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If you were to travel in a car built twenty years ago, you would be amazed by the level of noise. Major improvements have been made since then, so that now a quiet cabin is no longer an exclusive luxury. This shift has put pressure on manufacturers to ensure that new models continue to meet the increasingly stringent expectations of the car-buying public.Noise inside the cabin of a road vehicle arises from three main sources: the engine, the surrounding airflow, and the interaction between tyres and road. The 'road noise' from the latter reaches the cabin in two ways. The first is airborne transmission, where sound radiated externally by tyre vibrations can be heard in the cabin. The second is structure-borne transmission. Here vibrations pass from the tyre to the vehicle suspension, and thence to the car structure, again generating noise in the cabin. This project aims to develop calculation techniques that will help engineers predict, and improve, the structure-borne road noise characteristics of new car designs.Such a complex problem must be broken down into simpler components. In this case, one can consider the wheels, vehicle structure and passenger cabin separately. The first two are linked by the unsteady force exerted by the wheels on the hubs, and the second two by the vibrations of the cabin. Vehicle manufacturers already have methods to analyse the generation of structural vibrations by unsteady hub forces, and their conversion into cabin noise. However, calculation of the hub forces, depending as it does on the properties of the tyre and the suspension, is a topic that falls between tyre and car companies. It has consequently received less attention. A related problem has, though, been addressed by our group when studying exterior road noise. Here it is the vibrations of the tyre surface that matter, but one similarly needs to be able to predict its behaviour as it rolls along a road. The techniques used are thus directly applicable to hub force prediction.To predict the tyre vibrations and hub forces caused by rolling on a road, we will produce two computer programs, one to calculate the tyre and hub force responses to a general form of forcing known as an 'impulse', and one to calculate the specific contact forces and response as the tyre rolls on a given road surface. The advance calculations performed by the first program will greatly improve the numerical efficiency of the second, which would run too slowly for practical use without them.Next, our programs must be tested against experiment to assess their accuracy. This process will be carried out in collaboration with our industrial partners, Goodyear and LandRover. We will test our tyre/hub force impulse response program by attaching shakers to a stationary tyre, applying a known forcing, and measuring the resulting vibrations and unsteady hub forces. The rolling simulation will be compared with two experiments. The first will assess its success in predicting tyre vibrations, by measuring the accelerations of the tyre belt when it is run on a rolling road test rig. The second will address its prediction of hub forces. Direct measurement of the forces on a rolling hub is fraught with practical difficulties, so we will instead measure axle vibrations, and from these deduce the corresponding hub forces.Once the testing is complete, Goodyear and LandRover will have gained the ability to predict the hub forces generated by a tyre rolling on any road surface. This will enable Goodyear to improve tyre designs, and LandRover to design quieter cars more efficiently. Furthermore, the impulse response program, as well as feeding directly into this process, will also provide an alternative to existing, proprietary tyre vibration calculation techniques. This will be a particular benefit to LandRover, who currently have access to such methods only through the results provided to them by tyre manufacturers.
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