Purpose, device and types of car suspensions

6.1.1 Purpose of the chassis 

There is a body and there are wheels (propellers, if in a scientific way). The question arises: how to connect the wheels to the car body, so that it is possible to drive a car, continuously transmit the traction from the engine to the drive wheels and at the same time comfortably overcome all the unevenness of the roads with various surfaces and without pavement? In this case, the connection of the wheels with the car body must be sufficiently rigid so that the car, while performing any maneuvers, simply does not turn over. The answer is simple - install the wheels on the intermediate link. A suspension performs a function of such a link.

Suspension elements should be as light as possible and provide maximum isolation from road noise. In addition, it should be mentioned that the suspension transmits the forces arising from the contact of the wheel with the road to the car body. Therefore, it is designed in such a way that it has increased strength and durability (see figure 6.1).

Figure 6.1 Forces on the wheel when it’s moving on the road.

Due to the high requirements for the suspension, each of its elements must be designed according to certain criteria, namely: the hinged joints used should easily rotate, but at the same time be sufficiently rigid and provide noise insulation for the car body; levers must transmit the forces arising from the operation of the suspension in all directions, as well as perceive the forces that arise during braking and acceleration; however, they should not be too heavy or expensive to manufacture.

6.1.2 Components

Any suspension, whatever it may be, must include the following elements:

• guides/ties (levers, rods);
• damping (shock absorbers);
• springing (springs, air bags, carriage springs, etc.).

We'll describe each of these elements below, so don't be alarmed.

6.1.3 Suspension classification

First, let's consider the classification of existing types of suspensions that are used in modern cars.

6.1.3.a Dependent and independent suspension

Thus, the suspension can be dependent and independent. In case of using a dependent suspension, the wheels of one axle of the car are connected. That is, when the right wheel is moved, it will start changing its position and the left one, as is clearly shown in Figure 6.2. In case if the suspension is independent, then each wheel is connected to the car separately (Figure 6.3).

6.1.3.b Double-link and multi-link suspension

Suspensions are also classified by the number and location of the levers. Thus, if there are two levers in the design, then the suspension is called double wishbone. In case if there are more than two levers, then the suspension is called multi-link. In case if two levers, for example, are located across the longitudinal axis of the car, then its name will appear with the addition - "with transverse levers". However, there are a lot of designs, because the levers can be located along the longitudinal axis of the car, then in the characteristics they will write: "with a longitudinal arrangement of levers". And if not so and not that way, but at a certain angle to the axis of the car, then one says that the suspension is with "oblique levers".

Interesting
It is impossible to say which of the suspensions is better or worse. It all depends on the purpose of the car. In case if it is a truck or the most brutal SUV, then dependent suspension will be irreplaceable for simplicity, rigidity and reliability of the structure. In case if it is a passenger car, the main qualities of which are comfort and handling, then there is nothing better than individually suspended wheels.

Figure 6.4 Example of spring suspension on two transverse levers.

6.1.3.c Damping element

Suspensions are also classified according to the type of damping element used - shock absorber. Shock absorbers can be telescopic (resemble a "telescope" fishing rod or a spyglass), as in all modern cars, or lever, which you cannot find now with all the desire.

6.1.3.d Type of applied elastic element

And the last sign, according to which suspensions are classified into different classes, is the type of springing element used. It can be a carriage spring, a coil spring, a torsion bar (it is a rod, one end of which is fixed and does not move in any way on the car body, and the other is connected to the suspension arm), pneumatic element (based on the ability of air to compress) or hydro-pneumatic element (when air "duets" with hydraulic fluid).

6.1.3.f Signs of differences in suspensions

So, let's summarize.

Suspensions can be distinguished according to the following features:

• design: dependent, independent;
• the number and arrangement of levers: single-lever, double-lever, multi-link, with transverse, longitudinal or oblique arrangement of levers;
• the type of damping element: with telescopic or lever shock absorber;
• the type of springing element: carriage spring, spring, torsion bar, pneumatic, and hydro-pneumatic.

