Auto Service World
Feature   May 1, 2008   by Andrew Ross

Shock And Strut Tech

Despite the fact that that ride control technology has been around in its current form for nearly a century, damping suspension movement by squeezing oil though small holes in a tube, there continue to be misconceptions about it and, occasionally, a total misunderstanding of the function of shocks and struts.

I have always found it helpful to recall the earliest examples in describing the role of the shock or strut in limiting suspension movement. The oldest form, the friction shock, is still used by some street rod builders. Friction shocks rely on little more than a set of levers that scissor open and closed with the suspension movement; the amount of resistance depends on how tightly they are squeezed together. (If you want to tighten up the suspension movement, simply give a bolt another turn.) While this technology was very rudimentary, it did provide some suspension damping.

The Gabriel Snubber, introduced in 1907was, curiously enough, an aftermarket accessory that could be added to a car at the wish of the owner. It consisted of a length of leather belt coiled into a housing and kept under tension by a spring, something like a tape measure; the leather strip extended or retracted with suspension movement, thus damping movement. In action, as the suspension dropped into a pothole, the Snubber would provide some resistance to slow that movement. Alternatively, when the car went over a bump, the resistance to “uncoiling” the strip from the housing would slow the downward movement of the wheel suspension, preventing the tire from landing on the road surface so hard that it would bounce of the surface, again and again. The problem with this technology was that it appeared only to work in rebound.

In fact, getting the right balance between jounce (hitting the bump), and rebound (controlling the return of the wheel and tire to its at-rest position), is quite possibly the toughest job in ride control even today.

The reason it is helpful consider that old technology is it is clear that the Snubber and its ilk focused purely on limiting the speed of suspension movement. Modern ride control technology, though much more sophisticated, performs exactly the same function.

What shocks and struts do not do is affect the ride height of a vehicle or support its weight. Often, especially with gas-charged shocks and struts, there is a misconception that they provide an appreciable degree of weight bearing. While high-pressure monotube designs do provide some support, they do not replace the springs.

In most cases, you could remove the shocks from a vehicle (and the struts also, if it weren’t for the fact that they provide suspension location ), and it would sit at precisely the same ride height. You could also drive it, and on a smooth road you might not even notice any difference. Hit the first bump, however, and the rest of the trip is likely to be quite a pogo ride. Today, there are essentially only two basic shock and strut categories commonly offered in the aftermarket: twin-tube and monotube.

Twin-tube designs feature a pressure tube and a reserve tube. The piston and rod ride in the pressure tube. The reserve tube is where the hydraulic fluid goes as it is displaced by the piston rod.

Monotube shocks, as their name indicates, consist of just one tube. To allow space for the fluid displaced by the piston rod as it moves with the suspension, a secondary, floating “piston” divides the lower portion of the cylinder from the working piston’s swept area. On the side opposite the fluid is gas under pressure, which can be compressed, while fluid cannot.

All shocks in common use today employ the principle of hydraulic resistance to damp suspension movements. In each, a piston is fitted with a set of disc valves and orifices. There is also a similar strategy used in the “base valve” in twin-tube ride control units, which limits fluid movement from the pressure tube to the reserve tube.

As suspension movement forces the piston up and down, fluid is forced through small holes in both piston and base valves, providing resistance. (The use of pressurized nitrogen reduces the possibility of foaming.) In general, the smaller the holes, the greater the resistance; the faster the movement, the more resistance there is to that movement.

In practice, great effort is made to provide a range of ride control characteristics that react to several types of road and suspension movement.

A combination of thin discs provides a greater range than a simple set of orifices in the piston or base valve could ever do. For example, a sharp road-impact shock that might otherwise be transmitted directly to the car could create instability. A disc valve could be used that will deflect under these “high velocity” conditions, and allow fluid to pass through holes that it keeps covered under normal driving conditions.

In a ride control course I attended with Tenneco engineers some years ago, four control stages were specified covering low-, mid-, high-, and very high-velocity ranges of suspension movement. The low was 0 to six inches per second (equivalent to a gently rolling road surface), to the very high, upwards of 50 inches per second, which might be encountered in pretty extreme road conditions.

To illustrate how these differ from the actual speed of the vehicle involved, short-track stock cars generally look at a range from 0-4 in/sec for roll, squat, and dive, while rough surfaces (by race track standards) might push a shock into the 6- 12 in/sec velocity range.

While the exact construction of the makeup of control valves varies from one shock to another and between twin-tube and monotube designs, the point is that it provides a way to tune the performance of a shock or strut to the specific needs of a given application or set of applications.

There are of course, a few creative ways that ride control manufactures have used to extend the operating range. Adjustable shocks allow drivers to decide for themselves.

Generally restricted to the performance and racing markets, adjustable shocks adjust to a preset number of positions, affecting both compression (or bump) and rebound at the same time, one or the other, or, on high-end competition units, separately. The actual mechanism uses either a tapered needle that protrudes into a fluid channel, thus restricting or increasing flow, or allows the user to rotate elements of the foot valve to open or close orifices, adjusting rebound.

There are a great many variations on the market, enough for a completely separate article.

In the meantime, it is important to understand that the real workings of a shock or strut are enclosed and invisible to the eye; and to also understand that their real purpose is to keep the tires on the road, not just to keep the driver comfortable.

Semi-Active Ride Control

The advantage of being able to adjust the performance characteristics of a shock or strut to match conditions has long been recognized, but it is hampered by the cost and complexity of making the required mechanical adjustments on the fly.

A few years ago, Delphi introduced a semi-active suspension technology with no electro-mechanical valves and no small moving parts; the technology has found its way into Cadillac, Audi, and Ferrari models.

The system consists of four magneto-rheological (MR) fluid-based monotube dampers, a sensor set, and an on-board Electronic Control Unit (ECU).

MR fluid is a suspension of magnetically soft particles in a synthetic hydrocarbon-based fluid. When the coil in the piston, through which the MR fluid flows,

is not energized, the fluid behaves like conventional damper fluid. However, when the coil is energized, the magnetic field causes the particles to align into fibrous structures in the direction of the magnetic flux. The strength of the bond between the particles in the structures is proportional to the strength of the magnetic field. The result is a variable resistance to fluid flow within

the damper piston, which provides a variable damping capability. Fine-tuning the current supplied to the coil in the

damper piston allows the generation of a wide range of damping force. Changes in the damping force occur

nearly instantaneously; the result is continuously variable, real-time damping.