Suspension (vehicle)
The suspension is part of the chassis of vehicles. It bears the weight of the structure and is intended to ensure that the structure remains calm and that stimuli from the road are not transferred directly to the structure ( vibration isolation ). Passengers and cargo are protected from high loads. Higher speeds are only possible with vehicles with suspension. Together with the shock absorbers, the suspension ensures a compromise between driving comfort and driving safety. On the one hand, the occupants should be protected from unpleasant lifting, pitching and rolling vibrations as well as impacts, and on the other hand, the most uniform possible grip should be achieved.
At the time of the carriages , the suspension was only used for driving comfort. In the case of faster moving motor vehicles , it was also essential for driving safety.
Spring and shock absorber
The elastic yield of the sprung structure in the event of a shock caused by the roadway is the deflection of its oscillation. The purpose of converting the high impact force into a slightly longer lasting, smaller load on the body and its occupants is achieved after a deflection. Prolonged swaying is useless and the occupants find it uncomfortable. Without special measures, the structure - a sprung mass - vibrates too long, which is why vibration dampers ( called shock absorbers in motor vehicles ) are installed.
However, the oscillation must also be dampened quickly in order to ensure driving safety, because the force between the road and the wheels decreases when the oscillation is in the opposite direction and can no longer be sufficient.
stabilizer
With the help of stabilizers , the lateral inclination ( roll ) of the vehicle is reduced when cornering and the self-steering behavior at high lateral accelerations is coordinated. Stabilizers that are too hard, however, impair driving comfort on one-sided or alternating uneven surfaces. The vehicle body “copies” the road. Active stabilizers can defuse this conflict of objectives when driving straight ahead.
Unsprung and sprung mass
The superstructure to be kept as steady as possible when driving over bumps is the sprung mass . The unsprung mass, on the other hand, follows the uneven road surface and performs a vertical movement relative to the body.
The unsprung mass also includes the wheels
- the wheel carriers and the
- Brakes (provided that they are not "internal" in the case of driven and individually suspended wheels, i.e. not acting on the drive shafts at the differential output).
Components whose center of mass does not fully follow the uneven road surface are only partially counted as unsprung masses:
- the handlebars ,
- the springs (including stabilizers and balancing springs ),
- the shock absorbers and
- the axle shafts (driven axles with independent suspension; rigid axles are completely part of the unsprung mass).
The undesired wheel load fluctuations and, to a lesser extent, the impacts on the structure are less, the smaller the unsprung mass is in relation to the sprung mass of the vehicle. To obtain such a favorable, that is small to achieve ratio of unsprung to sprung mass, of sports and racing cars especially lightweight wheels made of aluminum and magnesium alloys or carbon fiber reinforced plastics (CFRP) and lightweight braking systems, for example, carbon fiber-ceramic - brake discs used .
Suspension travel
The distance that the wheel covers between an unloaded (zero wheel load) and a loaded position (usually double the static wheel load) is called the total suspension travel . In the idle state, the mass of the vehicle rests on the springs and reduces the total suspension travel by the negative suspension travel to the positive suspension travel .
The negative suspension travel is measured with the vehicle mass, i.e. including the driver, etc., and stated as a percentage of the total suspension travel. A value of 25 to 30 percent is recommended for motorcycles . If the compression is too strong - high percentage - usable spring travel is lost, the center of gravity is lowered and driving comfort and cornering stability are impaired. The same applies to insufficient negative spring travel - the telescopic fork then has too high a spring rate (or preload), which, depending on the condition of the road, can lead to the bike jumping with poor guidance and grip, as well as poor ride comfort.
A large spring deflection makes it possible to compensate for large unevenness in the road surface, but results in a high center of gravity of the vehicle and therefore prevents high speeds in curves in two-lane vehicles. Sports cars therefore usually have a small spring deflection. The shorter the spring travel, the stiffer the springs have to be. This applies to conventional steel linear springs, the force of which increases with the shortening of the path, and to steel torsion bar springs, the torque of which increases with the angle of rotation, but it does not always apply to air, gas, hydraulic or combination systems.
Spring characteristic
For a good ride comfort, the suspension is designed for a reference load (construction load) for a body resonance frequency to be specified. This results in a relatively soft spring constant of the suspension spring, in which the contribution of the link bearings, which are twisted during the spring (secondary spring rate), must also be taken into account. In order not to exceed the available spring deflection when carrying a load, the characteristic curve, usually by means of rubber buffers in simple cases, is e.g. B. leaf springs also designed by the spring itself, increasingly progressive. When rebounding, the wheel is held in place by a rebound stop. A rebound brake can also make the rebound branch progressive. In two-lane vehicles, the roll angle when cornering and the wheel load distribution between the front and rear axles are influenced by additional stabilizers.
