Rover's Vikingship       On The Level       Rover's Vikingship

Y ou don't need to have studied the motor car for a lifetime or to have acquired an honours degree in engineering to be correct in assuming that the softer you make the springs of a car the better its ride is likely to become. Nor do you need to be very bright to realise that there is a limit to the spring softness possible if the pride and joy of your factory is not to have certain embarrassing characteristics like heeling to one side when carrying nothing more than a large, heavy driver, or scraping its bottom along the ground when fully loaded with passengers and luggage.

If you are the constructor of relatively small, relatively cheap cars, you have little alternative but to remain content with fairly stiff springs. The maker of relatively large, relatively expensive cars can afford, on the other hand, to consider more sophisticated solutions to the problem, such as the adoption of a selflevelling springing system.

In such a system the springs are not directly connected to the suspension links, but instead are operated through tubes of hydraulic liquid. Liquids, as you will remember from your school days, are to all intents and purposes incompressible, and so form admirable low friction pushrods for tasks of this sort hence their use in braking systems.

But for this application the basic advantage of such hydraulic pushrods or rams is the ease with which they can be varied in length simply by removing some liquid or squirting in more under pressure.

This is what happens in a selflevelling springing system:
The lengths of the hydraulic rams interposed between the springs and the body are continuously and automatically adjusted to compensate for spring deflection under load and maintain a constant ride height or ground clearance.

Achieving all this generally calls for an engine driven pump (1), some height sensing valves which measure the distance between the body and the suspension links and some fairly complex plumbing to distribute the hydraulic fluid. Usually, there are two separate circuits. One for the front of the car and another for the rear, but in all cases the adjustment rate must be slow, otherwise the system would fight normal suspension movement over bumps in the road surface; there is therefore no compensation for dive under braking or squat under acceleration. For similar reasons there must be only one height sensor for each axle so that no attempt is made to counteract roll. It is certainly possible to devise a much more advanced selflevelling arrangement which will compensate both for roll and for dive and squat the Automotive Products Active Ride Control system does so but that's another story.

Another feature generally, but not always, associated with a selflevelling system is the use of gas rather than steel as the springing medium.
If you don't think a gas is springy, try depressing the plunger of a bicycle pump when your thumb's over the business end.
Usually a small steel spherical container, sealed by a flexible diaphragm and containing nitrogen at 400--800 psi, a gas spring is easily integrated into the hydraulic system. And when acting at a mechanical disadvantage or un-favourable leverage so that it is required to exert large forces through small displacements, it also becomes very small and compact, for reasons I won't go into here.
Nor have I space to go into the further characteristics of the gas spring: its increasing stiffness with increasing load-which is good and its greater stiffness for sudden deflections than for slow ones which is bad.

But I do wish to draw your attention to the very real advantages conferred by a properly designed selflevelling system in-corporating gas springs. Citroen, of course, are the acknowledged pioneers and masters of such systems, but for example I choose the obsolete DS rather than any car currently in their range. I do so because I recall the ride of the DS as being quite outstandingly, quite radically superior to that of any contemporary model, except when caught out on hump back bridges and the like.
But since the heyday of the DS, two things have happened: there has been a tremendous improvement in the ride of conventionally sprung cars, and Citroen have increased both the spring and the roll stiffness of their hydropneumatically suspended cars to improve handling by limiting roll in corners.
Of course, if they adopted modern suspension linkages which keep the outside loaded wheels more upright in bends, they wouldn't need to make such sacrifices, but that's yet another story.
For the less compromised design of the DS, though, the facts and figures are as follows:
Total front-wheel suspension travel: 6.5in.
Total rear-wheel suspension travel: 8.7in.

Suspension travel.

Front laden
Front unladen
Rear Laden
Rear unladen
Static deflection
Frequency (cycles per minute)
Load lb.

To put these figures into per-spective remember that there are two interrelated ways of defining the softness or stiffness of a car's ride. One is by a quantity known as the static deflection, which is simply the deflection of the spring under the static load it has to support; the greater the static deflection, therefore, the softer the ride.
This is a theoretical rather than a practical quantity which can't necessarily be measured by jacking up a car to take the load off its wheels. One reason, as the DS figures show, is that the static deflection may exceed the total suspension travel.
Few cars have springs anything like as soft as those of the DS, though usually the static deflection is around threequarters of the total suspension travel and even for the Jaguar XJ 4.2, one of the best riding cars of today, the static deflection at the front is only 11in. for a total suspension movement of 6.9in.
Another measure of ride softness is the frequency of oscillation of the car on its springs: the lower the frequency the better road surface roughnesses are attenuated. Mathematicallyminded readers may like to know that the static deflection, d, in inches is related to the frequency, f, in cpm (cycles per minute) by the formula d = (188/f)2.).
Note that it's usual and useful to consider the masses and springs at each end of the car as being independent with separately calculable frequencies and that a car with frequencies as low as 60 cpm will be very comfortable indeed. This with suspension frequencies of around 40 cpm for a reasonable partly laden condition and static deflections ranging from 13.8in to 29.1in, it's hardly surprising that the DS had a magic carpet ride.