In addition to all of the above, it should be mentioned that suspensions are also distinguished by controllability, that is, by the degree of controllability of the state of the suspension: active, semi-active and passive.

Note
Active suspensions include suspensions in which the stiffness of shock absorbers, ground clearance, stiffness of the stabilizer bar can be adjusted. The control of such a suspension can be either fully automatic or with the possibility of manual control. Semi-active - these are suspensions, the control possibilities of which are limited to adjusting the ground clearance height. Passive (inactive) are regular suspensions that perform their role in their pure form.

It is also worth mentioning suspensions with electronically controlled shock absorbers, that are able to change their stiffness depending on road conditions. These shock absorbers are filled not with ordinary, but with a special liquid, which, under the influence of an electric field, can change its viscosity. If you simplify the principle of operation, you get the following: when there is no current, the car very gently drives over all the irregularities. But once the current is applied over the irregularities, it will not be very pleasant to drive. It will become very pleasant to drive the car on highways and in turns.

6.1.4 Steering knuckle

The steering knuckle is the link between the suspension arms and the wheel. A schematic view of this part is shown in Figure 6.4. Generally, such part is called axle journal. However, in case if the axle journal is mounted on a steering wheel suspension, then it is called a steering knuckle. In case if the wheels are not steered, then the name "axle journal" remains.

The knuckle is steering, because it is turning. It also participates in the process of changing the direction of movement. It is to the steering knuckle that the steering linkage elements or steering rods are attached (these elements are described in detail in the "Steering" chapter). The steering knuckle is a massive part, because it absorbs all shocks and vibrations from the road.

The design of the steering knuckles depends on the type of vehicle drive. Thus, in case if the drive is combined (when the wheels are both steered and traction at the same time, which is typical for front-wheel drive cars), then the steering knuckle will have a through hole for the outer part of the drive shaft, as shown in Figure 6.4. In case if the wheels are only steerable, then the steering knuckle will have a support axle with a tapered section, as, for example, shown in Figure 6.7.

6.1.5 Wheel hub

The wheel hub (shown in Figure 6.4) is the link between the wheel and the steering knuckle/axle journal. The steering knuckle only transfers forces to the suspension elements, but it does not rotate itself. A hub is required in order to ensure free rotation of the wheel. A brake disc* (or brake drum) is installed on the hub. The wheel is attached to it, and the hub itself is installed in the steering knuckle in the case shown in Figure 6.4, on bearings that ensure smooth rotation of the wheel.

* The brake disc and brake drum are described in detail in the "Brake System" chapter.

Note
The brake disc can be structurally made as one piece with the wheel hub. Depending on the design, the hub bearings can be roller or ball bearings.

Good to know
Always after removing and installing a hub or replacing bearings, it is necessary to adjust the preload (which is, see the note below) of the hub bearings.

Note
In simple terms, the preload is the force with which the hub bearings are compressed while tightening the fastening nut. The amount of preload affects the force of resistance to wheel rotation. Each manufacturer provides its own recommendations regarding the value of the resistance to wheel rotation. Therefore, while performing repair works that are related to the removal of the hub, always ask whether or not the wheel hub bearing has been adjusted or not.

6.1.6 Guiding / connecting elements

The wheel is attached to the car body or sub frame using guides and connecting elements. These connecting elements are divided into levers and rods. The bar is a hollow profile, usually of a circular cross-section, less often of a square. In fact, it is just a tube with lugs that are welded to both ends for installing rubber bushings in them, with which it is attached to the car body and the steering knuckle or axle journal. Levers are structurally more complex elements. They can be welded from tubes (this design is used mainly in sports cars), cast, for example, from an aluminum alloy (to make them lighter) or stamped from sheet metal (to be cheaper). The number and location of levers affect the ride and handling of the car.

