When a car turns a corner, a centrifugal force arises, which tends to tilt the car or, as a last resort, knock it over. The corresponding formulas for calculating these forces are given in the appendix. The amount of roll depends on the magnitude of centrifugal forces and the distance between the point of application of centrifugal forces (i.e., the center of gravity of the car) and the metacenter of the car, i.e., on the magnitude of the overturning moment of the car.

A car with an elastic suspension rolls relative to the metacenter, the position of which depends on the way the wheels are connected to the sprung mass of the vehicle. Figure 1 shows how to determine the metacenter position for the most typical wheel installation patterns.

Rice. 1. Determination of the metacenter using various methods
wheel mounts

In the first figure we are talking about a short swinging axis, the center of swing of which is designated S 1. The coordinates of the metacenter are determined as follows: the point of contact of the tire with the ground is connected to the center of swing of the wheel axle shaft; the point of intersection of this line with the plane of symmetry of the car will give the position of its metacenter S.

The same is done in the second case, when the wheel is suspended on two wishbones of different lengths. The upper lever rotates around point S 1, and the lower one - relative to point S 2. On the continuation of the axes of these levers at the intersection point there is the actual instantaneous center of swing of the wheel S 3. By connecting it to the point of contact of the wheel with the road, find the metacenter S at a height h 2 above the ground at the point of intersection of this straight line with the plane of symmetry of the car.

The instantaneous center of wheel swing when using MacPherson suspension is found as follows: draw a perpendicular to the axis of the telescopic elastic suspension element at the upper point of its attachment and extend the axis of the lower arm swinging relative to point S1. The actual instantaneous center of the wheel's swing is located at their intersection, i.e., at point S 2 ; the position of the metacenter S is determined by the method already described: it is located at a height h 3.

When turning, the centrifugal force is applied at the center of gravity of the car, and the closer in height the center of gravity is located to the metacenter, the smaller the overturning moment. An example of a shortened swing axle of a car is shown in Fig. 2.

The distance from the center of gravity T to the metacenter S in this case is equal to t, the magnitude of the overturning moment is equal to Ot, where O is the centrifugal force of the sprung mass.

This moment must be perceived and extinguished, in which the so-called return moment arises. Its value in this case is equal to 2h "ca", where h" is the compression of the elastic suspension element; c is the stiffness of the suspension element.

Obviously, in this case the car roll will be small.

If the metacenter is located low, then the shoulder t will be large. Low rigidity of the elastic elements of the suspension also leads to an increase in vehicle roll.

To reduce the roll of the car, especially if it has a soft suspension, a stabilizer is installed on it. Torsion stabilizers are most often used (see Fig. 3).

Stabilizer 1 also has a torsion bar. To adjust the load, one of the upper arms 2 has an adjustable length.

This is a special torsion spring installed across the car and connected by levers to the wheels. If both wheels hit an obstacle at the same time, the stabilizer will rotate, but will not twist. If one wheel hits an obstacle, the stabilizer, twisting, tends to lift the other wheel. When a car turns, the elastic element of the suspension of the inner wheel (in relation to the turn) is compressed, the stabilizer tends to compress the elastic element of the suspension of the outer wheel (towards the turn), thereby preventing excessive roll of the car. By twisting, the stabilizer more strongly compresses the outer (toward the turn) elastic element of the suspension, while the inner (toward the turn) is unloaded.

There are many different ways to stabilize a car. When using hydraulic or pneumatic suspension, you can install the simplest stabilizer - a transverse leaf spring, which is mounted in two rubber blocks, as shown in Fig. 4.

Rice. 4. Front axle of a Fiat car with a transverse leaf spring installed in two rubber blocks and serving as a stabilizer

When one wheel is lifted, the spring will bend, its center will move down, and the end of the spring on the other side will move up.

A rear-engined car has shortened swing axle shafts at the rear, and the front wheels are mounted on two wishbones. According to Fig. 1 in the first figure, the height of the metacenter h 1 is large, and that of the front axle in the second figure is small h 2. If we consider the car as a rigid whole, then its roll will be limited mainly by the rear axle, which is manifested by an increased load on the outer rear wheel. Since the stabilizer redistributes the loads on the wheels to some extent, and increases, and the car acquires some oversteer. If the stabilizer is installed on the front axle, the value of the return moment (Nm/°) and the vehicle's stability against roll will increase. This will increase its load and lateral drift, as a result of which the car’s oversteer may change to understeer.