Many modern car designers, however, question the value of a full selflevelling system like Citroen's. Most of the load variation due to the addition of passengers and luggage, they point out, takes place at one end of a car only: at the rear of a front engined car and at the front of a rear-engined car. Then there is the expense and complexity of an engine driven pump plus accumulators, valves and plumbing.

But an enterprising designer can obtain the best of both worlds by fitting a sophisticated form of suspension strut with all the "selfs": it is selfcontained (no pumps or plumbing), self energising and selflevelling. It doesn't really generate its own energy, of course but, more ingeniously, uses the free energy put into it by the bumps in the road. Most of the big shock absorber companies have experimented with such self-levelling struts, but the only one currently in production is made by Boge (pronounced "Bowger"). a German company so far virtually unknown in Britain except for this speciality, but in fact one of Europe's biggest manufacturers of dampers and related components.
Boge selflevelling struts have been used from time to time by Mercedes and BMW, but the first British car to be fitted with one was the Range Rover. This has normal coil springs and hydraulic dampers at the ends of its live rear axle, but in addition a centrally mounted and angled Boge Hydromat unit (2) which has no damping effect but maintains the axle at a constant distance from the chassis.

Rear axle from the Range Rover
If this unit were merely a hydraulic ram, it would short circuit the coil springs, but it contains an additional gas spring of its own. Thus it can be regarded as being a kind of helper spring, combined with a hydraulic strut varying in length which is self pumped by ordinary suspen-sion movements.
The live rear axle of the new Rover 3500 which was designed to provide the utmost comfort by the simplest means is also selflevelled but by two Boge units which also provide damping.

Each of these Nivomat units (3) essentially consists of a pump combined with a conventional hydraulic damper. Like all dampers it has an operating cylinder filled with hydraulic fluid and containing a piston which is displaced by suspension movements. Orifices in the piston pro-vide the damping effect, and can be controlled, if necessary, by valves which open at high velocities to allow the characteristics to be tailored in the usual way. And like conventional dampers the Nivomat's operating cylinder is surrounded by an annular equalising space, partly filled with low pressure gas but one which encircles the upper part of the unit only into which the hydraulic fluid can be displaced by the gradually increasing protrusion of the piston rod volume as it is forced into the cylinder.

But the unit differs significantly from an ordinary damper in having a lower annular chamber filled partly with oil and partly by a gas spring contained within a flexible rubber diaphragm. In addition, the piston rod is hollow, and forms a pump in conjunction with a fixed shaft attached to the upper endplate and some valves.
The downward stroke of the piston rod draws oil from the upper annular space into the operating cylinder and lower annular space, raising the pressure in both these parts of the unit, compressing the gas spring and exerting a force on the lower end of the hollow piston rod tend to raise the car.
Upward movement of the piston rod shuts an inlet valve to maintain the pressure. Successive strokes continue the process, forcing the piston rod down until it reaches a mean position at which a control aperture in the pump shaft is exposed. The oil drawn in on the down stroke is then taken from the high pressure area, the operating cylinder and lower chamber and returned to it on the up stroke so that pumping action ceases.

One objection that has been raised against a unit of this kind is the time it takes to pump the body up to the correct level. If, for example, you're unlucky enough to start off along a surface of near billiard table smoothness when heavily loaded, you may have to cover 2000 metres,
Boge say, before the body reaches its correct level. But at the same time there is little risk of bottoming or suspension damage on very smooth surfaces, and the rougher the road the more rapid the pumping action, so the unit is in this respect once more self compensating. Normally the body reaches its correct height within about 200 metres.
The action of the pump has the desirable characteristic of increasing the damping forces at a rate which rises with the load. On rough roads, too, the pump will tend to deliver more oil than can escape through the control orifice, raising the body a few millimetres above the preset level another useful feature.

Like all gas springs, the one contained within the Nivomat gets rapidly stiffer with load, but for the Rover 3500 the tendency is diluted, as it were. By the constant rate of the coil springs giving a rear suspension system with a frequency which remains nearly independent of the weight carried, ranging from 80 cpm with the driver only, to 84 cpm when fully laden. Although the frequency of the front suspension is 66 cpm well within the range appropriate to a more sophisticated luxury car. The Rover's ride doesn't compare with that of the old Citroen DS. But is certainly very good indeed and achieved at far less cost and with far less complexity.

THANKS TO: Anthony Curtis and Shirley Rimmer.


How the Nivomat works.

1. Downward stroke draws oil from annular low-pressure chamber A, through passage B, hollow pump shaft C and inlet valve D into hollow piston rod E.
Oil is also transferred from lower annular chamber through damping orifices to upper part of operating cylinder.

2. On upward stroke inlet valve closes and oil is forced between the piston rod and pump shaft, through a pressure relief valve into the operating cylinder F,
passing through the damping orifices G and the ports H,
Into the lower annular chamber I, compressing the gas spring J. 3.

When the car reaches Its preset level the high pressure oil escapes through the control port K,
though on rough surfaces the rate of delivery exceeds the leakage rate and the car rides a little Range Rover.

It is higher as shown. The gas spring is differential housing annular in form, but is shown compressed one side, expanded the other


The Monroe Replacement set.
© rwp nov 2004.