Modern Tech
Ken Adams
The ABCs of Active Body Suspension
Active suspension systems are becoming more popular these days, so it seems appropriate to review the technology-rich Active Body Control systems (ABC) and Airmatic.
Mercedes-Benz has been a pioneer in ABC since the 1960s. One example that comes to mind is the mighty 300SEL 6.3 sedan, a classic that was supported at all four corners solely by airbags. A belt-driven air compressor provided pressure via a series of valves, lines, and a dryer to protect the system from moisture.
In 1975, the 450SEL 6.9 was updated to a newer fully hydraulic system that used an engine-driven hydraulic pump, level control valves, nitrogen-filled chambers, and fluid reservoir and valves to control ride height. The system also found its way onto some wagons and coupes as a rear-only load-leveling system.
The W129 V-12 had the first real “active” system for U.S. production vehicles. It lowered vehicle ride height to predetermined levels based on road speeds. It could be raised manually for driveway clearance. This was a partial hydraulic system; steel springs were used in conjunction with hydraulics.
The generation of active suspension that followed was dubbed Active Body Control. It was used on CLs (C215), SLs (R230), and some S-Classes (W220). Mercedes-Benz really stepped up the technology on these systems.
In a nutshell, here’s the basic layout: A belt-driven high-pressure seven-piston radial pump combined with a pressure regulator, pressure sensor, nitrogen-filled bladder, and a fluid reservoir keep approximately 200 bar, or nearly 3,000 pounds per square inch, on tap at all times. Sound like a lot? It is. Having this high pressure enables the system to respond quickly. So fast, in fact, that anti-sway bars were eliminated.
Does that sound crazy? It’s not, really. Remember, this system also uses steel springs as well, which means the hydraulic portion functions as an enhancement, not for total vehicle support.
The suspension struts have what appears to be the normal steel-spring configuration. The key to fast response lies in the hydraulic ring that serves as an adjustable spring pad. This hydraulic sleeve moves up and down to tighten or soften the spring preload. The hydraulic fluid chamber is quite small in terms of fluid volume, hence the quick response. Hydraulic changes can occur up to eight times per second (8 hertz). The system requires many inputs to operate properly: a level sensor at each wheel with opposing redundant signals, three body acceleration sensors, a plunger travel sensor in each strut, and pressure and temperature sensors.
What caused the technology to leap so far this time? Simple: a two-wire electronic data exchange network developed by Robert Bosch, called the Controller Area Network Bus (CAN Bus), that allows all control units to share vital information at very high speeds.
For all four corners of the suspension to work in harmony, the suspension control unit needs a lot of information to calculate suspension demands properly. Lateral and longitudinal sensors keep tabs on vehicle pitch and roll. If the vehicle is making a hard left, a command is sent to increase pressure to the right-side struts and decrease pressure on the left. This actually eliminates the need for anti-sway bars.
If the longitudinal sensors determine the vehicle is under hard braking, a command is sent to both front struts to increase pressure, and rear struts to reduce pressure to counter the nosedive.
The suspension control unit also needs to share its information with other systems as well, including headlights. For example, as the ride height changes, the headlights must be able to stay in adjustment. The CAN Bus sends a digital signal containing ride-height information to the headlight-range adjustment control unit and makes appropriate adjustments to the headlights.
Let me offer a quick opinion on ABC suspension in the C215 CLs. In 2001, when they were being introduced, the training centers allowed us to gather in groups of four and take one out on an extended drive to see how well they handled. I was in a wide-open area, with oversized cloverleafs, where we could really test the limits of adhesion. Known to push the envelope, I came into a corner pretty hot and continued to accelerate to full throttle through the turn. The CL stuck to the ground as if it were on the proverbial rails – absolutely no body roll. It seemed as if you couldn’t lose control of the car if you wanted. When everything is operating properly, the car handles and performs exceptionally well. But the system is complex, and when it breaks, so does your bank account.
Now we fast-forward to current production. Airmatic – the new, “less expensive” active suspension system – has found its way into nearly every model lineup. The components consist of an electrically powered air compressor, a pressure reservoir, a silicate air dryer to prevent moisture contamination, level sensors, a pressure sensor, and the well-known lateral and longitudinal sensors.
Airmatic operation is similar to the ABC system, but with some interesting twists. A key difference is that most Airmatic-equipped vehicles have no separate steel springs for suspension suppport.
Airmatic suspension struts have two different-sized air chambers with an internal valving system consisting of two solenoid valves and four bypass valves controlled via a pulse width modulation signal. This combination is capable of four different suspension settings, which are driver-controlled via the ADS II switch or the sport/comfort switch.
Several new CAN Bus inputs are used to enhance performance and safety. These include wheel-speed data from the electronic stability program (ESP), intake air temperature and barometric readings from the engine control module (ECM), and steering angle from the steering control module (SCM) to name a few.
An important note to remember on both ABC and Airmatic vehicles is the added expense to align them properly. Many extra steps are required to ensure that the proper base ride height is correct before an alignment can be performed. If your vehicle is equipped with Distronic, additional time may be necessary – Distronic requires precise aiming to function properly. A correctly performed alignment can take two to three hours if all the steps are done to Mercedes-Benz specifications. Service should be performed by an authorized dealer for best results.
There’s no question that the added complexity of these systems is one reason why repairs can be more costly on newer models. However, they are designed to function a long time, if checked periodically and maintained correctly, and the difference that they make in vehicle control and ride comfort could never be achieved without their presence.
