Multibody dynamics aids in development of customizable commercial vehicles

  • 08-Aug-2013 05:46 EDT

TerraStar bare chassis model.

In the commercial vehicle industry, vehicles are typically made-to-order for specific vocations such as severe service, intermodal transportation, and line haul. The development of a vocational truck typically starts with available off-the-shelf-modular components, with the primary task of vehicle-level integration being owned by the OEM engineers. However, past the bare frame, the OEM engineers have to deal with variations of hundreds of parts available to configure the vehicle for the end customer. To leverage economies of scale, it is common practice in the industry to share the same platform for multiple vocations, and also adapt different components such as suspension, frame components, engine, cab, and other body components. To accomplish this, one platform has to be developed to have robustness for many different applications, such as bus and/or utility vehicles.

Different components such as battery boxes, exhaust systems, fuel tanks, and crossmembers can be used across one application to another, saving development cost per vehicle in addition to shortening development and validation time, with minor adjustments to accommodate different vocations and applications. For example, the International TerraStar Light medium-duty platform is designed to be a versatile platform that can be adapted across different vocations.

The complexity of the interaction of various components typically means that the vehicle has to be verified for both performance and robustness for operation in the field, either virtually (when loads and durability information have been correlated from the track) or via testing to identify any undesirable dynamics before the test vehicle is built.

With the computation advancements today, various parts can be designed and evaluated to ensure that all the component requirements are met virtually even before it is evaluated on full-vehicle models. Although FEA has been available for this purpose for some time, the recent advances in computational power have made it possible to create a 3-D model of a part, discretize the geometry, and perform simulations that will verify the virtual design in a matter of hours. After the FEA is performed to make sure the design is robust within the intended service life, the component can be applied to the rest of the vehicle system via multibody dynamics to create a virtual vehicle model, which can serve as a first pass at the integration of the vehicle.

One of the primary possibilities that can fully utilize the versatility of a virtual multibody model is in investigating the integration of a component in a full-vehicle model. In this particular case, it was planned that the battery box bracket was to be a carryover component from the TerraStar 4x2 to the TerraStar 4x4. It was necessary to evaluate the bracket design virtually before it is even used in the prototype vehicle to ensure that the risks of not meeting durability requirements are minimized. Since the battery box is mounted on the vehicle frame, which is supported by the front and rear suspension, it was determined that it was best to evaluate the box in a full-vehicle setting virtually before it was tested on the track. This would allow for any final tuning or redesign of the suspension components (if needed), which would impact vehicle dynamics and ride.

The models were correlated with an initial build of the vehicle to ensure that the accelerations experienced by the model have the same frequency content and amplitude across the entire vehicle, especially on the battery box.

The correlated vehicle model was run in a virtual track with the durability profile at two different speeds that were deemed to most likely generate the highest g-loads on the bracket over the designed life of the component to evaluate design changes and concepts. During the virtual durability run, it was discovered that the g-levels in the TerraStar 4x4 were higher than the TerraStar 4x2.

To trace the source of the accelerations, the data were analyzed further, especially the accelerations at the frame, and the axle, to perform root cause analysis.

During the root cause analysis, it was discovered that the accelerations are originating from the axle motions, up through the frame into the bracket. This means that one of the things that has to be done to minimize the risk of subjecting the bracket to the potential of a durability failure is to reduce the acceleration level traveling up to the frame by controlling the axle motion.

In this particular case, one of the proposed solutions was found in adding a set of shock absorbers for the rear suspension of the vehicle to reduce the vertical and pitching of the vehicle during the run on the durability track, and thus reducing the shock load on the battery box. In the original concept, it was thought that the friction from the rear vari-rate suspension was adequate for the vocation, including all misuse and durability loads. Another concept is reducing the front shock compression damping, as it is thought that the increased compression damping will transmit the loads up to the frame. Lastly, an optional antiroll bar was considered to be made standard on the vehicle to improve durability performance. To get an idea of the probability of success using these solutions before testing the real vehicle, the concepts were first tested in simulation on the virtual durability track.

During the simulation, the bracket can also be analyzed in the time simulation to see which areas “light up” at peak accelerations. It can be helpful to visualize the hotspots as the simulations evolve through time to analyze the risk areas of a particular design.

From the virtual durability analysis performed, there were several design changes that were evaluated on the vehicle to reduce the accelerations that were propagated to the frame, as well as adding more robustness to the battery box bracket. The addition of rear shocks proves to provide more bang for the buck by taking the accelerations down the most. Since this is a released option for the TerraStar 4x4, the vehicle dynamics effects have already been verified.

In this study, the combination of FEA to generate the flexible body and the multibody model to combine all the bodies opens up a world of possibilities to save time and cost on problems in durability. In today’s competitive engineering development, the ability to test virtually and obtain directional guidance for development is one that will pay itself in dividends.

This article is based on SAE International technical paper 2013-01-2370 by Brendan Chan of Navistar Inc., to be presented at the SAE 2013 Commercial Vehicle Engineering Congress.

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