To more accurately calculate the lateral stability of a car, it is necessary to take into account the torsional elasticity of the body. Both axles are connected by one torsion spring. It is necessary that the body has sufficient torsional rigidity and does not act as some kind of elastic, undamped element that affects the handling of the car. The torsional rigidity of the body is expressed by a moment Nm, which causes a relative rotation of 1° of two planes of the body, 1 m apart from each other. The body rigidities of some cars are given in Table 7.

Table 7. Car body rigidity

Options	Car models
	Simka 1000	Tatra 603	Wartburg	Mercedes Benz 220 SE
Front wheel track (mm)	1250	1403	1190	1470
Rear wheel track (mm)	1234	1400	1260	1485
Wheelbase (mm)	2220	2750	2450	2750
Engine displacement (cm³)	944	2472	1000	2195
Gross vehicle weight (kg)	1040	1960	1300	450	880	625	590	1080	675	970
Load force (N)	4000	6000	4000	6000
Load moment (Nm)	4000	4000	2000	3000
Maximum deflection (mm)	1,08	0,52	0,64	0,67
Maximum torsion (°)	-	0°9.5"	0°13.4"	0°8.28"
Bending stiffness (N/mm)	4820	11500	6000	13320
Torsional rigidity (Nm/°)	-	25300	8950	21700

Car Simka 1000 from table 7

By car roll, we usually mean its tilt relative to its axis in any direction. Moreover, such a tilt can be not only to the right, but also to the left. The roll of the car can also be present both in front and behind, and can also be combined depending on the load or sagging of one of the wheels.

How can the car roll? Types of roll

It is important to consider that the car’s roll can be either permanent or temporary. But in each case, you should be careful about this phenomenon, because the presence of even a small deviation from the norm significantly reduces the level of safe and comfortable driving and can cause an accident on the road.

Let's start with a temporary phenomenon. It can often be observed on trucks when the body is unevenly loaded. In such cases, the likelihood that the vehicle will roll over increases significantly. In this case, such situations can arise not only when driving on uneven roads (especially on the side of an incline), but also when performing maneuvers on turns (especially when driving at high speed). The fix is ​​very simple - just correctly distribute the load over the body - this reduces the risk of an accident, and also significantly reduces the load on individual parts and components of the car.

Constant roll may also vary. If, for example, a car owner deliberately raises the rear of the car a little higher than the front, thereby increasing the stability of the vehicle during high-speed turns, that’s one thing. In the same vein, we can note a slight rise in the front part, which improves the controllability of the car even in extreme situations (for example, driving on a slippery or uneven road).

An artificial roll can also be practiced if a sufficiently obese person is driving a passenger car. In this case, to maintain balance while driving, you can slightly raise the driver's side.

It’s worse if the roll is the cause of long-term use and wear, or poor quality work on assembling and fastening one of the wheel or suspension units. In this case, the wear of parts and assemblies located in the area of ​​greatest load (in fact, at the lowest point) increases significantly.

It is important to understand that driving comfort and safety in such cases remain in question (often a car with such an “illness” simply begins to “drive” in the direction of the car’s tilt, and at high speeds the likelihood of an accident increases significantly).

No matter how this happens, whether you did the roll deliberately, or whether it arose due to wear of the components, you can be sure of one thing, the wear of rubber on the wheels located in the lower part will be significantly higher. Therefore, the practice of deflecting a vehicle from the normal axis must be done wisely and preferably temporarily. Otherwise, “show-offs” will ultimately become very real troubles in the form of a damaged car, or significant costs for replacing individual parts that have failed prematurely.

In the automotive world, certain ideas have long been formed regarding the use of one or another type of suspension: double wishbone - for sports models, dependent - for SUVs, semi-independent - for compact cars... But what are the reasons for these ideas, and are they even true?

In the suspension of a car, three groups of elements can be distinguished: guides - levers, elastic - springs and stabilizers, and damping - shock absorbers. The last two, that is, stabilizers, springs and shock absorbers, are the cornerstone of most debates about car performance. And this is largely true, because the listed details determine such tangible and important parameters as smoothness, rollability and handling characteristics. The design of the suspension - the geometry of the levers - often remains in the shadows, although in terms of its significance and influence on the behavior of the car it is in no way inferior to other factors.

