Heavy vehicles can be equipped with different types of spring arrangements for absorbing shocks caused by unevenness of road surfaces. Commonly occurring spring arrangements are either leaf springs or air springs. Combinations of these springs are also used; e.g., vehicles with a leaf-suspended front axle and one or more air-suspended rear axles. Air suspension results in a soft and shock-free ride both laden and unladen, with consequently good traveling comfort and less stress on chassis and tires.
Air-suspended vehicles are often equipped with manual or automatic level control. Automatic level control not only enables the vehicle to be kept on level even when unevenly laden but also enables the height of the vehicle to be kept constant irrespective of load. When the vehicle is standing still, its height may also be adjusted manually whereby the vehicle can be raised, lowered or caused to tilt forwards or rearwards; e.g., to adapt the vehicle to a loading dock for the purpose of loading or unloading.
Air suspensions typically include rubber bellows situated between the frame and the wheel axles. As the height of the chassis may change, the vehicle is equipped with at least one level sensor which detects the height between the frame and a wheel axle. One level sensor is sufficient for a vehicle with a leaf-suspended front axle and an air-suspended rear axle, but a vehicle with an air-suspended front axle and two air-suspended rear axles requires three level sensors to enable reliable monitoring of the air suspension system.
Heavy vehicles are commonly equipped with more than one rear axle. An arrangement with more than one rear axle is called a bogie, which may comprise two or three rear axles. The most common arrangement is a bogie with two rear axles comprising either two powered rear axles or one powered rear axle and an trailing or pusher axle. Vehicles with two powered rear axles are called 6×4 and vehicles with one powered rear axle and a trailing axle are called 6×2. A bogie may be designed in various ways depending inter alia on the intended load capacity.
When the longitudinal tilt of a vehicle with two rear axles and air suspension changes, it is important that the maximum permissible chassis height at each axle is not exceeded, i.e. that the distance between the frame and each axle does not exceed a maximum permissible value. If that value is exceeded, the axle installation is subjected to impermissible stressing which may result in mechanical damage. The axle installation is more or less sensitive to incorrect stressing, depending on the type of bogie. For example, an axle installation where the rear axle can be raised by a bogie lift may be sensitive to stressing in a wrong direction. Possible forms of damage are shock absorbers being pulled apart or damaged or the fastenings of the V-stay being incorrectly stressed.
Exceeding the maximum permissible chassis height may occur at the rear wheel axle if, for example, the whole vehicle is first raised to the maximum and the chassis height is thereafter reduced at the front axle to cause the vehicle to tilt forwards. The result is that the vehicle pivots about the forward rear axle, which means that the distance between the rearmost axle and the frame will increase, with consequent risk of the maximum permissible chassis height being exceeded at the rearmost axle. On vehicles with bogies, the stresses may be distributed among the wheel axles.
On a vehicle with a two-axle bogie it is usually the rearmost axle which can be relieved of stress. This entails having a level sensor on each axle in order to be able to monitor the distance between the frame and the wheel axles.
A disadvantage of using a level sensor for each axle is that the cost of each level sensor is high. As a level sensor for a heavy vehicle is subject to severe environmental effects, meeting the requirements is expensive. Another disadvantage is the need for the vehicle to comprise an extra installation which comprises various lever arms and stays and is therefore expensive and occupies space.