Situation Critical

If you’re in the business of light vehicle service, SUV’s and light trucks are a major part of your operation. And if you’re like most, ride control is a major part of your chassis service segment. Depending on who you talk to, a third to half (some claim more) of light vehicles go to the wrecker at the end of their lives wearing OEM shocks and struts. What’s going on? While the parts are better, there’s no question that there are millions in unsold ride control service dollars left on the table every year, and that’s about consumer knowledge. Educating motorists about the need for effective ride control means techs and service writers need to understand the issues themselves. Major manufacturers like KYB, Gabriel and Monroe offer useful training programs, which progressive shops exploit, but underneath the features and benefits and sales strategies, what is ride control, and why is it especially important for trucks and truck-based vehicles? Here’s a quick primer on the subject that should convince the most jaded owner.

Why are shocks/struts there?

It’s a basic question, but one at a surprising number of techs can’t answer in a way that’s convincing to consumers. Every tech has seen a vehicle bobbing up and down when stopped, a sure sign of dead shocks or struts, but why does this happen? It’s a natural consequence of the springing that makes up vehicle suspension systems. Springs aren’t just metal suspenders that keep the bumpers off the ground, but are complex devices that isolate drivers and passengers from bumps and potholes, while allowing tires to spend more time in contact with the road, enhancing steering, acceleration and braking. The key concept is energy. When an SUV or light truck hits a bump, part of the forward motion is converted into compression of the spring, coil or leaf, or torsional “winding up” of a torsion bar. Air suspension systems share this energy between the air bladder and metal springs. Once the bump is passed, the stored energy in the spring has to go somewhere, and it is released by pushing back against the suspension in the reverse of the compression process. Unfortunately, when the energy is completely used up in extending the spring, or unwinding the torsion bar, the process repeats, as the springing elements “snap back”, recompressing the suspension without any additional inputs from road surface bumps or potholes. It’s the see-sawing action of energy switching back and forth from spring extension to compression and back again that creates the bobbing motion seen when the ride control fails completely. The job of shocks and struts is to manage that energy as it swaps back and forth between spring compression and extension to stop the oscillations in a reasonable time.

Manage energy? Engineers call it “damping”, which is what Europeans have called “shock absorbers” since the beginning. Damping happens inside the shock or strut by the pumping action of oil through a hole or holes (don’t let the term “orifice” fool you, it just means hole) that are too small to allow it to flow easily. The resistance creates the damping action, and much of the different shock technologies advertised by manufacturers are about varying the rate of oil flow by changing the effective “hole” area under different driving conditions. There are four basic technologies. The simplest is a fixed orifice design that damps at the same rate regardless of compression/ extension rates or piston rod position. This is entry-level product and should be avoided for all but the most cash-strapped consumer, and even then shouldn’t be installed where the O.E. fit uses better technology. The second is valving that varies by the position of the piston rod, generally by increasing damping rates (“stiffening”) as the unit is compressed. This helps handling since rates rise on the corners compressed during hard maneuvering, a factor that’s especially important with heavy, top heavy SUV’s and light trucks. A third type changes effective valving based on the rate of piston rod compression or extension. This allows the unit to respond like a performance shock or strut in sudden, violent maneuvers like accident avoidance or lane-changing, while maintaining a compliant ride under normal driving. If available for the application, this velocity-determined valving is an excellent compromise for solid-axle SUV’s where consumers want car-like ride quality, but superior handling to conventional light trucks. The fourth category is growing: Electronically controlled damping. These units use driver inputs or active computer control, often as part of a load-leveling or stability control system and are both expensive to replace, and difficult to substitute for with other technologies. For some applications, (Lincoln is one) troublesome air suspension systems can be retrofitted with a conventional spring/shock kit, but where the damping is integrated into the vehicle electronics, O.E. is often the only option. Manually or driver-adjustable systems are available for performance users, but they’re rare in typical street-level SUV usage. Unfortunately, in too many cases, technicians and service writers don’t understand what ride control does within the suspension system and fail to communicate the message to motorists.

Shock absorbers don’t absorb road shocks, springs do. This understandable misconception too often leads to consumer dissatisfaction with their ride control purchase. A better handling, safer car may not feel smoother in straight ahead driving on average roads. What “shocks” and “struts” will do, however, is absorb the energy of the spring oscillation by converting it into heat. Why? Because like everything else in physics, there’s no free lunch, i.e. the energy of the spring oscillations can be converted into another form, like heat, but it can’t disappear. Where does the heat go? Into the air, at least it should, and if too much builds up within the body of the shock or strut, the consequences can degrade the units’ performance. It happens by changing the viscosity of the oil as it heats, or in extreme cases, by boiling it. Most modern ride control technologies use pressurized nitrogen gas to keep the oil as a non-aerated liquid. For really extreme use, like hard off-roading or rallying, remote reservoir shocks separate the oil supply from the shock body, adding a reserve quantity and keeping it cool. Both single and twin-tube designs are available, and pressurization may be high or low pressure. Each design has advantages, but from a consumer satisfaction point of view, the key is to create a vehicle with noticeably better ride and handling.

Technicians continue to fail to diagnose failures properly. Leaking oil is a sign of a bad unit, but the absence of oil on the shock or strut body doesn’t mean the unit is still serviceable. An intelligently performed road test is necessary to diagnose bad ride control. Similarly, when replacing worn struts and shocks, too many techs still ignore the rest of the system.

Technicians too often ignore the springing elements in the suspension system. Broken or collapsed springs affect ride control in several ways. One is to change the vehicle’s static ride height, which alters suspension geometry and can allow the ride control to exceed its travel limits. Bump stops are not part of the suspension system. A broken coil, for example, may lower ride height by less than an inch, but as the coils bottom the spring rate goes up, meaning the ride control is asked to effectively damp higher frequency spring oscillations for a stiffer unit. Conversely, weak springs tax ride control at the other extreme, with lower frequency, high amplitude (read “bottoming out”) oscillations that can over extend or compress the unit.

Technicians don’t pay attention to the rest of the system. In auto/light truck suspension systems, there are more springs than just the springs. Any route that can feed vibration into the body structure is a possible owner complaint or comeback. Strut mounts are critical, for example as a noise and harshness issue but are n
ot always inspected during strut replacement. Deteriorated suspension bushings are another under-inspected contributor to NVH, as are spring isolators. Even tires are a factor. Consumers read the maximum inflation pressures on the tire’s sidewall and often take this figure as the recommended pressure. And with some SUV tires carrying 40-plus psi ratings, grossly over inflated tires can pass a lot of vibration into the cabin. Similarly, under inflated tires will kill handling, regardless of the health of the suspension system. It’s an easy check.

Technicians don’t train adequately for ride control sales and service. “Re and re” of ride control is generally a simple service, so there’s a tendency for technicians to ignore training in shocks and struts. All the major manufacturers offer training, but too few shops embrace it as a way to boost sales and increase customer satisfaction. Several manufacturers are launching major initiatives, which will be covered soon in SSGM.

Are you getting your share of the millions in underperformed SUV/light truck ride control service? The numbers suggest that most Canadian shops aren’t, a situation that’s strange considering the high level of perceived customer satisfaction that comes with this service. Few repair jobs generate the seat-of-the-pants feel of better ride and handling that shock or strut service, an effect that’s amplified by the generally poor capabilities of light truck-based SUV’s. It’s out there, if you’re willing to go and get it.