Road unevenness relation to road safety - a vehicle dynamics study

Abstract: The purpose of this Master thesis in Vehicle Engineering, is to study the road unevenness relation to road safety. The long term objective is to be able to prioritize which road section that is in the need of repair and maintenance prior to other road sections. This study focus on how close to an acceptable safety limit the vehicle is handled when it is run over different road surfaces. This applies to straight road sections as well as cornering, where the road surface is uneven and bumps/pits occurs. No driver behaviour or random actions are analysed but these aspects will be included in the overall discussion. The method to analyse this is through computer simulation. From a Volvo S40 a computerised vehicle model has been developed in Matlab and the effect of different road unevenness has been implemented and analysed. Forces that are generated by the unevenness of the road are compared with the normal forces that a driver needs to correct the course based on the friction between tire and road surface. On this basis, a margin to the risk of losing the grip can be estimated. In this way it can be interpreted how a road section contributes more or less, compared to another section, to whether the vehicle is closer to a safe limit from a vehicle dynamic perspective. The vehicle model has been analysed at a speed of 70 km/h with the simplification that the irregularities can be described by sinusoidal shapes. For larger bumps or dips in the road the results show that both front and rear tires can absorb side forces so that stability can be achieved. If the grip would deteriorate due to gravel, ice, etc. there is a risk that the vehicle loses steering control and/or cord leading to damage of the tyre and consequently an accident will occur. For the analysed road unevenness in the form of bumps and pits the tires do not have any ability to absorb required side forces during an avoidance manoeuvre when travelling over the road due to the tyre model used. It is therefore important that a section with varying unevenness are analysed to determine a maximum speed so that the control of the vehicle during the whole distance can be maintained regardless of whether control needs to be done in connection with the unevenness. A recommendation of future work in this area is to develop this model to make it more robust and to update the input data with relevant data for one today representative car and to carry out a more detailed full-scale modelling with also lateral simulations. If the model was verified with measured normal forces for a test car that has travelled over various bumps and pits, this would also be valuable to confirm the validity of the model. There would also be improvements if available road profile is implemented in the analysis so that realistic examples can be analysed for better real-world analysis.