Designs
In the first automobiles , the suspension was achieved, as in carriages, by a pair of leaf springs per axle. This type of construction is still used on commercial vehicles today. Before the First World War, the suspension with coil springs came up, then also with torsion bar springs. In the meantime, coil springs have largely gained acceptance.
One of the best-known cars with torsion bar suspension is the VW Beetle with a “spring sword” as a trailing arm on the rear axle, to which the half-shaft pendulum is attached to the wheel bearing.
From the 1930s, but especially in the 1950s, many European and American car manufacturers built vehicles with longitudinally arranged torsion springs, some even with level control.
The air suspension widely used in buses and trucks , in which a compressor generates air pressure, which causes a comfortable suspension with the option of level control via bellows, was first used in Germany by Borgward . In the sixties, some of the luxury class sedans from Mercedes-Benz had air suspension.
Since the Citroën DS, Citroën has been installing hydropneumatic suspension ( hydropneumatics ) in its mid-range and luxury cars . In the past, Mercedes-Benz and Rolls-Royce used it under license. With the Citroën XM , Citroën introduced the electronically controlled Hydractive . The Citroën Xantia Activa was also active stabilizers , the body roll almost completely suppressed when cornering.
In 1998, Mercedes-Benz was the first German manufacturer to equip the S-Class with the "Airmatic", an air suspension with adjustable dampers (ADS). An externally guided rolling bellows with axial reinforcements was used for the first time. In the meantime, various manufacturers offer the upscale models optionally with air suspension, which contributes significantly to comfort. The level can be adjusted to the driving situation with the air suspension ( level control ). For example, the level can be raised in the field or lowered on the motorway, which reduces air resistance.
A computer-aided active suspension is the Active Body Control . Mercedes-Benz introduced this hydraulic system for adjusting the spring base in 1999 in the C 215 .
Electric suspension, in which an electric linear motor pushes the wheel down or pulls it up, is not yet found in series vehicles. Unevenness in the road should be leveled out so that the occupants of the vehicle do not feel it. This system also makes it theoretically possible to let the vehicle jump over obstacles.
Two-wheelers
For bicycle suspension and motorcycles different suspension systems are used. A telescopic fork can often be found at the front, which, in addition to the suspension, also takes over the wheel guidance by means of telescopic tubes.
Rear suspension systems are usually designed as a swing arm. In motor scooters , this rocker arm is often at the same time the drive carrier with the motor and gearbox, but this increases the unsprung masses; this arrangement is called the drive train swing arm . Suspension and wheel guidance systems are also seldom separated, such as the stub axle steering on the Yamaha GTS 1000 or the Bimota Tesi . On the other hand, the " Telelever " systems from BMW , which decouple the wheel guidance on the front wheel from the suspension , are successfully established . In earlier times, front wheels were often guided by swinging; it is divided into long and short wings as well as pushed and pulled wings; Swings with particularly short swing arms are also known as rocker arm forks . The parallelogram forks and pendulum forks of motorcycles from the 1920s and 1930s are special designs of the swing arms .
The running wheels of bicycles nowadays are often sprung; this means an enormous gain in comfort.
Tricycles
Trikes (one front wheel, rear axle), on the other hand, usually do not have a telescopic fork (it would be too unstable for the heavy weight), but swing arm guides on the front wheel. The asymmetrical variant of the “tricycle”, the motorcycle combination , often has an “Earles fork” at the front, a pushed long- swing fork with separate components for wheel guidance and suspension, for reasons of shortening the caster and easier maneuverability; on the rear wheel, the arrangement specified by the motorcycle manufacturer is usually used, and the sidecar usually has a longitudinal swing arm or trailing arm .
Threewheelers have a motorcycle-like swing arm for the single driven rear wheel, while the two front wheels are individually suspended on double wishbones with leaf or coil springs, or, as with Morgan threewheelers, on sliding sleeves.
Web links
- Paul Balzer: Animations of the damped vibrations in a "quarter vehicle" (masses of: 1 wheel, 1/4 of the body and 1/4 of the driver)
Individual evidence
- ^ A b Hans-Hermann Braess, Ulrich Seiffert (ed.): Vieweg Handbook Automotive Technology . 6th, updated edition. Vieweg + Teubner, 2011, ISBN 978-3-8348-1011-3 , pp. 580–582 ( limited preview in Google Book search).
- ↑ M. Mitschke, H. Wallentowitz: Dynamics of motor vehicles. Springer, 2004.
- ^ Benny Wilbers , Werner Koch: New chassis technology in detail, ISBN 3-929534-17-7