6.1.7 McPherson's suspension

McPherson strut (Figure 6.5) is perhaps one of the most common designs of suspensions at the present time. It is also called "candle" (the most striking example is the front suspension of the VAZ 2109 and the like). It is distinguished by its simplicity of design, cheapness, maintainability (which means that it will be easy to repair it) and relative comfort. The so-called McPherson strut is attached from above to the car body and has the ability to rotate in the support, and from below to the steering knuckle. The steering knuckle, in turn, is connected to the lower wishbone, which is connected to the car body - that's it, the ring is closed. Sometimes, in order to provide additional rigidity, a longitudinal thrust is installed in the structure, connecting it to the transverse lever (again, as an example, VAZ 2109). The strut has a lever, to which the track rod is attached. Thus, when driving a car, the entire shock absorber rotates, turning the wheel, without stopping to compress and stretch, overcoming the unevenness of the road surface. But you should also pay attention to the shortcomings of a single-link (and in the case described above, it is just a single-link) suspension. These are the "pecking" of the car when braking and a small energy consumption of the suspension.

Figure6.5 Suspension with McPherson strut.

6.1.8 Suspension breakdown

Note
"Pecking" means the following: in case of heavy braking, the weight of the car shifts towards the front end. Because of this, the front part sags, and after stopping abruptly returns to its original position. The energy content of the suspension is the strength of the entire structure, the ability to withstand all shocks and moments arising from these shocks without breakdowns. Suspension breakdown is a short circuit, contact of metal suspension elements with each other with a sharply increasing shock load. This usually happens when the wheel hits a road obstacle of impressive size. It declares itself with a characteristic ringing metallic sound from the support (or supports) of the suspension.

6.1.9 Suspension on two wishbones

In order to get rid of "pecks", improve handling and increase energy intensity, one of the oldest suspension structures is used, which has come down to our times with significant transformations - a suspension on two wishbones (an example of which is shown in Figure 6.6).

Figure 6.6 Front suspension on two transverse levers with McPherson strut.

I this design there are a support lever (lower) and a guide lever (upper) that are attached to the steering knuckle. The lower part of the shock absorber is installed on the support arm, or a separate spring and a separate shock absorber. The upper lever serves as a direction of the wheel movement in the vertical plane, minimizing its deviation from the vertical. The way the levers are positioned relative to each other has a direct impact on the behavior of the car while it is moving. Pay attention to Figure 6.6. Here the upper lever is maximally retracted from the lower lever upwards. It was necessary to lengthen the steering knuckle in order to reduce the impact of forces on the car body during suspension operation. In addition, this lever is installed at a certain angle to the horizontal axis of the vehicle in order to avoid the notorious "pecks". The essence remains the same, but the appearance, geometric and kinematic parameters change.

Note
There is still one very significant drawback in this design despite all the advantages. This is the deviation of the wheel from the vertical axis during suspension operation. There seems to be a solution - lengthening the levers. This is good in case if the car is frame-based. But in case if the body is load-bearing, then there is nowhere to lengthen - further is the engine compartment. Thus manufacturers approach the solution in a non-standard way: they try to make the lower lever as long as possible, and the upper one - set as far as possible from the lower one.

Note
It should be mentioned that in case if the spring and shock absorber or McPherson strut are attached with their lower end to the upper lever (as in the case shown in Figure 6.7), it is the upper lever that becomes the support lever, while the lower one goes into the category of guides.

Figure 6.7 Layout of suspension of 1968 Ford Mustang.

6.1.10 Multi-link suspension

Once the resources for the development of any one plan for solving a problem are exhausted, and the goals are not achieved, the design has to be complicated, despite the increase in cost. Designers took this path while developing a multi-link suspension. Yes, it turned out to be more expensive than a two- or single-lever suspension, but the result was almost perfect wheel movement - no deviations in the vertical plane, no steering effect when turning (more on that below) and stability.

6.1.11 Rear semi-independent suspension

Note
Almost all the layouts described above can be used in the rear suspension design.

This is one of the simplest, cheapest and most reliable rear suspension solutions. But is still has many disadvantages. The essence of the design is that two trailing arms, on which the springs and shock absorbers are supported, are connected by a beam, as shown in Figure 6.8. The suspension partly turned out to be dependent, because the wheels are interconnected. However, due to the properties of the beam, the wheels have the ability to move relative to each other.

Figure 6.8 Example of rear semi dependent suspension.

