Auto Service World
Feature   June 1, 2000   by Jim Anderton

The Shocking Truth

Heat, oil and energy conversion make suspension damping work, and multiple product options make it pay.

Like many automotive systems, ride control works by a set of basic principles that can be understood by every technician, and by most consumers as well. Installing the right product on a given vehicle is a simple call to a jobber, but matching the product to the vehicle’s owner, and just as importantly, making the consumer understand the product choice, takes a little knowledge.

How shocks and struts work

To get a grip on the secrets of shock and strut technology, it helps to think a little about what’s happening when a vehicle drives over a bump or undulation in the road surface. When the wheel encounters the bump, the spring (and tire sidewall) compress, softening the impact transmitted to passengers and chassis, but the problem occurs when the wheel passes over the bump. Springs act like a form of battery, the difference being that batteries store electrical energy, and springs store mechanical energy. That energy is significant, as any technician who has witnessed a strut spring compressor accident will confirm. The trouble is, the spring wants to give back the mechanical energy it stored (by compressing over the bump) immediately, causing the familiar “pogo” effect. That pogo action not only looks funny, it loads and unloads the tires as the car’s mass cycles between suspension compression and extension, drastically reducing vehicle control during cornering and braking. While this isn’t news to technicians, think of it from an energy point of view. Stopping the pogo effect means slowing down the rates of jounce (compression) and rebound to get rid of some of that troublesome mechanical energy. Unfortunately, the energy has to go somewhere, and where it goes is into the shock or strut as heat. Before the invention of the modern shock absorber, the problem was much simpler. Leaf-spring front and rear suspensions used leaves that slid against each other during compression and rebound, scrubbing off some of the energy as frictional heat. Even then, suspensions were harsh and vehicle handling was poor. Coil springs and torsion bars dissipate little energy as heat, so the shock/strut system has to work harder.

How they do that hard work is simple, at least in principle. A piston forces oil through holes too small for the liquid to easily pass through, creating a resistance which converts some mechanical energy into heat. The faster you pump the piston, the more energy is dissipated, and the more the unit heats.

Terry Heffelfinger, chief of aftermarket engineering for Tenneco Automotive (Monroe) explains: “All shocks are velocity sensitive devices, so that the faster you stroke a shock in compression, or in rebound, the more the damping force. There are three areas that shock engineers use to tune shock absorbers for a specific ride. They are bleeds or bleed areas, discs, which are bending discs, and holes or orifices. You change the diameter of the bleeds or orifices, or the thickness of the discs in order to tune the ride to a specific vehicle application.”

Over and under damping

The tuning that Heffelfinger describes goes on in the new vehicle development phase, and is one reason why manufacturers with OEM experience claim to produce better, application-specific products. The real world, however, can be different from a manufacturer’s test track. A key factor is unsprung weight. The wheel/tire/ knuckle and brake assembly act just like the weight on a pendulum, and like an old clock, they want to swing at specific rates or frequencies. A vehicle’s shell also has its own set of natural flexing frequencies or harmonics, which, like the wheel assemblies, have preferred frequencies. Exceed these frequencies and the suspension can’t react fast enough, causing poor ride and handling. Values vary by the model of vehicle, but for cars, suspension compression and rebound frequencies faster than 12 to 14 cycles per second, and body frequencies exceeding the three to four “cps” range will become essentially solid, resulting in very poor ride and handling qualities. Where would the average driver encounter these frequencies? The “washboard” segments of typical dirt roads are the most typical examples, while urban drivers will experience the effect through “ripple strips” on highways, or from expansion joints and drainage grooves in concrete highway surfaces. For the installer, the point is this: The “correct” application for a given vehicle does depend on how and where it’s driven. A straight OEM-type replacement will give an excellent fit, but may not give the customer excellent satisfaction.

For hard-working suspensions, oil foaming of entry-level products, as well as premature leakage and loss of adequate damping is a definite possibility. Both nitrogen gas pressurization and larger oil capacity fight foaming, while dust boots, where available, can help keep abrasive grit out of the piston seals. Boots are popular with the off-road community, and while effective, should be used with caution since a small tear will create an environment which will trap dirt and moisture against the piston rod. Periodic inspections are the only way to tell before the unit loses oil, inspections that should be recommended to any customer with heavy-duty needs.

Another factor is vehicle modifications, and modification can be as simple as a new set of tires. Why? Because tires have their own “suspension properties”, with unique sets of resonant frequencies and damping characteristics. An OEM tire fitment, for example, might be chosen for ride comfort, and a customer purchasing run-flats often finds that the stiff sidewalls give the suspension system more effective spring rate. Unless the strut damping can accommodate the additional stiffness, ride and handling can suffer. While it’s unlikely that a customer will replace serviceable shocks and struts with a tire purchase, a look at the vehicle’s rubber when quoting replacements can suggest the right level of damping needed. Plus-sized imports with very low unsprung weights can regain some ride quality with the right products, while at the other end of the suspension spectrum, a four-by-four swinging 40-inch mudders will likely overwhelm OEM-type shocks.

Affordability vs. quality

Ride control technology means nothing if the customer can’t or won’t purchase products that will produce satisfactory ride and handling characteristics. The key to getting consumers to make a choice, however, is to let them know that there is a choice to be made. Going straight to the entry-level product may work, but as a blanket strategy it rejects the possibility of a more profitable job and a more satisfied customer.

Pete Mitchell, technical analyst for Sachs North America, has some definite opinions about “white box” components: “The people who are private labeling or white boxing are building for the aftermarket. The product isn’t necessarily built to O.E. tolerances. That doesn’t mean that the product won’t work, or that it won’t last, but you get what you pay for. If you’ve got an older car, and it doesn’t have much in the way of resale value, it makes sense to use a lower end product, but there is a performance tradeoff. If you’re using a European application, you’ll find that the (European manufacturers) build a high quality unit. The downside is that it takes a bunch of money to make these things. You could probably buy a white box shock for a Rabbit for less than it costs to make it in Germany. So do you plan to keep it forever?”

Mitchell is describing the other part of strut and shock marketing: unless the part fails catastrophically, owners of older vehicles can, and do, delay needed strut replacement, or prioritize other repairs first. At the entry-level, consumers faced with repairs at the limit of affordability may opt for off-brand products, although in struts, remanufactured products are a viable option. Savings can approach 30 percent over new product. Can consumers (and installers) expect the same service life from reman struts? “Sure, absolutely”, says Chris Lopez, product manager for ride control products for Cardone Industries. “What’s goin
g to fail on a strut is the seal. We replace all seals 100 percent, and add new oil. Those are the major parts of a strut, so you have as good a strut as possible.” Cardone’s reman line, Ride Saver Plus, features a limited lifetime warranty. Is there a reason for any customer to leave a shop with leaky, worn or damaged ride control components? Probably not, especially given the wide range of performance and pricing options open to consumers. There will always be the customer at the bottom end for who even white box or reman is unaffordable, but at the other end of the spectrum, marginal product on luxury and performance vehicles is still too common. The product is out there, and it starts by a simple suggestion by the installer. SSGM

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