So what determines suspension design? First of all, it sets the trajectory of the wheel during compression and rebound. Ideally, this trajectory should be such that the wheel always remains perpendicular to the road, so that the contact area of ​​the tire with the surface is maximum. However, as we will see later, this is rarely achieved: usually, during the compression of the suspension, the wheels change camber, and when turning, they tilt to the side along with the heeling body. And the greater their deviation from the vertical, the smaller the tire contact patch. Thus, the stability of the car and the level of its grip on the road are parameters entirely determined by the design of the suspension.

Camber and Toe

The two main parameters of the suspension are camber and toe. Camber is the inclination of the wheel plane to the perpendicular restored to the plane of the road. If the top of the wheel is tilted outward of the car, then the camber angle is considered positive, if inward - negative. Toe is the angle between the direction of movement and the plane of rotation of the wheel. It can be measured in both degrees and millimeters. In the latter case, toe is understood as the difference in distances between the leading edges of the disks and the rear ones.

In a similar way, the geometry of the levers affects controllability, only here the instability of the wheel alignment affects it. It is not difficult to imagine the consequences - the car begins to yaw on uneven surfaces, and when turning, a tendency to oversteer or understeer appears. However, this phenomenon can be used for good, compensating, for example, for the tendency to drift in front-wheel drive models.

As a rule, the car's track also turns out to be unstable - even a small suspension travel can lead to a change in it by a couple of centimeters. All this, of course, leads to an increase in driving resistance, and ultimately to an increase in fuel consumption and accelerated tire wear. But what is much more dangerous is the fact that this reduces the stability of straight-line motion, because the traction properties of the tires are “spent” not on holding the car, but on resistance to the wheels diverging to the sides.

Against rolls

Along with the lateral roll center, the suspension design also sets the longitudinal roll center - the point around which the body tilts during braking or acceleration. And at a certain position of this point, the suspension can prevent the increase in roll, pushing or pressing the body in the right places. However, not all pendants have such capabilities. The most effective in this regard are suspensions on oblique levers, double levers and multi-link. They allow you to place the roll centers exactly where you need them. McPherson's capabilities are more modest - its range of adjustments is narrower. But the suspension on the trailing arms does not need adjustments - the center of the longitudinal roll is already located in the optimal place. Dependent and semi-independent suspensions do not allow you to fight roll - their roll center is at infinity.

The design of the suspension also affects the smoothness of the ride. Firstly, by the size of the unsprung masses, which includes the mass of all the levers (although not completely, since they are attached to the body at one end), and secondly, by their internal friction. The fact is that many modern suspensions, especially multi-link ones, have the ability to move only due to the deformation of rubber-metal hinges and silent blocks used to attach the levers. Replace them with hard bearings - and the suspension will petrify, lose the ability to move, since each of the levers describes a circle around its attachment point, and these circles intersect at a maximum of two points. By using rubber-metal hinges (with varying stiffness in different directions), it is possible to achieve a more complex kinematics of the levers and still provide suspension travel, although at the same time increasing friction. And the higher it is, the worse the filtration of irregularities.

But what is more surprising is the effect of the suspension on the level of car roll. Please note that we are not talking about springs and shock absorbers, but rather about the layout of the levers! It turns out that their design sets the center of the lateral roll. Simply put, the point around which the body rolls. Usually it is located below the center of gravity - the point of application of the inertial force, and therefore the car leans outward when turning. However, by changing the location and angle of the levers, the roll center can be increased, reducing or even completely eliminating body lean. If this point is above the center of gravity, then the roll will appear again, but in the opposite direction - into the turn, like a motorcycle! This is in theory, but in practice, attempts to increase the roll center are accompanied by a number of problems, such as changing the track too much, and therefore we are only talking about a slight reduction in roll, but it is certainly worth it.

Thus, designing a suspension is a responsible and difficult task, and its implementation is always a search for a compromise. We will look at what solutions this search leads to in the next issue.

Author Edition Autopanorama No. 3 2016 Ways to prevent a car from rolling over off-road

“Ears”, “somersault”, “flip-mortale”, “overkill”... How many names does such a simple and, unfortunately, common off-road phenomenon as capsizing have? And the more serious the preparation of the car, the more chances the pilot has to earn the title of “Carlson who lay on the roof.” Traditional methods of combating rollovers are well known. But are they effective (and if so, how effective)? In general, you already understand that we decided to try to deal with this issue to the best of our ability. As they say, for the benefit of the prosperity of jeeping, and out of excessively developed natural curiosity, of course...

