| There are three types
of shock absorbers -- hydraulic twin-tube, twin-tube low-pressure gas and
mono-tube high-pressure gas. The twin-tube hydraulic shock is the oldest
and least complex design and the cheapest to produce. That's why nearly
all American cars receive them as original equipment. The twin-tube has
an inner pressure chamber that houses a piston and oil; a concentric outer
tube serves as oil reservoir. Depending upon piston speed, oil is forced
through orifices in the piston or through a valve in the base of the pressure
tube. The twin-tube shock can easily be valved to deliver good ride quality
and it works well with the long suspension travel and moderate spring rates
found in most stock vehicles. Properly designed and built, such a shock
can give admirable performance with good longevity.
One design limitation of the twin-tube shock is that the outer chamber acts as an insulator, slowing the rate of heat transfer from the oil inside as it's churned by the fast-moving piston. Heat buildup is the bane of shock absorbers, eventually leading to aeration of the oil. Oil can't be compressed but air certainly can; mixing the two causes a sharp drop in damping performance. Aeration can be kept in check by using a larger-diameter piston, high-quality oil and premium materials, particularly for the piston seal. But a bargain basement shock -- constructed with low-rent materials and indifferently assembled -- will quickly overheat and wilt at the first hint of spirited driving. Efforts to limit heat build-up led to the creation of the mono-tube high-pressure gas shock. German manufacturer August Bilstein licensed the design and perfected it. During the process they learned that a key element is the piston seal; pressurized gas or not, if the seal failed to reliably separate oil from gas, aeration would still occur. Bilstein devoted considerable resources to developing a rugged, self-lubricating and long-wearing seal that optimized their monotube high-pressure gas design. When it appeared in 1983, the new C4 became the first U.S. vehicle to offer Bilsteins as original equipment. In 1989 Bilstein also supplied the optional FX3 electrically adjustable shocks. Since then some 30 other GM vehicles have also worn Bilsteins ranging from the Z71 pickup to the police pursuit 2 WD Tahoe. In recent years the German manufacturer's OEM U.S. business has blossomed to the point where an Ohio manufacturing plant was constructed to handle the demand. Bilstein continues to concentrate on high-pressure gas mono-tube models for both street use and racing with several available for Corvettes from model years 1963 and up. The mono-tube design has other advantages. For example, given tubing of equal bore diameter, it can accommodate a larger-diameter piston in comparison to a twin-tube shock. Its greater working area allows more damping control without having to scale-up the package with tubing of larger diameter, bigger components and more oil. The advantage: less weight and smaller packaging. And without the thermal barrier imposed by an outer tube, a high-quality mono-tube shock can more easily dissipate heat. Not surprisingly, the mono-tube gas shock is widely used on racecourse competing in series ranging from the Winston Cup to the Baja 1000. The twin-tube low-pressure gas shock is the third type. Available from several companies, many of the cheaper variants are simply hydraulic designs with a plastic bag of low-pressure nitrogen placed above the oil in the pressure tube. Although allowing the manufacturer, at least technically, to claim it as gas-pressure shock, this Baggie design deteriorates quickly, marked by a short service life and limited resistance to aeration, hardly a surprise given the harsh conditions found inside a shock. Adjustable shocks have some
enticing advantages. On a street driven vehicle they allow compensation
for normal wear, maintaining optimal performance for the life of the shock.
Better yet, a Corvette owner looking to do some weekend autocrossing can
dial in stiffer valving for the event, then restore stock settings afterward,
eliminating the need to buy a second set of shocks.
The C4's single-adjustable FX3 Bilsteins with touring, normal and sport settings had a noticeable effect on ride and handling. This was accomplished, according to John Dullam, Bilstein's OE/Aftermarket Ride Development technical wizard, despite the conservative range of adjustment dictated by Corvette development engineers -- 15 percent from full soft to full hard. For the serious owner who increases spring rates, alters roll stiffness or otherwise tinkers with stock parameters, adjustable shocks can greatly ease the task of tuning the suspension. A single-adjustable Koni can be set to deliver up to a 100 percent range of adjustment from full soft to full hard. That's enough to cope with major changes in tire construction, size and profile, not to mention spring rates, anti-roll bar stiffness and bushing durometer. While Bilstein offers no adjustable street shocks for Corvettes at present, this gap in the product line hasn't escaped their notice; we won't be, uh, shocked if an adjustable Bilstein were to appear in the near future. Bilsteins and Konis aren't cheap; a single shock can easily cost more than two complete sets of lesser models. But if all of these competing shocks offer lifetime warranties, why not buy the lower cost unit? The answer depends on how often you'll feel like removing them from your Vette, schlepping them back to the retailer for replacements, then installing the new set, knowing the process will occur periodically until you finally tire of the routine.
High-end shocks can last 150,000 miles or more while delivering consistent performance throughout. And they can be rebuilt or custom valved if desired. That's because of the quality of their materials and superior construction. Bilstein tubes, for example, are extruded from hockey puck sized chunks of aluminum. The resulting close tolerances optimize sealing and prolong component life. Like Konis, their induction-hardened piston rods are lavished with chrome plating of extremely high quality to limit friction and wear. Costly Viton™ piston seals -- Teflon impregnated for low friction and durability -- are used in lieu of the cheap rubber O-rings found in lower priced shocks. Choosing the right shock for you application makes an appreciable difference in handling and cornering performance as well as ride comfortable. Obviously, price and quality go hand in hand. Just remember that when it comes to purchasing shocks for your Corvette, compromising on quality is not an option. CM com |
Single-adjustable Koni Sport model (front shock for '89 - '96 Corvette pictured) features a larger bore and piston for greater damping performance. Its more aggressive valving makes it suitable for weekend competition and high-performance street use.  Red non-adjustable Koni Special model (front shock for '84 - '87 Corvette pictured) is intended as an OEM replacement unit and valved with a bias toward ride comfort versus all-out performance.  Damping of the adjustable Koni Sport (left) can be controlled by turning a knob atop the piston rod. Factory-set to be installed at full-soft setting, its wide range of adjustment makes it ideal for suspension tuning. The Koni Special shock is an OEM replacement unit with valving biased more toward ride comfort than performance |
