Air Suspension

Air suspension or air spring is a type of vehicle suspension that is powered by an air pump or a compressor. They are commonly used in heavy duty vehicles, but over the last decade it has also made its mark in the smaller commercial vehicles.

Air suspension is made of tough rubber and plastics inflated at a certain pressure to withstand the shocks. Air springs have various advantages over a shock absorber and coil spring arrangement. Several automobile manufacturers use air suspension today such as Rolls Royce, Mercedes-Benz, Maybach, Land Rover, Jeep, Lexus, Volkswagen, etc.

Air Suspension System Design:

As already mentioned, that air suspension is made of tough rubber and plastic (polyurethane) bags that are inflated to a correct pressure with the help of a compressor.  

The bags come in 3 different shapes:

·         Double-Convoluted Bag: This type of bag is in the shape of an hour glass or a double cheese burger. It provides better lateral flexibility than other types and has more load capacity. It has a short stroke and a progressive spring rate. It is best suited for front suspension.

·         Rolling Sleeve and Tapered Sleeve Bags:  These have a smaller diameter and also the stroke length is longer. The linear spring rate is higher and more suited for rear end applications.

Rolling Sleeve

Taper Sleeve

Compressor Motor:

Most of the air suspension systems are provided with an electric compressor motor. It draws the air from the atmosphere, compresses it and then sends the compressed air to inflate the bags through a series of compressed air lines. Apart from compressing the air, it also maintains the pressure to be supplied to the bags. The pressure has to be varied for different driving conditions. Therefore, the compressor provides a transition in pressure.

In vehicles fitted with advanced air suspension system, a separate air tank is provided to maintain the compressed air pressure and also to provide a smoother change in pressure. The pressure of the air might vary from 5 bar to 12 bar and the tank capacity might vary from 7 liters to 20 liters depending on the load that a vehicle carries.

Desiccant Air dryer:

The air drawn from the atmosphere might have moisture which can damage the system. To remove the moisture, air dryer is installed to absorb the moisture before it is sent to the system.  

Air Lines:

High pressure lines made of rubber and polyurethane composition are used to carry the compressed air to the bags. Advanced systems often use steel lines.

Valves (Two way or Four way):

Valves play a crucial role in controlling the amount of air to be supplied to the bags. A two way valve system uses one valve per axle. During cornering, let’s assume a vehicle taking a left turn; the left air bag will be more compressed than the right air bag. Two way valve systems allow the air to transfer from left to right, thus increasing the tendency of body rolls. Body rolls bring a huge disadvantage to a air suspension system.

To overcome the problem, four way valve systems were introduced. It offered separate valve for each bag. It also requires a separate air line for each bag, hence canceling the possibility of body rolls due to air suspension.   

Electronic Control System (Solenoids):

Solenoids are used to electronically control the amount of air supplied to an air bag. The electronic control module receives inputs such as ride height, and pressure through sensors and switches the compressor ON and OFF as required to vary the pressure.

There is also a completely manual setup to regulate the rife height and pressure. This is done with the help of pneumatic valves.

There are 3 types of Electronic Control Systems:

·         Pressure Based System: The pressure based electronic control system relies solely on the air pressure inputs. Based on the air pressure inputs, the control module has to calculate the ride height of the vehicle. It is more of an assumption than calculating the exact ride height value. The system might not perfectly vary the ride height required based on the pressure inputs alone.

·         Ride Height Based System: With a ride height control system, the issue of assuming the ride height is overcome. Ride height sensors accurately measure the ride height of individual wheel. But without a pressure based system, the vehicle might face cross loading. Cross loading is the difference in ride height between the tires in one diagonal and the tires in the other diagonal. Two diagonal corner tires might be over inflated while the other two diagonal corner tires might be under inflated. To overcome the issue, air suspensions are fitted with a combo of both pressure based system and ride height based system.

·         Combo System: Both pressure based system and ride height system are installed together to overcome both cross loading and ride height calculation issues.

Advantages of Air Suspension:

·         Superior ride quality. Passengers don’t feel like their car is being driven over bumps.

·         It can improve towing capabilities

·         It can maintain the firmness of a vehicle when carrying heavy loads.

Disadvantages:

The only major disadvantage is the cost of buying an air suspension unit. It is mostly used in luxury cars. People who can spend extra money would generally go for this variant. Air suspension is standard in Mercedes S class and Range Rover.