An expedition Toyota Land Cruiser 105 with a 1KZ engine was used as a “falling rabbit” for our unusual test. The choice is due to the fact that this car, with all its ceremonial, glossy appearance, at one time underwent quite serious off-road training, and accordingly, its center of mass “galloped” far upward. This is due to wheels with a diameter of 35 inches, a 3-inch lift, and even a 7-centimeter body lift. The result was a version of a typical car used by lovers of short and long trips to places not covered with asphalt. Who said: “What about Land Rover”? No, let's agree: today we are not arguing about who is more capable and expeditionary, but we are only talking about ways to prevent capsizing. In general, the introductory steps are as follows: there is a lifted TLC105 (but, I repeat, in this case the brand and model are not important), there is a platform for turning the car over, there is a lot of enthusiasm and a couple of ropes. Which means you can start!

Exotic observation technique

As a starting point, we turned over the car in its, so to speak, untouched form. That is, the cabin and cargo compartment are empty, and there is nothing on top of the expedition trunk either. This will be something like a “stove” from which we will have to “dance” in an attempt to draw some logical conclusions. Actually, it should be said that all rollover tests look pretty much the same. First, the car is placed on a platform with the wheels of one side resting on a special limit rail, then the limiter belts are secured to the body. Then, at the touch of a red button, the powerful hydraulic system begins to tilt the platform. At this moment we just wait, enjoying the spectacle. But, it must be said that at first there is nothing special to enjoy: the car just stands firmly with its wheels on the surface. But when the angles reach about 25-30 degrees, interesting things start to happen. At first, the body rolls reluctantly (suspension strokes are being worked out).

Then, if it is an SUV with a dependent suspension and a heavy engine, the front wheel usually begins to lift off the platform. This is the so-called “first bell”, indicating... no, not the beginning of a rollover, but only that the rebound stroke of the front suspension has ended. But nevertheless, a moment of extreme tension comes. How many times have I seen this, but I still can’t get used to it... And then the car finally tore the wheels off the surface of the platform, swung sharply towards the roll and hung helplessly on the belts... This is the rollover point. The time has come for measurements and recordings. And this time we recorded the following numbers: 42°13’ – platform roll and 48°35’ – body roll. That is, the relative body roll was 6°22’.

Yes... The indicators, to put it mildly, are not record-breaking. No, this seems to be normal for a lifted car, but it is completely unacceptable, for example, for high-speed maneuvers on hard surfaces. By the way, having overturned the car on the other side (keeping in mind the Panhard rod, which gives asymmetrical operation of the suspension), we got slightly different results: the empty car fell onto the left side already at an angle of 41°19’, and the roll was 6°45’. We will carry out all further experiments with a tilt on the right, passenger side, but remember that when left, all “left-hand drive” cars with a similar type of suspension turn over about one degree earlier in static conditions. By the way, in dynamics the difference will be even more noticeable.

Brave "Vasi"

The next stage of our experiments was to simulate the actual loading of a car under conditions of an expedition or trophy raid. Let's try the maximum first. We guessed it was four people, with about 100kg in the cargo area and another 100kg or so in the roof rack. Measuring bags of sand (25 kg each) worked as “kilograms”. Four weight and size mannequins filled with water with the original Russian names Vasily were “seated” on the chairs. Humans are also almost 90 percent water, so they are almost like brothers to us. Therefore, it was not difficult for the author to imagine himself as a mannequin. So, read the fantasy on the theme “What if we were sitting inside”... To the sounds of imaginary marches, the back row was “filled” with two volunteers, one of whom is now writing these lines. Well, shall we get started?

Oh, and what a feeling... It is known that rolls inside a car are perceived much stronger than they actually should be. The vestibular apparatus is such a reinsurer, don’t tell me... I remember, I once started in a trial... So, wait, what is the angle now? How is it only 30 degrees?!! I can barely stay in the car, but it still stands still! And from above, Andrei Kuprin looks appraisingly (I don’t know why, but I wanted to include this character in my story). Well, Andrei sat on the left and, it seemed, was quite consciously going to fall on me... The car is still standing, and he is holding on...

