickups and full size vans. It’s strong, but leaf springs have to simultaneously locate the axle, suspend the vehicle and absorb the torque induced by both acceleration and braking. The problem is serious enough that manufacturers have used supplemental devices like axle housing snubbers and staggered shock absorbers to limit the axle wrap phenomenon. Full size SUV’s have similar weights, but are moving to fully independent rear suspensions with struts or spring/shock combinations that are commonly located by four or more trailing arms. They’re essentially front suspension systems for the rear, with many using conventional tie rods for toe location, or toe-compensating trailing arms. Good designs isolate springs and struts from braking and acceleration torques and keep their motion as close to linear as possible. Service, however, involves a close inspection of bushings and mounting bolts as well as alignment to keep the various suspension forces isolated.

A crucial aspect is ride height. Ride height has always been important for proper alignment, but with modern systems there are camber and toe implications front and rear, as well as potential damage to shocks or struts. Air suspension systems add another dimension to service, since they have to work properly before any other part of the system can be diagnosed. They’re also expensive to repair, leading to aftermarket coil spring conversion kits for older Lincolns, for example. Hummer H3 is available with either system, and as these vehicles age, look for conversions to become a possibility as the vehicle’s residual value drops.

While the coils, leaves and torsion bars are the textbook suspenders of most modern light vehicles, the actual spring rate at each corner is the sum of the spring rates of the individual elements. The additional springing comes from several sources. The most obvious is the sway bar and its bushings. Actual rates are hard to describe since they depend upon the suspension compression of a vehicle corner relative to the spring on the other side. Driving over a “speed bump” straight on, for example, should add zero rate, but driving over a bump with one wheel only would add maximum rate. That’s the basis of roll control and while it aids flatter cornering, the bar has to be relatively free in its bushings to load the corners consistently. End-link bushings are a cheap and easy fix, as are the less frequently replaced inner mounts. The inners are just as important, however, and are frequently bathed in engine oil and undercoating, petroleum products that eat rubber. They’re a logical replacement item when “outers” are deteriorated and eliminate a potential noise source too. Why are anti-roll bar mountings so important? Because anti-dive suspension geometry, softer spring rates and long suspension travel, combined with a stout ‘bar gives decent handling with a smooth ride. There is much more to modern suspension service than can be covered in any single article; attention to detail and a methodical approach to diagnosis goes a long way toward happy consumers and profitable operations. Look for more on suspension and ride control issues coming up in SSGM.

Suspension Bushing Service Tips

Mounts and bushings can be a simple bolt-on or a frustrating force-fit. When removing worn or deteriorated parts, avoid burning them out with oxy-acetylene torches. Besides the fire risk, many suspension parts are now forged and cast from aluminum. Unlike steel, aluminum melts or loses its heat treat imperceptibly, and looks cold even when it burns unprotected skin. Drilling multiple holes in a bushing with a sharp bit will aid removal safely. During installation, rubber parts can benefit from a light coating of a non-petroleum grease, like the silicone-based brake lubes, during assembly. They’re also handy for installation of press-fit bushings. Sometimes it’s hard to tell if a rubber or poly part is deteriorated to the replacement point by inspection alone. If a spring/shock or strut assembly is out of the vehicle, take advantage of the lack of preload to move the suspension throughout its travel. Any squeaks or groans now take strut mounts out of the equation. Besides, relative to the cost of proper ride control service, replacement of marginal rubber parts is cheap insurance, especially if strut mount noise is masking other sounds. Consumers will believe that the new sound is a sign of defective ride control products and the comeback visit might be difficult. Several ride control and chassis part manufacturers offer training and instruction manuals on proper diagnosis of suspension and ride control. Check with your jobber or on manufacturer Internet sites for training opportunities.