Устройство автомобиля

This suspension – also known as a crank axle – consists of a control arm lying longitudinally in the driving direction and mounted to rotate on a suspension sub-frame or on the body on both sides of the vehicle. The control arm has to withstand forces in all directions, and is therefore highly subject to bending and torsional stress. Moreover, no camber and toe-in changes are caused by vertical and lateral forces.

The trailing-arm axle is relatively simple and is popular on front-wheel drive vehicles. It offers the advantage that the car body floor pan can be flat and the fuel tank and/or spare wheel can be positioned between the suspension control arms. If the pivot axes lie parallel to the floor, the bump and rebound-travel wheels undergo no track width, camber or toe-in change, and the wheel base simply shortens slightly. If torsion springs are applied, the length of the control arm can be used to influence the progressivity of the springing to achieve better vibration behavior under load. The control arm pivots also provide the radius-arm axis O; I. e. during bracing the tail end is drawn down at this point.

The tendency to over steer as a result of the deformation of the link (arm) when subject to a lateral force, the roll centre at floor level, the extremely small possibility of a kinematic and elastokinematic effect on the position of the wheels and the inclination of the wheels during cornering with the inclination of the body outwards (unwanted positive camber) are disadvantages.

 

This is a special type of trailing-arm axle, which is fitted mainly in rear-wheel and four-wheel drive passenger  cars, but which is also found on front-wheel drive vehicles. Seen from the top, the control arm axis of rotation EG is diagonally positioned at an angle α=10^o to 25^o,and from the rear an angle β≤5^o can still be achieved. When the wheels bump and rebound-travel they cause spatial movement, so the drive shafts need two joints per side with angular mobility and length compensation. The horizontal and vertical angles determine the roll steer properties.

When the control arm is a certain length, the following kinematic characteristics can be positively affected by angles α and β:

• height of the roll centre;
• position of the radius-arm axis;
• change of camber;
• toe-in change;

Camber and toe-in changes increase the bigger the angles α and β: semi-trailing axles have an elastokinematic tendency to over steering.

 

 

A form of multi-link suspension was first developed by Mercedes-Benz in 1982 for the 190 series. Driven and non-driven multi-link front and rear suspension have since been used.

Up to five links are used to control wheel forces and torque depending on the geometry, kinematics, elastokinematics, and force application of the axle. As the arrangement of links is almost a matter of choice depending on the amount of available space, there is extraordinarily wide scope for design. In addition to the known benefits of independent wheel suspensions, with the relevant configuration the front and rear systems also offer the following advantages:

• Free and independent establishment of the kingpin offset, disturbing force and torque developed by the radial load.
• Considerable opportunities for balancing the pitching movements of vehicles during braking and acceleration (up to more than 100% anti-dive, anti-lift and anti-squat possible).
• advantageous wheel control with regard to toe-in, camber and track width behavior from the point of view of type force build-up, and type wear as a function of jounce with almost free definition of the roll centre and hence a very good possibility of balancing the self-steering properties.
• Wide scope for design with regard to elastokinematic compensation from the point of view of (a) specific elastokinematic toe-in changes under lateral and longitudinal forces and (b) longitudinal elasticity with a view to riding comfort (high running wheel comfort) with accurate wheel control.

As a result of the more open design, the wheel forces can be optimally controlled, i. e. without superposition, and introduced into the bodywork in an advantageous way with wide distances between the supports. The disadvantages are:

• Increased expenditure as a result of the high number of links and bearings;
• Higher production and assembly costs;
• The possibility of kinematic overcorrection of the axle resulting in necessary deformation of the bearings during vertical or longitudinal movements;
• Greater sensitivity to wear of the link bearings;
• High requirements with regard to the observation of tolerances relating to geometry and rigidity.

 

 

 

 

3. Rigid and semi-rigid crank axles

3.1. Rigid axles

 

Rigid axles can have a whole series of disadvantages that are a consideration in passenger cars, but which can be accepted in commercial vehicles:

• Mutual wheel influence.
• The space requirement above the beam corresponding to the spring bump travel.
• Limited potential for kinematic and elastokinematic fine-tuning.
• Weight – if the differential is located in the axle casing, it produces a tendency for wheel hop to occur on bumpy roads.
• The wheel load changes during traction and (particularly on twin types) there is a poor support base bs_p for the body, which can only be improved following costly design work.

The effective distance bs_p of the springs is generally less that the tracking width b_r, so the projected spring rate c_ϕ is lower. The springs, and/or suspension dampers, for this reason should be mounted as far apart as possible.

The centrifugal force acting on the body’s centre of gravity during cornering increases the roll pitch where there is a rigid axle.

Thanks to highly developed suspension parts and the appropriate design of the springing and damping, it has been possible to improve the behavior of rigid drive axles. Nevertheless, they are  no longer found in standard-design passenger cars, but only on four-wheel drive and special all-terrain vehicles.

Because of its weight, the driven rigid axle is outperformed on uneven roads (and especially on bends) by independent wheel suspension, although the deficiency in road-holding can be partly overcome with pressurized mono-tube dampers. These are more expensive, but on the compressive stroke, the valve characteristic can be set to be harder without a perceptible loss of comfort. With this, a responsive damping force is already opposing the compressing wheels. This is the simplest and perhaps the most economic way of overcoming the main disadvantage of rigid axles.