Well, finally... 36°31’, and the wheels came off the floor. And the body roll at the moment of lift-off is more than 10 degrees! These are the indicators... If we had really sat inside, we would hardly have been able to hold out. But the car “fell” extremely early, at the same time taking out the entire suspension travel.

Okay, now we try without the “plastic people” sitting in the back, but with the “crew” of two “Vasilievs” in the driver’s and co-driver’s seats. Yes, and, of course, with a load. The rollover angle immediately jumped to 39°08’ with a roll of 7°03’. That is, with a standard load of “2 people plus cargo,” we have a decrease in stability by 3 degrees. Quite a lot. But we will take this value as the starting point for all further torment as the closest to reality.

Folk signs

I admit, despite the fact that I am a person of the material world, I still believe in some signs. There is a sin. Moreover, rumor says about their exceptional, almost one hundred percent “salability”. Actually, what am I talking about? Oh yes, about popular methods of fighting the coup. The first method is this: if you do not want to roll over, reduce the pressure in the tires on the upper side of the slope. The machine will level out, and the chances of overkill will decrease to microscopic. Shall we check? Toyota hisses air through the inverted valves and is preparing to demonstrate the wonders of stability. The pressure in the left wheels is 0.6 atm, and the body roll on a flat surface is almost 4 degrees.

We press the button, and the platform slowly pushes the car towards the irreparable. And here we see an interesting picture. After working out the suspension moves, the wheels begin to... “inflate”. That is, the load on the side changes, and flat tires no longer affect anything! And indeed, we recorded a complete loss of stability at 38° 35’. The picture was as follows: with flat tires, the car fell earlier than with inflated ones, by more than half a degree. Maybe not by much, but sooner! That is, we are simply not talking about improving stability in this case. So, we cross out one “correct” way...

Next method. A navigator hanging on the step (jeepers picked this up from yachtsmen). The method is said to be quite effective. But we, according to the laws of the genre, doubt it. And we will doubt it while the compressor “pumps highway pressure” into the tires of the experimental Land Cruiser. Well, when he finishes... In general, I stand on the inexorably moving upward bandwagon and feel sad. Jokes and jokes about the heavy lot of an ORD columnist fly by without bouncing... The platform rotates with a characteristic roar, the car beneath me goes down, and the world turns upside down. No, gentlemen, honestly, I took such an unnatural pose only for the sake of the purity of the experiment, in order to tilt the car to the maximum.

And, you know, all this torment was not in vain: working with a living counterweight had an effect! As a result, the angle increased to 40° 14’! We lower the platform a little, and another test participant jumps onto the force threshold. Now there are two of us, but this measure increases the angle to only 40° 54’, that is, by less than a degree. From which we conclude: carrying two navigators for ballast is wasteful. But in any case, we must admit that the method works. Because returning one and a half degrees of stability to a car at critical angles is, to put it mildly, a lot. Let’s summarize: the effectiveness of “human rejection” is quite high.

Now let's take a look at the overseas expanses, where restless spotters climb over rocks, periodically keeping cars from turning over with muscular, so to speak, force. Moreover, they often succeed... So, we need a thick rope and a dynamometer. We tie our “rope” to the expedition trunk, attach a dynamometer to it and... In general, I stand, holding the rope in my hands, and wait for the moment when I need to show the miracles of heroic strength. And he showed it! With an effort of 50 kg, I “saved” as much as 1° 34’ of steady state, and when I pushed myself and “took the weight” of 100 kg, it turned out to be as much as 3° 40’. Well, aren't I great? Frankly, they helped me lift 100 kg (we were already lifting together), but the result was positive in any case. Conclusion: the method of pulling a car with a cable is alive! At least of those tried, it is the most effective.

In fact, of the folk remedies, only the “barbaric” method remains. We are talking about artificially limiting the suspension travel on the upper side of the slope. No sooner said than done. And now I dive under the car and use banal tie-down straps with a “ratchet” to compress the springs. But since after the exercises with the rope I didn’t have much strength left, I managed to achieve a roll angle “towards the slope” of only 2 degrees. However, this is quite enough for the experiment... The grumbling of the hydraulic drives of the stand this time pleased us for quite a long time, but the difference was completely invisible to the eye. But all is not lost. We take measurements... No, a miracle, of course, did not happen, but with this simple action we achieved the same result as with two “live counterweights”! The numbers appeared in the notebook: 40°49’ with a body roll of 4°46’. Very good result. Of course, not as in the “with rope” option, but also quite acceptable. Well, three out of four methods is a very good result. I would even say positive.

