Auto Service World
Feature   July 1, 2001   by Jim Anderton

Compression, jounce or Bump

Knowing ride control begins with basic strut/shock types and terminology

Strut and shock technology is undergoing a renaissance these days with multiple offerings and attributes that are sometimes confusing for consumers. There are customers that know nothing about their ride control purchase, or worse, know just enough to cause confusion. What can you do? Learn as much as you can about the products, starting with the basic strut/shock types, and the correct terminology.

The basic types

There are several ways to break down the different internal workings of struts or shock absorbers, but it’s useful to think about three basic types: mono-tube high-pressure gas, Twin-tube low pressure gas, and twin tube hydraulic. Mono tube high pressure units use the shock or strut body as the main chamber, with a relatively large area piston, which displaces the strut’s oil to a chamber at the unit bottom. The oil is pressurized by nitrogen gas at pressures that can exceed 250 PSI, and a floating piston physically separates the gas and oil.

Twin-tube low-pressure gas struts and shocks use an inner and an outer chamber. The outer acts as the oil reservoir, while the inner cylinder houses the piston and rod. Oil is displaced between the inner and outer cylinders, and the oil is pressurized with nitrogen gas, at pressures typically between 100 and 200PSI. Depending on the manufacturer, the gas may be physically separated from the oil by a plastic barrier, or may be in solution.

Twin-tube hydraulics have the same internal architecture as twin-tube low-pressure types, but operate without a gas charge.

Which is best?

There is no clear answer, mainly because of the wide differences in real-world service. Gas pressure was designed to give strut and shock action that remained consistent with rising internal temperature by keeping the oil under enough pressure to keep it from boiling. Another advantage in many designs is the ability to mount the unit at any angle, or upside down, an old racing strategy to reduce unsprung weight and invert the shock’s jounce/rebound characteristics. While it’s less common with today’s adjustable performance products, some “muscle car” owners and drag racers invert shocks with good results, so don’t be surprised to find “flipped” units in these applications. Only gas pressure shocks can be inverted, however, since conventional hydraulics use gravity to keep the oil settled in the outer tube reservoir.

Do Canadian drivers need high-tech gas pressure suspension damping? Probably, for three main reasons: the first is that adjustability of premium products gives the customer some control over ride quality and performance. Install units that are too stiff for daily driving? Just dial in a softer ride, or vice-versa. The second reason for advanced units is the extra sensitivity offered by the nitrogen gas dissolved in the oil, which can act as a “pre-strut” by reacting to road surface irregularities before the mass of the piston and rod moves extensively. The third is the simplest: they’re OE specified in most applications, and should be replaced with at least the same performance characteristics as the original strut or shock.

There are a multitude of issues and technologies to talk about in ride control, but making consumers aware of the basics is essential to the move away from purely price-driven purchase decisions. Watch SSGM in the coming months for more coverage of this important issue.

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