Suspension systems are in charge of the interactions between a moving body and the terrain where it drives. This interaction always involves several compromises on how the body is physically linked to the road and what is the response to the irregularities on it.
There is also to consider that suspension takes care of three of the vehicle body movements such as its vertical movement, pitch and roll. Nevertheless, the fact that most vehicles have four wheels introduces an additional item of complexity.
Suspension Modes
When a vehicle has 4 or more wheels, tire load can vary depending on surface irregularities. Car positioning in respect to the road involves three degrees of freedom, and using 4 or more wheels implies an hyperstatic system.
We can understand the suspension of a 4-wheel vehicle as a 4-freedom degrees system defined by the independent movement of each wheel in respect to the vehicle body.
Normally we can only think of three body movements, the vertical, the roll and the pitch movements. So we need to incorporate a fourth "movement" that is independent of car position: the axle crossing, sometimes called "warp" movement of the wheels.These movements are the so called "suspension modes", this is combined wheel movements that are associated to:
- Body Vertical movement
- Body Pitch movement
- Body Roll movement
- Axle crossing
Conventional suspension systems make a simplified approach, talking each wheel aside - quarter of vehicle - and placing a resilient element and a damping element associated to such wheel alone. This is usually insufficient, and thus antiroll bars are widely used.
Very few systems, mostly active systems, make use of suspension modes to specifically control each of these movements and therefore gaining higher levels of vehicle dynamics performance.
Creuat Suspension System
Although many interconnected suspension systems can achieve a certain degree of control of each suspension modes, most of these systems are rather complex and require additional control devices to make it operative, thus losing most of the benefits of a purely passive system.
The advantages of the Creuat Suspension System are linked to its simplicity and its straightforward design. The system layout is simple and can be adjusted to many types of vehicles with minimum changes in the system design.
Conventional suspensions are based on individual wheel suspension elements that provide a given resiliency and a given damping to the movements between the wheel and the vehicle body. These elements are usually independent of each other, except for specific transverse linkages that increment the stiffness to roll. In most cases these elements are the so-called anti roll bars, that increase the elasticity rate for the opposite wheel movements of each side.
Beyond that it is difficult to find any other linkage. Only a
few odd cars like the Citroën 2CV and the Mini Morris implemented a longitudinal
connection with no real improvement if compared to more conventional systems.
The philosophy in these cases was to soften pitch movements
instead of increasing roll stiffness. The ratio between pitch and
roll stiffness could be adjusted, but then the vertical movement
was stiffer than if anti roll bars had been used.
To really optimize the suspension performance it would be desirable to control the vehicle dynamics regarding all body movements, not only roll and pitch movements. in a conventional suspension, body movements are controlled by the elasticity and damping characteristics of all wheel suspension elements, and these do not know whether the wheel movement is caused by the car roll, pitch or just a bump in the road.
To control body movements separately it is necessary to interconnect not only the elastic components but the damping too. In fact it seems reasonable that damping elements rates match the elastic elements rates so the vehicle dynamics can be optimized to the best stiffness in any conditions.
Creuat suspension technology provides a simple and effective solution to obtain the maximum control over the vehicle dynamic response to every body movement as well as the weight distribution among all wheels.
Active and Passive systems
Active systems can provide unlimited control on the vehicle dynamics. They rule the energy exchanges associated to wheel movements, and therefore control the position of the chassis in respect to the road surface.
Active systems require two expensive things: sensors and actuators. Sensors to determine the body movements, and actuators to respond to external input forces. Actuators require also a powerful energy supply. Active systems have been implemented on high end cars but have not proven reliable and cost effective to the market. Very few automakers have introduced it in series cars.
In 1981 Lotus introduced active suspension systems to the F1. The performance obtained was not matched by any other suspension, and because of their success, it was banned from F1.
Active systems Pros and Cons can be summarized as follows:
Full
control vehicle dynamics
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Reliability
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