In contrast to standard-design vehicles, the use of the rigid rear axle in front-wheel drive vehicles has advantages rather than disadvantages. The rigid rear axle weighs no more than a comparable independent wheel suspension and also gives the option of raising the body roll centre. Further advantages, including those for driven axles, are:

• They are simple and economical to manufacture;
• There are no changes to track width, toe-in and camber on full bump/rebound-travel, thus giving;
• Low tyre wear and sure-footed road holding;
• There is no change to wheel camber when the body rolls during cornering, therefore there is constant lateral force transmission of tyres;
• The absorption of lateral force moment M_y=F_txh_ro,r by a transverse link, which can be placed at almost any height;
• Optimal force transfer due to large spring track width bsp;
• The lateral force compliance steering can be tuned towards under- or over-steering.

There are many options for attaching a rigid axle rear suspension beneath the body or chassis frame. Longitudinal leaf springs are often used as a single suspension control arm, which is both supporting and springing at the same time, as these can absorb forces in all three directions as well as drive-off and braking moments. This economical type of rear suspension also has the advantage that the area on lorries and the body of passenger cars can be supported in two places at the back: at the level of the rear seat and under the boot. This reduces the stress on the rear and of the car body when the boot is heavily laden, and also the stress on the lorry frame under full load.

The longitudinal leaf springs can be fitted inclined, with the advantage that during cornering the rigid rear axle (viewed from above) is at a small angle to the vehicle longitudinal axis. To be precise, the side of the wheel base on the outside of the bend shortens somewhat, while the side on the inside of the bend lengthens by the same amount. The rear axle steers into the bend and, in other words, it is forced to self-steer towards ‘roll-under steering’. This measure can, of course, have an adverse effect when the vehicle is traveling on bad roads, but it does prevent the standard passenger car’s tendency to over steer when cornering. Even driven rigid axles exhibit – more or less irrespective of the type of suspension – a tendency towards the load alteration (torque steering) effect, but not to the same extent as semi-trailing link suspensions.

On front-wheel drive vehicles, the wheels of the trailing axle can take on a negative camber. This improves the lateral grip somewhat, but does not promote perfect tyre wear. This is also possible on the compound crank suspension (a suspension-type halfway between a rigid axle and independent wheel suspension) which, up to now, has been fitted only on front-wheel drive vehicles.

 

 

 

3.2. Semi-rigid crank axles

The compound crank suspension could be described as the new rear axle design of the 1970s and it is still used in today’s small and medium-sized front-wheel drive vehicles. It consists of two trailing arms that are welded to a twistable cross-member and fixed to the body via trailing links. This member absorbs all vertical and lateral force moments and, because of its offset to the wheel centre, must be less torsionally stiff and function simultaneously as an anti-roll bar. The axle has numerous advantages and is therefore found on a number of passenger cars which have come onto the market. From an installation point of view:

• The whole axle is easy to assemble and dismantle;
• It  needs little space;
• A spring damper unit or the shock absorbed and springs are easy to fit;
• No need for any control arms and rods; and thus;
• Only few components to handle.

From a suspension point of view:

• There is a favorable wheel to spring damper ratio;
• There are only two bearing points O_l and O_rs, which hardly affect the springing
• Low weight of the unspring masses; and
• The cross-member can also function as an anti-roll bar.

From a kinematic point of view:

• There is negligible toe-in and track width change on reciprocal and parallel springing;
• There is a low change of camber under lateral forces;
• There is low load-dependent body roll under steering of the whole axle; and
• Good radius-arm axis locations  O_l and O_rs, which reduce tail-lift during bracing

The disadvantages are:

• A tendency to lateral force over steer due to control arm deformation;
• Torsion and shear stress in the cross-member;
• High stress in the weld seams; which means
• The permissible  rear axle load is limited in terms of strength;
• The limited kinematic and elastokinematic opportunities for determining the wheel position;
• The establishment of the position of the instantaneous centre by means of the axle kinematics and rigidity  of the twist-beam axle;
• The mutual effect on the wheel;
• The difficult decoupling of the vibration and noise caused by the road surface;
• The considerable need for stability of the bodywork in the region of those points on the front bearing at which complex, superposed forces have to be transmitted.

 

 

 

 

 

 

4. front-mounted engine, rear-mounted drive

In passenger cars and estate cars, the engine is approximately in the centre of the front axle and the rear wheels are driven. To put more weight on the rear axle and obtain a more balanced weight distribution, Alfa Romeo, Porsche (928, 968 models) and Volvo integrated transmission with the differential.  This is also the case with the Chevrolet Corvette sports cars. With the exception of light commercial vehicles, all lorries have the engine at the front or centrally between the front and rear axles together with rear-wheel drive vehicles. The long load area gives hardly any other option. Articulated lorries, where a major part of the trailer weight – the trailer hitch load – is carried over the rear wheels, have the same configuration. On buses, however, the passengers are spread evenly throughout the whole interior of the vehicle, which is why there are models with front, central and rear engines.