Tyre wind tunnel simulation

Wednesday, October 23, 2013

Tags :  testing

Formula One cars are probably the most definitive example of detail engineering taken to the nth degree anywhere in the world. This is not through choice for the engineers involved in designing the cars but a necessity in order to remain competitive under a stable regulation regime. Without the scope to introduce big changes to components, they instead have to optimise the areas they can develop in pursuit of tiny percentage gains.

A close look at the floor of cars such as the 2013 Red Bull or Ferrari bears out this approach, revealing a plethora of tiny flicks and winglets, each deemed to have produced some gain in performance in the wind tunnel. However, in order to ascertain whether such subtle changes are effective, the correlation between results obtained with a 50 or 60% scale model, CFD simulation and the car on track must be precise. This means that as many factors that dictate aerodynamic performance on the full-scale car must be replicated on the scale model, including the tyres.

Producing scale-model tyres that behave in the same way as their full-sized counterpart is not easy. The high sidewall tyres used in Formula One present some interesting challenges in this area due to the way they deform under load, changing their aerodynamic characteristics. Accurately modelling these changes in the wind tunnel can be beneficial to improving overall aerodynamic performance. In order to achieve this accuracy, teams and tyre suppliers have gone to great lengths to produce scale wind tunnel tyres that can replicate these changes and methods of testing that best replicate on-track performance.

In the past, scale models were run with solid composite or aluminium ‘tyres’ that produced repeatable results but did not accurately mimic the aero effect of a real tyre. It is only relatively recently that tyre manufacturers have begun to produce scale-model tyres that represent their full-scale counterparts.

Producing these tyres is not simply a case of scaling down the construction of a full-scale item, as the forces a tyre is subject to in the real world differ considerably from those found in the wind tunnel. For example, if the construction of a full-scale tyre were to be reproduced in scale form, the resulting tyre carcass would be far too stiff and would not create the required deformation. Also, whereas a ‘real’ tyre is optimised to provide maximum grip, this is not desirable in a wind tunnel where the model may be required to yaw from side to side across the tunnel’s rolling road. Thus the scale-model tyre needs to be as low grip as possible.

In order to generate the required vertical loads to get the tyre to deform, teams can use various systems of actuators in the scale model’s suspension to load the tyre against the rolling road floor. However, these loads cannot be too high as this would be detrimental to the life of the tunnel’s rolling road belt, which is normally made from either steel or fabric. To counter this, model tyres will be run at very low inflation pressures to reduce the loads needed to cause sufficient deformation.

While it is one thing to make the tyre deform, it is another to ensure it is deforming in the right way. This means the deformation needs to be measured, and one way this can be achieved is by using stereoscopic cameras. The images taken are processed using software that identifies the location of specific pixels from frame to frame, and the level of deformation at different loadings can be measured and compared to data provided by the manufacturer for the full-scale tyre.

While all of this effort may seem excessive given the relatively small difference accounting for changes in a tyre’s shape has on the overall aerodynamic package performance, in a sport where performance between rivals can sometimes be measured in thousandths of seconds, it is an expenditure in time and resources that teams feel is justified.

Written by Lawrence Butcher

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