Systematic Damping

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When it comes to driving comfort, dynamics and safety, the sMOTION active damping system has raised the bar. The prototype is an enhanced version of the tried-and-trusted damping systems from ZF, as a more detailed look at the technology shows.

Martin Westerhoff,

Martin Westerhoff studied technology journalism and writes about vehicles and technologies since then. He has a soft spot for motorsports and racing cars.

No doubt about it, vehicle damping requirements are high. The main task of the shock absorbers is to quiet down the vehicle body as quickly as possible after driving over bumpy road surfaces. Moreover, the shock absorbers must ensure that the wheels always have contact with the ground. Both tasks have a considerable impact on driving safety and ride comfort. Though hard damping increases both driving safety and dynamics, it also reduces comfort for the passengers. The reverse also applies: Soft damping increases driving comfort, but driving safety and dynamics suffer. What is needed is a balanced damping behavior. To successfully walk this fine line in all vehicle categories, ZF offers different types of shock absorbers.

Premium add-on for standard and semi-active shock absorbers

Today, modern passenger car chassis are almost exclusively equipped with hydraulic shock absorbers in a single or twin-tube design. A compromise always has to be made between a sporty and comfortable shock setup. In terms of design, these shocks consist of a cylinder in which a piston moves up and down, pushing the oil through openings or valves in the process. The resistance as the oil flows through the openings essentially determines the damping properties. Over the last 20 years, in addition to standard shock absorbers with a fixed characteristic curve, ZF has been offering shocks with Continuous Damping Control (CDC) as well as semi-active damping systems. sMOTION – still in the prototype phase – now completes this product range as an active system at the top end.

Damping systems for any purpose

ZF damping systems vary in their complexity, thus giving auto manufacturers the freedom to equip the same vehicle platform with one of its systems based on the model, engine type or configuration. The decision on which shock absorber will be installed also depends on whether a person will be driving the vehicle or whether it will be operated autonomously. Even for autonomous vehicles, an active chassis is important because the driver will no longer be permanently looking at the road. In light of this, sMOTION – soon to go into volume production – will generate a noticeable gain in comfort, safety and driving dynamics.

How the technology of the three systems works

Standard twin-tube shock absorber

The piston rod of the twin-tube shock absorber is attached to the car body and its casing is attached to the steering knuckle. The casing contains a second tube completely filled with oil, which is the working chamber. The piston moves up and down in this cylinder. The space between the inner tube and the casing acts as a compensating chamber, which is not completely filled with oil. When the piston rod moves downward, it receives the oil pressed out of the working chamber. The bottom of the working chamber and the piston contain valves that normally restrict the oil flow more in the rebound direction (when the piston comes up) than in the compression direction (when the piston goes down). When the piston rises, it sucks oil out of the compensation chamber back into the working chamber.

Twin-tube shock absorber with CDC valves

A shock absorber with Continuous Damping Control (CDC) uses either one or two electrically controlled solenoid valves. The CDC valves continuously vary the hydraulic resistance. Low resistance produces soft damping and high resistance, hard damping.

Twin-tube shock absorber with sMOTION

The sMOTION shock absorber has an electric pump that pumps the oil from the upper working chamber to the lower chamber or vice versa. This effects active forces. A bypass upstream from the pump makes it possible to activate two CDC valves starting at a frequency of five Hertz. Such high frequencies are stimulated, for example, by driving over manhole covers, transverse joints, chip seal or gravel. The video shows how the system and shock absorbers work.

How sMOTION works

In brief: With sMOTION, ZF intends to add an active system to its damping solution portfolio. This system is ideal for meeting the most diverse customer needs. It ensures the highest possible driving comfort as well as the greatest possible driving safety and dynamics. sMOTION is an important contribution to highly automated and autonomous driving. When sMOTION goes into volume-production, ZF will offer not only standard shock absorbers with a fixed characteristic curve and its semi-active Continuous Damping Control (CDC) system, but also the new active sMOTION system. From a technological standpoint, the systems build on one another and the shock absorbers work according to the same basic principle.