6.1.12 Damping elements

Damping elements are suspension elements that are capable of damping its vibrations while the car is moving. Why should these vibrations be damped? The springing suspension element, whatever it may be, is designed to negate all shock loads that arise when the wheel collides with obstacles on the road. But whether it is a spring or air in an air bag, after compression or expansion of the springing element, it will immediately return to its original position. Squeeze any spring in your hands, and then release it - it will fly as far as the forces that have arisen during the release will allow it. Another example: take an ordinary medical syringe, draw in clean air, clamp the outlet and try to move the piston. It will move, but until a certain point (until you have enough strength to compress the air), after releasing the rod, the air will start expanding, returning the piston to its original position. The same happens in a car: when hitting an obstacle, the spring in the suspension will compress. But then, under the influence of elastic forces, it will start expanding. Since the car has a certain mass, the spring, while straightening, will be forced to overcome its inertia, which will be expressed by swaying with a gradual damping of oscillations.

Due to the constant multidirectional movements of the suspension, such swaying is unacceptable, because at a certain moment resonance may occur, which ultimately will simply destroy the suspension partially or completely. Another element was introduced into the suspension design - a shock absorber – in order to prevent such fluctuations.

The principle of the shock absorber operation is simple. Let's try to explain this using the example of the same syringe. But this time we will fill it with, for example, water. The rate of collection and discharge of liquid in this case is limited by the viscosity of the water and the throughput of the syringe opening.

In the suspension, a shock absorber was combined with a spring (or other springing element) and received an excellent "mechanism" in which one element does not allow swinging, and the second takes on all the loads.

Below we will describe the damping elements of the suspension using the example of a telescopic shock absorber.

Double-tube and single-tube gas-filled shock absorbers are the most common types of shock absorbers in passenger cars.

Note
Any shock absorber has two essential characteristics: rebound force and compression force.

Interesting
The compression force of the shock absorber is less than the rebound resistance. This is done so that when hitting an obstacle, the wheel moves up as easily and quickly as possible. And when driving through a pothole, it sinks into it as slowly as possible. The best driving comfort can be achieved in this way.

6.1.13 Double-tube hydraulic shock absorbers

The name of this type of shock absorber speaks for itself. The simple type of shock absorber is two pipes, external and internal (shown in Figure 6.9). The outer tube also acts as a housing for the entire shock absorber and as a reservoir for working fluid. The inner tube is called a cylinder. A piston is installed inside it, made as one piece with the rod. The piston has holes in which non-return valves are installed. Some of the valves are directed in one direction, the rest are directed in the opposite direction. Some valves are called compensating valves; others are rebound valves.

Figure 6.9 Double-tube telescopic shock absorber.

Note
Non-return valve is a valve, which only opens in one direction. When applied to a shock absorber, the valves are called rebound and compression valves. Rebound and compression is the stretching and compression of the shock absorber, respectively.

The cavity between the cylinder and the body is called compensational (it is not completely filled with fluid). This cavity, as well as the shock absorber cylinder, are filled with working fluid. On the one side, the cylinder has a hole for the piston rod, and on the other, it is plugged with a plate with holes and one-way valves in them (compensation and compression valves).

When the piston moves in the cylinder, the oil flows from the cavity under the piston to the cavity above the piston, while part of the oil is squeezed out through the valve, which is located at the bottom of the cylinder. Part of the fluid flows through the compression valves to the external expansion tank. There it compresses the air, which was previously under atmospheric pressure in the upper part of the shock absorber body. Since this liquid has a certain viscosity and fluidity, then faster than predetermined, the overflow process will not take place. The same, only in the opposite direction, occurs on the rebound stroke when the piston moves up. In this case, the compensation valves of the cylinder plate and rebound valves inside the piston are activated.

However, this design has one, but a significant drawback: in case of prolonged operation of the shock absorber, the working fluid heats up. It starts mixing with air in the compensation tank and foams. As a result of this, a loss of work efficiency and failure occurs.

6.1.14 Double-tube gas-hydraulic shock absorbers

It was decided to pump an inert gas into the compensation tank instead of air (usually nitrogen is used) in order to solve the problem of foaming of the working fluid in the shock absorber. The pressure can range from 4 to 20 atm.

