Wave theory

Streamlining suspension development on a vertical dynamics test stand

The goal of optimising a suspension system – in a performanceoriented context – is to minimise wheel load fluctuations

Test drive, adjust, test drive, repeat. This was the common pattern for suspension development and tuning in the past. Recognising the inefficiency of this process, suspension specialist, KW Automotive, has successfully streamlined these iteration loops using modern technology. This has not only saved time, but also significantly reduced costs.

The suspension system is one of the most crucial components of any vehicle, encompassing all components that connect the vehicle to the road. These include the wheels, tyres, suspension, damping, stabilisers, axle design and steering as well.

It is often said that there are suspension designs that can do it all but, truthfully, this is not the case. All suspension systems are basically a compromise, developed to suit their market and relevant applications.

To minimise this compromise, OEMs, aftermarket companies, motorsport teams and automobile manufacturers invest a great deal of effort. Just as racecars and production vehicles differ, so do their dampers.

Although they fulfil the same tasks of damping shocks and the resulting vibrations, supporting the body structure and, ultimately, as a wheel-guiding element responsible for vehicle stability and wheel control, the way they achieve this, and how they are built, are fundamentally different.

To further refine these characteristics and deliver precisely tailored suspension solutions for the most diverse requirements, KW Automotive specialises exclusively in the development and production of dampers for motorsport, small-series equipment and aftermarket coilover kits.

Complex interplay

Located in Fichtenberg, southern Germany, the company uses a seven-post vertical dynamics test rig for suspension development. Simply put, this facility, also known as a vehicle dynamics test stand, analyses the complex interplay of elastokinematics, damper forces, spring rates, tyre carcasses and vehicle structure directly on the vehicle.

Why? To determine the ideal spring / damper configuration for the respective application of the vehicle, driver and tyres with the least amount of time and effort and under any weather conditions.

Of course, such a test stand cannot replace final road or track tests, but the results obtained indoors come very close to the real-world optimum, drastically shortening development time.

The goal of optimising a suspension system – in a performance-oriented context – is to minimise wheel load fluctuations, thereby improving traction between the road and tyre. This not only enhances acceleration and braking traction, but also allows for the generation of higher lateral forces in corners.

The more lateral forces the suspension and tyres can generate, the faster cornering speeds can be achieved by a racecar. Lap times then invariably improve, and the car becomes faster in the race.

Perfect balance

Simply put, the task of dampers and springs is to maintain all parameters in perfect balance. In essence, vehicle dynamics engineers seek the best possible compromise to dampen vibrations without risking a loss of tyre grip or compromising body control due to excessive body roll or pitch.

For data acquisition, each racecar is equipped with various sensors, such as acceleration sensors and linear potentiometers for spring travel, all wired and additionally secured on the test rig.

The rig itself consists of a robust base, on which four powerful, dynamically controllable hydraulic cylinders are mounted, one under each wheel. In addition to integrated position sensors, these four pistons, on which the wheels sit, also function as wheel load scales.

Three additional cylinders, fixed directly to the chassis, can simulate additional forces acting on the vehicle.

In test operation, each individual piston is moved hydraulically, generating pressures of up to 230bar in the lines and hoses. The vertical movement of the pistons induces vibrations throughout the entire chassis. During this process, engineers analyse resonance frequencies, where the amplitude of the forced excited body is maximised. In this so-called hub sinusoidal oscillation, the vehicle passes through a frequency band from zero to 20Hz, at a constant speed in the phase null passage. If the inherent oscillation is not dampened, the entire system becomes uncontrollable.

Abstract thought

This might sound a bit abstract, and it is.

Let’s simplify it a bit. Engineers use the vibrations and data frequency bands to see how even small transverse joints, or wavy asphalt surfaces caused by weather conditions, affect the vehicle. For example, it helps understand what happens when a regular road car encounters a bump, or a pothole. Engineers can then see how quickly this force impulse can be balanced and dissipated through the springs and dampers. Only with optimal damping can the vehicle maintain its stability and stay on the ideal line, especially at high cornering speeds.

The individual measurement of frequency bands and resonances takes only 64 seconds. This is sufficient for the experienced KW vehicle dynamics engineers to uncover even the slightest suspension weaknesses.

In addition to measurements at constant speeds of 75, 150, 200 or 250mm/s (excitation speed of the pistons at the phase null passage), the test rig can also replicate various racetracks, or sectors, using a ‘track replay’ feature. This allows the simulation of stresses encountered during challenging drives, such as navigating the famous Fuchsröhre section of the Nürburgring.

The system must be operated in the ‘seven-post mode’ to achieve this. Here, the suspension is connected to the vertical dynamics test rig at two additional points – in the rear third of the vehicle and at the front. With these additional force application points, the entire aerodynamics, including lift or downforce, as well as moments of roll around the longitudinal axis and pitch around the transverse axis, can be simulated.

When a current GT4 racecar, or a high performance sports road car such as a BMW M4 CSL for example, which does not have as much aerodynamic downforce as a GT3 racecar, is put on the test rig for shock absorber and suspension development, the system is used in the vertical four-post mode without simulating an aeromap.

The test rig can also replicate various racetracks, or sectors, using a ‘track replay’ feature. This allows the simulation of stresses encountered during challenging drives

KW Automotive uses a seven-post vertical dynamics test rig to develop its damper and suspension systems. The company says the results obtained correlate very well with real-world track testing

In pursuit of the ideal suspension set-up, vehicle dynamics engineers seek the optimal balance between tyre grip, body control and comfort, leading to the development of diverse damper designs. This four-way adjustable road coilover kit shows such innovation

These diagrams provide an overview of the diverse range of data used to determine the suspension set-up and enhance the driving dynamics of both road and motorsport vehicles The raw data provides valuable insights into the vehicle’s dynamic behaviour under different conditions, enabling engineers to fine tune damper settings to achieve the desired handling characteristics for the intended racing environment

A frequency dependent analysis of the amplitudes and phase relationships between the front and rear axles of a vehicle, especially pitching behaviour, is a crucial step in understanding and optimising the vehicle’s dynamic behaviour

A roll stiffness diagram shows the relationship between roll stiffness, jounce travel and wheel load. Engineers use it to evaluate the handling characteristics of a vehicle

Comparison of dynamic wheel loads under different track and weather conditions with a dedicated focus on the front and rear axles

The vertical movement of the hydraulically-operated pistons induces hub sinusoidal oscillations throughout the chassis

On a seven-post rig there are three additional posts, fixed directly to the chassis, that simulate additional forces acting on the car

Engineers then analyse the vibration frequencies to see how quickly force impulses can be balanced and dissipated through the dampers