All for the same goal

And now attention: instead of drawing conclusions and smearing thoughts in pseudoscientific expressions, we decided to do something simpler. What happens if we apply all the methods of combating rollovers that have yielded positive results as a united front? To the perplexed “how is this?” I answer: point one - complete unloading of the car, including dismantling the spare wheel, point two - three on the steps, point three - one person with a rope, ready to give a calibrated 50 kg of force. And you know, despite the laughter and jokes that poured out during the process of casting off the platform from the horizontal “pier,” we held on, as they say, until the last. Apparently, they didn’t hold on in vain: the result was 52 degrees!!! At such angles, small crossovers turn over, and here is a frame-lifted SUV!

That is, we were able to add as much as 13 degrees to the maximum angle of stable position of a car equipped with a full arsenal of means to... worsen this parameter. So traditional methods work, and how they work! Just don't try to let the tires go down.

  A new way to combat roll
The money for the car has already been spent, and you have finally moved to the stage of active motorism - you have started driving. In addition to the feeling of comfort that a good car will give you right away, over time it will give you a more important feeling - a feeling of security. Reliability. Confidence. What does it consist of? You know that there is an anti-lock brake system, and the car will no longer skid during sudden braking. There is a traction control system - it will allow you to move off without any problems on any surface. There is a convenient and simple automatic transmission, and the steering wheel turns easily because it is equipped with a hydraulic booster. Then on the list are other advances: four-wheel steering (Honda does it) and all-wheel drive (Audi was the first to install it on a production passenger car). Add hydropneumatic suspension like Citroen. And also, perhaps, air conditioning and heated seats - all this is a completely achievable dream of an ordinary motorist.
Until recently, perhaps only one inconvenience remained unresolved: the lateral roll of the car that occurs when cornering. The feeling that this gives passengers is unambiguous - a tilting car is unreliable. Indeed, the behavior of the car in this case is unpredictable and difficult to control.

What happens to a car when turning? When moving along a curve, as is known, centrifugal force arises. It tends to push the car out of the turn, which is prevented only by the reaction at the point of contact of the wheels with the road (in cases where the centrifugal force exceeds the adhesion force of the tires to the surface, the car skids).
The wheels of a car, rising and falling on uneven roads, make rather complex vertical and lateral evolutions. If we consider the movements of the point that is located in the center of the contact patch of the wheel with the road, then in the suspension you can find a certain center relative to which these movements occur along an arc of a circle. This is called the suspension roll center. The straight line connecting the roll centers of the front and rear suspension is called the vehicle's roll axis.
The centrifugal force arising during a turn acts laterally on the center of gravity, or, more correctly, the center of mass of the car body. It is located about half a meter above the ground, but always above the roll axis. The lateral force applied to the center of mass creates a tipping moment relative to this axis, which tilts the body during a turn or swings it from side to side during a series of turns.
Centrifugal force doesn't just tilt the car. It also affects passengers, throwing them from side to side and forcing them to grab handles in search of support. It would seem easier for the driver: the fulcrum - the steering wheel - is always at hand. However, he can instinctively hang onto it and involuntarily change the trajectory of the car.
Body roll doesn't just happen when cornering. It can also be caused by uncoordinated movement of wheels on one axle, for example, if one of them falls into a hole or onto a bump. The suspension does not have time to work, and one side of the car bounces slightly. If the road is very uneven, the wheels each dance on their own (a phenomenon called “shimmy” - from shimmy, there was once such a dance). The car body sways from side to side, and it is clear that the trajectory of its movement is not stable.
One of the main ways to reduce roll is to equip the suspension with an anti-roll bar. As a rule, it is a curved rod of complex shape attached to the body, which connects opposite suspension arms. The stabilizer bar does not prevent the wheels from rising and falling together, but as soon as one of them hits, for example, a bump and begins to rise separately from the other, it twists (hence the name of the bar - torsion bar) and prevents the wheel from lifting, which would lead to body swaying .
The installation of such a stabilizer, although it gives the car resistance to rolling, has its drawbacks. Connecting the suspension arms to each other makes it not as independent as the name suggests. Since the rod is an elastic element, it vibrates at its own frequency, which disrupts the operation of the suspension. And in very sharp turns, such a stabilizer is even harmful - it additionally transfers the load from the inner wheel to the outer one - the outer tire is literally smeared on the road, while the inner one is about to come off it.
Is it possible for a car not to tilt at all when turning? Theoretically, yes. For example, if you lower the center of mass of the body to the roll axis, like in Formula 1 cars, which do not roll when cornering. But for ordinary passenger cars, this method is not suitable for obvious reasons.