The principle of operation is no different from a double-tube hydraulic shock absorber, with the only difference that the working fluid does not foam so intensively.

6.1.15 Single-tube gas-filled shock absorbers

A distinctive feature of these shock absorbers from the aforementioned designs is that they have only one tube. This tube plays the role of both the body and the cylinder. The arrangement of such shock absorber differs only in that it has no compensation valves (Figure 6.10). The piston is equipped with rebound and compression valves. However, a floating piston, which separates the reservoir with the working fluid from the chamber with gas, which is injected under very high pressure (20-30 atm), is a feature of this design.

Do not think that the price is lower if the case is not double. Since only the piston performs all the work, the bulk value of the shock absorber is the cost of calculating and selecting the piston. True, the result of such laborious work is the increased efficiency of all characteristics of the shock absorber.

One of the advantages of this layout is that the working fluid inside the shock absorber is cooled much better due to the fact that there is only one wall in the body. Further advantages are the reduction in weight and size and the possibility of installation "upside down" - thus it is possible to reduce the amount of unsprung weights*.

Note
* Unsprung weight is everything between the road surface and the suspension components. We will not delve into the theory of suspension and vibrations. Let's just say that the smaller the unsprung weight, the less its inertia and the faster the wheel will return to its original position after hitting any obstacle.

However, there are also significant disadvantages of gas-filled shock absorbers, namely:

• vulnerability to external damage: any dent will result in a replacement shock absorber;
• sensitivity to temperature: the higher it is, the higher the gas back pressure and the harder the shock absorber works.

6.1.16 Springs

Spring is the simplest and most commonly used springing element, which is used in suspension design. The simplest version uses a tubular coil spring. However, due to the race for optimizing and improving the performance of the suspension, the springs can take on a wide variety of shapes. Thus, the springs can be barrel-shaped, concave, conical and with a variable diameter of the coil section. This is done in order for the spring stiffness to become progressive. That is, with an increase in the compression ratio of the springing element, its resistance to this compression should also increase. The dependence function should be nonlinear and continuously increasing. An example of a diagram of the dependence of the arising stiffness on the amount of compression is shown in Figure 6.12.

In ordinary tubular coil springs, this dependence is linear. Designers started changing the section and pitch of the coil in order to somehow solve this problem.

While changing the shape of the spring (Figure 6.13), they try to bring the stiffness closer to the ideal, guided by the diagram (Figure 6.12).

Barrel springs are sometimes called "miniblock" (see figure 6.14 for an example). Such springs, with the same stiffness characteristics as for a conventional coil spring, have smaller dimensions. Contact of the coils is also excluded when the spring is fully compressed.

6.1.17 Carriage spring

Carriage spring is the simplest and oldest version of springing element in car suspensions. What is easier: take several steel sheets, connect them together and hang suspension elements on them. In addition, carriage spring has the property of damping vibrations due to friction between the sheets. The leaf suspension is good for heavy SUVs and pickups, that have no special requirements for ride comfort, but there are high requirements for carrying capacity.

Until recently, carriage spring was also used in such a car as Chevrolet Corvette. However, there it was located transversely and was made of composite material.

Figure 6.15 Chevrolet Corvette with transverse carbon-plastic carriage spring.

6.1.18 Torsion bar

Torsion bar is a type of springing element, which is often used for saving space. It is a rod, one end of which is connected to the suspension arm, and the other is clamped with a bracket on the car body. When the suspension arm is moved, this rod twists, acting as a springing element. The main advantage of torsion bar is the simplicity of its design. The disadvantages include the fact that the torsion bar must be long enough for normal operation. But because of this, problems arise with its placement. In case if the torsion bar is located longitudinally, it “eats up” space under the car body or inside it. In case if it is transverse, it reduces the parameters of the geometric cross-country ability of the vehicle.

Figure 6.16 Example of suspension with longitudinal torsion bar (long rod, which is fixed on the lever in front and on the car body cross-arm in rear).

