DTM in the 90s: Modelling Mercedes-Benz’s active ballast system

Evolving out of the ferociously popular German DTM series in the mid 1990s, the International Touring Car Championship (ITC) featured exotic and sophisticated racing cars; certainly, the most advanced production-based racing cars ever seen. Ballooning budgets from 3 large European manufacturers: Mercedes-Benz, Opel and Alfa Romeo, allied to a beautifully liberal rule book, enabled various innovative developments to flourish.

Back in August 1996, I had just turned 4 years old. Even at that age I was obsessed with racing cars. I  was lucky enough that summer to attend my first ever racing event, when the ITC visited Silverstone. Uniquely, it would be the only time they would visit the home of British motorsport, as the championship collapsed due to the massive costs at the end of the season. Naturally, it stuck in my head. Packed with various gizmos and technologies, I thought it would be a fun exercise in simple multibody modelling to build some of the famous features of these machines. 

Figure 1: Innocuous looking family car...? More technology than an F1 car at the time. Image: Martin Lee (Wikimedia creative commons).

Figure 1: Innocuous looking family car…? More technology than an F1 car at the time. Image: Martin Lee (Wikimedia creative commons).

Looks can deceive!

The basic premise was simple. Take a 4 door saloon (sedan) vehicle from the show room line up, source a 2.5L V6 engine from one of the manufacturers’ products and from the road going chassis upwards, build a thoroughbred racing machine. Restrictions meant the overall outside shape of the bodywork (the “silhouette”) had to be retained, although full freedom for the aerodynamics was found from the centre of the wheel downwards. Apart from that, pretty much anything went. Predictably, the results were spectacular and eye wateringly expensive! Technologies banned in Formula 1, such as traction control and anti-lock brakes (ABS) were permitted, as well as active suspension. Drivetrain choice was open, with Mercedes sticking with the traditional RWD and Opel and Alfa Romeo opting for 4WD.

Perhaps the most extreme innovation could be found on the Mercedes, active ballast. Commonly ballast is used in a racing car to bring it to the minimum weight limit. For handling purposes, it is moved around the car, depending on where is best for a particular car/track combination. Traditionally it is bolted to the floor of the car for safety reasons, which means its location is a compromise. Being able to move this ballast around the car during a lap would be beneficial; during acceleration, more load on the rear wheels enables greater traction to be found, whilst under braking, moving load forwards can be beneficial. Moving the mass during cornering could also aid the car’s handling.

The Mercedes implementation featured two masses fitted inside the door sills (rails) of the car, which could then be moved (either hydraulically or electrically) forwards and backwards along the longitudinal axis of the vehicle. It has been said that the system was fully active; the car ran an installation lap to “learn” the track, with the system then understanding how to move the ballast during the lap. Engineers back in the pits would likely have been able to tune the parameters of the system to optimise it.

Modelling and testing

To model something like this in Dymola, a multibody mass needs to be moved along the vehicle centreline and move in relation to the vehicle. A simple prismatic joint, constrained to allow movement in the x axis, is mounted to the vehicle chassis frame which enables the mass to move in the x direction relative to the vehicle. Using the axis flange option of the prismatic, a positional actuator is used to drive the position of the mass, depending on a real control signal. The force required to move the mass is automatically generated for the mass position to conform to the positional demand; it is then reacted into the vehicle chassis through the frame_a connector. For simplicity, the control signal used is the longitudinal acceleration of the vehicle. It is filtered (to smooth fluctuations and noise as well as break any non-linear system formation) and scaled to an appropriate value via the filter gain. Finally, a limiter block prevents the positional command from pushing the ballast further than an estimation of the length of the chassis sill.

Figure 2: A relatively simple and easy to put together model. Compare this to the effort that would be required to test such a system physically!

Figure 2: A relatively simple and easy to put together model. Compare this to the effort that would be required to test such a system physically!

The RoadsterSportATRT example vehicle model from the Suspensions library was used as the basis for a rudimentary approximation of the Mercedes-Benz C-Klasse W202, with the engine relocated to the front. One of the advantages of using this example vehicle as the basis for the ITC car was that it has a very similar wheelbase to the real car. 2.7m versus the C-Klasse’s 2.67m, making the estimation of the length and location of the ballast in the sills easier. It was assumed the allowed travel of the ballast would be 1.2m; the sill would be located 0.75m behind the front wheel centreline, offset from the centre of the car by 0.6m to place it roughly at the extremities of the vehicle track. It was elevated 50mm above the wheel centre, consistent with the aerodynamic regulations allowing freedom below the wheel centre height.

Figure 3: Just like the real car, two chassis rails are present. They only need to be connected to the chassis frame via a single connection; if they were connected at the rear, this would cause a multibody loop in the system.

Figure 3: Just like the real car, two chassis rails are present. They only need to be connected to the chassis frame via a single connection; if they were connected at the rear, this would cause a multibody loop in the system.

To test the function of this model, the first thing to do is to exercise it with the vehicle in a static test. Commandeering a mass check experiment is perfect for this task, to check the initialisation and basic function of the model without having to deal with the complexities of a full dynamic experiment. Replacing the longitudinal acceleration signal input to a sinusoidal source enables the system to be exercised.

Figure 4: Using a dummy sinusoidal positional input into the ballast enables the effect versus being static to be observed.

Figure 4: Using a dummy sinusoidal positional input into the ballast enables the effect versus being static to be observed.

With the system shaken down and the ballast actuator reconnected to the longitudinal acceleration signal on the control bus, the effects can be observed on a full dynamic experiment. Here, the curved and sloped road test is used. This is more akin to a car driving down a country lane than a race track, but it provides a way of showing that the system is responding to the longitudinal acceleration signal.

Video: The two pink spheres represent the masses being actuated to actively adjust the loads on the wheels. The pink tubes are the fixed translations connecting the masses to the chassis frame, enabling the masses to move relative to the vehicle.

Final thoughts

Whilst this is a fun nostalgic exercise, it demonstrates the ease with which complex system effects can be modelled in Dymola. Such a model can be used as a proof of concept or design of experiment type study, with the results able to influence the design choices made for such a system. A more detailed model could be made, which would enable further mechanical optimisation to be made. Finally, a detailed system can be used to develop the control algorithm for such a system. Compare this to the difficulty of trying to develop such a system physically without a tool like Dymola, which is what the Mercedes engineers would have had to do back in the 90s!

Written by: Theodor Ensbury – Project Engineer

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