Ken Adams
The ABCs of Active Body Suspension
Active suspension systems are becoming more popular these days, so it seems appropriate to review the technology-rich Active Body Control systems (ABC) and Airmatic.
Mercedes-Benz has been a pioneer in ABC since the 1960s. One example that comes to mind is the mighty 300SEL 6.3 sedan, a classic that was supported at all four corners solely by airbags. A belt-driven air compressor provided pressure via a series of valves, lines, and a dryer to protect the system from moisture.
In 1975, the 450SEL 6.9 was updated to a newer fully hydraulic system that used an engine-driven hydraulic pump, level control valves, nitrogen-filled chambers, and fluid reservoir and valves to control ride height. The system also found its way onto some wagons and coupes as a rear-only load-leveling system.
The W129 V-12 had the first real “active” system for U.S. production vehicles. It lowered vehicle ride height to predetermined levels based on road speeds. It could be raised manually for driveway clearance. This was a partial hydraulic system; steel springs were used in conjunction with hydraulics.
The generation of active suspension that followed was dubbed Active Body Control. It was used on CLs (C215), SLs (R230), and some S-Classes (W220). Mercedes-Benz really stepped up the technology on these systems.
In a nutshell, here’s the basic layout: A belt-driven high-pressure seven-piston radial pump combined with a pressure regulator, pressure sensor, nitrogen-filled bladder, and a fluid reservoir keep approximately 200 bar, or nearly 3,000 pounds per square inch, on tap at all times. Sound like a lot? It is. Having this high pressure enables the system to respond quickly. So fast, in fact, that anti-sway bars were eliminated.
Does that sound crazy? It’s not, really. Remember, this system also uses steel springs as well, which means the hydraulic portion functions as an enhancement, not for total vehicle support.
The suspension struts have what appears to be the normal steel-spring configuration. The key to fast response lies in the hydraulic ring that serves as an adjustable spring pad. This hydraulic sleeve moves up and down to tighten or soften the spring preload. The hydraulic fluid chamber is quite small in terms of fluid volume, hence the quick response. Hydraulic changes can occur up to eight times per second (8 hertz). The system requires many inputs to operate properly: a level sensor at each wheel with opposing redundant signals, three body acceleration sensors, a plunger travel sensor in each strut, and pressure and temperature sensors.
What caused the technology to leap so far this time? Simple: a two-wire electronic data exchange network developed by Robert Bosch, called the Controller Area Network Bus (CAN Bus), that allows all control units to share vital information at very high speeds.
For all four corners of the suspension to work in harmony, the suspension control unit needs a lot of information to calculate suspension demands properly. Lateral and longitudinal sensors keep tabs on vehicle pitch and roll. If the vehicle is making a hard left, a command is sent to increase pressure to the right-side struts and decrease pressure on the left. This actually eliminates the need for anti-sway bars.
If the longitudinal sensors determine the vehicle is under hard braking, a command is sent to both front struts to increase pressure, and rear struts to reduce pressure to counter the nosedive.
The suspension control unit also needs to share its information with other systems as well, including headlights. For example, as the ride height changes, the headlights must be able to stay in adjustment. The CAN Bus sends a digital signal containing ride-height information to the headlight-range adjustment control unit and makes appropriate adjustments to the headlights.
Let me offer a quick opinion on ABC suspension in the C215 CLs. In 2001, when they were being introduced, the training centers allowed us to gather in groups of four and take one out on an extended drive to see how well they handled. I was in a wide-open area, with oversized cloverleafs, where we could really test the limits of adhesion. Known to push the envelope, I came into a corner pretty hot and continued to accelerate to full throttle through the turn. The CL stuck to the ground as if it were on the proverbial rails – absolutely no body roll. It seemed as if you couldn’t lose control of the car if you wanted. When everything is operating properly, the car handles and performs exceptionally well. But the system is complex, and when it breaks, so does your bank account.
Now we fast-forward to current production. Airmatic – the new, “less expensive” active suspension system – has found its way into nearly every model lineup. The components consist of an electrically powered air compressor, a pressure reservoir, a silicate air dryer to prevent moisture contamination, level sensors, a pressure sensor, and the well-known lateral and longitudinal sensors.
Airmatic operation is similar to the ABC system, but with some interesting twists. A key difference is that most Airmatic-equipped vehicles have no separate steel springs for suspension suppport.
Airmatic suspension struts have two different-sized air chambers with an internal valving system consisting of two solenoid valves and four bypass valves controlled via a pulse width modulation signal. This combination is capable of four different suspension settings, which are driver-controlled via the ADS II switch or the sport/comfort switch.
Several new CAN Bus inputs are used to enhance performance and safety. These include wheel-speed data from the electronic stability program (ESP), intake air temperature and barometric readings from the engine control module (ECM), and steering angle from the steering control module (SCM) to name a few.
An important note to remember on both ABC and Airmatic vehicles is the added expense to align them properly. Many extra steps are required to ensure that the proper base ride height is correct before an alignment can be performed. If your vehicle is equipped with Distronic, additional time may be necessary – Distronic requires precise aiming to function properly. A correctly performed alignment can take two to three hours if all the steps are done to Mercedes-Benz specifications. Service should be performed by an authorized dealer for best results.
There’s no question that the added complexity of these systems is one reason why repairs can be more costly on newer models. However, they are designed to function a long time, if checked periodically and maintained correctly, and the difference that they make in vehicle control and ride comfort could never be achieved without their presence.