6.1.19 Pneumatic element

As the car is loaded with hand luggage and passengers, the rear suspension sags. This decreases the ground clearance and the probability of suspension breakdown increases (we described what it is above). In order to avoid this, designers first decided to replace the rear suspension springs with air springs (an example of such an element is shown in Figure 6.17). These elements are rubber buffers, into which air is pumped. In case if the rear suspension is loaded, air pressure inside the air springs increases. The position of the car body relative to the surface and the suspension travel remain unchanged. The likelihood of interlocking of the chassis elements is minimized.

Powerful compressors and an electronic control unit, as well as the possibility of automatic and manual control of the suspension were installed in order to expand the capabilities of the air springs. This is how a semi-active suspension, which, depending on the driving mode and road conditions, automatically changes the ground clearance, was invented. After the introduction of shock absorbers with variable stiffness into the structure, an active suspension was obtained at the output.

6.1.20 Stretcher

In order to ensure noise and vibration isolation, suspension parts are often attached not to the car body itself, but to an intermediate cross member or sub frame (an example of which is shown in Figure 6.18), which together with the suspension elements forms a single assembly unit. This structure simplifies assembly on the conveyor (and therefore reduces the cost of the car), adjustments and subsequent repairs.

Figure 6.19 Example of stabilizer bar installation.

6.1.21 Stabilizer bar

While turning one way or another, the car tilts in the opposite direction of the turn, because centrifugal forces act on it. There are two options to minimize this effect: make a very rigid suspension or install a rod, which connects the wheels of one axle in a special way. The first option is interesting, but in order to overcome the car's roll in turns, one would have to make a very rigid suspension. This would negate the car's comfort indicators. Another option is to install a complex electronically controlled active suspension, which would make the outer wheel suspension more rigid in turns. But this option is very expensive. Therefore, manufacturers made the simplest decision. They installed a rod, which was tied through the racks or directly to the levers of the wheel suspension on both sides of the car (see Figure 6.19). Thus, in turns, when the wheels that were located on the outside relative to the center of rotation, rise up (relative to the car body), the rod twists and, as it were, pulls the inner wheel to the body, thereby stabilizing the position of the car. From this and the name - "stabilizer bar". 

The main disadvantages of a conventional stabilizer bar are a deterioration in ride comfort and a decrease in overall suspension travel due to a small, but still connection between the wheels of the same axle. The first flaw affects luxury cars, the second one affects SUVs. In the era of electronics and technological breakthroughs, designers could not help but take advantage of all the possibilities of engineering. Therefore, they invented and implemented an active stabilizer bar, which consists of two parts: one part is connected to the right wheel suspension, the other to the left wheel suspension. In the middle, the two ends of the stabilizer bar are clamped in a hydraulic or electromechanical module, which has the ability to twist one or another part, thereby increasing the stability of the car. When the car moves straight, it “dissolves” these two ends of the rod, thus enabling each of the wheels to generate the suspension travel assigned to it.

6.1.22 Geometrical Maneuverability of the Car

The geometric maneuverability of a car is understood as a set of its parameters that affect the ability to move freely in certain conditions. These parameters include the height of the vehicle's ground clearance, the angles of departure and approach, the ramp angle, the size of the overhangs. Road clearance or vehicle clearance is the height from the lowest point of the car body, assembly (for example, suspension parts) or assembly (for example, the crankcase) of the car to the ground The approach and departure angles are parameters that determine the ability of a car to climb a hill at a certain angle or descend it. The magnitude of these angles is directly related to another parameter, which is included in the concept of geometric maneuverability - the length of the front and rear overhangs. As a rule, in case if the overhangs are short, then the car can have large approach and departure angles, which helps it easily climb steep hills and descend from them. In turn, knowledge of the length of the overhangs is important in order to understand whether it is possible to park your car to a particular curb. Finally, one more parameter is the ramp angle, which depends on the length of the wheelbase and the height of the car body above the surface. In case if the wheelbase is long and the height is small, then the car will not be able to overcome the transition point from the vertical to the horizontal plane. In other words, the car, having climbed the mountain, will not be able to cross its peak and will "sit" on the bottom.