Last year, Citroen came up with a rather elegant technical solution to the problem of stabilizing body roll. The method is based on the unique properties of hydropneumatic suspension, which was first used on the experimental Citroen DS back in 1955, has since been significantly improved and is now widely used in cars of this company.
The elastic element in the Citroen hydropneumatic suspension ("Autopilot" #3), as is known, is a gas with which small spheres are filled. The load on the gas is transferred through the membrane to the fluid in the hydraulic system.
In early versions of the design, where there was only one sphere for each wheel, by changing the amount of fluid in the system it was possible to regulate only the ground clearance and the position of the car body depending on the load. Then (in the Hydractive suspension) additional spheres were installed, and the control was entrusted to the computer - it became possible to change the stiffness of the suspension. The next option is the Hydractive II suspension with a modified control algorithm.
This suspension, equipped with a rather complex system of sensors and a computer, monitors factors (crosswind, bumps, potholes) that tend to deflect the car from moving in a straight line. The car's speed, gas pedal position, steering angle and lateral acceleration are also taken into account. If there is an unfavorable combination of controlled parameters, the computer disconnects the additional sphere from the general circuit, increasing the stiffness of the suspension. Naturally, the stiffer the suspension, the less susceptible it is to roll, so a car with Hydractive or Hydractive II suspension, such as the Xantia VSX, is much more resistant to body roll than any other car.
Hydractive II works well, no doubt about it. But from the point of view of stabilizing lateral stability, this suspension, despite its name, behaves as passive - it only reacts to the lateral acceleration of the car that has already occurred. Naturally, with some delay.
Citroen specialists were not happy with this. In addition, it would have been a sin not to use the potential of the very idea of ​​hydropneumatic suspension. And a system of active stabilization of lateral stability of the car appeared, which received the ugly name SC.CAR. Since last fall, it has been installed on production Citroen Xantia Activa.
To be fair, it is worth noting that attempts to create an active stabilization system have been made before - such a system was first tested on the same experimental Citroen DS. But there were no computers then.
The Citroen Xantia Activa uses, with minor additions, the same suspension elements as in previous versions. But the system works differently. The first difference is that the electronics that control the suspension do not wait for lateral acceleration to occur, indicating that the car has already entered the turn. In Activa, the amount of lateral acceleration is predicted before turning, based on measurements of vehicle speed, angle and steering wheel speed - this increases the responsiveness of the system.
The car, as usual, is equipped with two - front and rear - torsion stabilizer bars. But only one end of each of them is rigidly attached to its suspension strut. The other is connected to the opposite post via a small hydraulic cylinder. The hydraulic cylinders are located diagonally, one on the left front pillar, the second on the right rear.
While the additional central sphere is connected to the general circuit and the suspension is in a “soft” state, the active stabilization system does not work - the hydraulic cylinders reduce the rigidity of the torsion bar and perform only damping functions, dampening its own vibrations.
If the combination of measured parameters indicates that the car has begun to turn, the computer turns off the additional central sphere. At the same time, as in the regular Hydractive II, the stiffness of the suspension increases. And the active lateral stabilization system is activated - along with the stiffness of the suspension, the rigidity of the hydraulic cylinders and, accordingly, the torsion bar increases, which begins to prevent body roll.
If a roll does occur, the sensor that measures it is triggered and an additional amount of fluid is supplied to the hydraulic cylinders - this turns them into a kind of jacks that forcefully level the body. The roll sensor is triggered when the body angle exceeds 1/2° - an amount so insignificant that it is not felt by either the eye or the stomach.
The result is that the Citroen Xantia Activa does not roll even during sharp turns, the wheels remain perpendicular to the road, and the car’s behavior is completely predictable. Probably, the premature expression “turns like it’s on rails” should actually refer to this car.

Alexander Pikulenko