Durability Tests with OpenCRG Roads

The VeSyMA libraries have been used for many different applications, including durability studies. Such studies usually involve recreating a range of different experiments, situations and sometimes very extreme tests designed to push components to their limits. The purpose of these tests is two-fold; firstly, evaluating vehicle response to normal and abnormal driving conditions; secondly, evaluating the degradation of components over the lifetime of a vehicle, often in a compressed time-frame. Fundamental to the accuracy of these tests is the road surface and the tyre models; test validity does not rest upon the accuracy of the vehicle model alone.

Tyre and Road Fidelity

Selecting the fidelity of the surface, tyre and vehicle models is dependent on the type and nature of the experiment, as well as the desired detail in the results. Consider a high speed experiment on a uniform road, such as high speed laps around the Milbrook High Speed Bowl. Road uniformity (in a general sense) enables an “ideal” surface to be used in conjunction with a roughness model to capture the “non ideal” aspects of the surface. A standardised roughness model, such as ISO-8608, can be “tuned” to replicate the correct amplitude and wavelength of the seemingly random road roughness. This allows for the most efficient simulation to reduce time and computing power.

By contrast, some experiments are dependent on specific surface events such as a kerb or speed bumps. As these experiments are usually low speed, higher fidelity road and contact models are required. The best way to recreate this virtually is with a scanned road model of the particular surface of interest. Many different scanned surface storage-file types exist. One open source file format is CRG, available with associated tools. VeSyMA – Suspensions includes the ability to read these files and use them as surfaces to drive over.

Utilisation of a higher fidelity road model therefore allows for a larger range of greater fidelity tyre models to be deployed effectively, including using Pacejka with a grid contact model. Harnessing the Pacejka formulas for the slip and vertical forces, it is enhanced with a grid of contact points, reducing the complex surface into a single point. This allows for a good compromise between requiring high fidelity tyre modelling software and single point contact models. A previous post has detailed the different tyre models, including FTire.

Application of OpenCRG and VeSyMA

An example of a common test is over a specific speed bump at a specified speed. Below is an image of a scanned surface available from the OpenCRG website, which includes both a speed bump and a pothole.

Figure 1: Image of a scanned surface which includes both a speed bump and a pothole.
Figure 1: Image of a scanned surface which includes both a speed bump and a pothole.

The vehicle was created using the VeSyMA – Suspensions library and utilises a full multibody suspension system and the grid contact models discussed above. Evaluation of the vehicle dynamics and the loads transferred through the suspension system components can be studied in this test, demonstrated below.

Visibly, the vehicle moves over the speed bump at slightly higher speed than would be comfortable, as both the front and rear wheels leave the ground after the speed bump. The test was completed at a constant speed of 12 MPH, whereas the comfortable speed would probably be under 5 MPH.

Viewing the same test above, in slow motion (1/10th speed), shows the loss of contact of the wheels with the ground. Spring and damper forces can be determined, in this case for both front corners. This can then be used in component evaluations along with lifetime evaluations to evaluate cumulative component degradation forecasts over the lifetime of the vehicle.

Figure 2: Suspension component forces over a speed bump and pothole
Figure 2: Suspension component forces over a speed bump and pothole

While this example has had the focus of suspension evaluation, the same models can be used for other areas. Possibilities include: drivetrain shock analysis with the loss and gain of traction while under power with a higher fidelity driveline, passenger comfort by looking at the body motions, drivability by looking at the steering input/force feedback and the accelerator requirements.

All of these evaluations are key to gain an understanding of ability and to predict lifetimes of components as early in development as possible.

Written by: David Briant – Project Engineer

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