Stick to the road

Want to now how well a tyre made of a certain rubber mixture will grip the road, without actually having to manufacture the thing? Well, Dr. Bo Persson of the Institute of Solid State Research in Jülich, Germany has developed a new theory that makes it possible to calculate and predict how to do just that.

Tyre companies test thousands of mixtures of the most varied types of rubber in order to develop new car tyres. Test tyres are then manufactured from these rubber mixtures in order to test their properties on asphalt. With his new theory of static friction, Bo Persson can make an enormous reduction in the long test series.

‘All the information I need for my calculations is how elastic a small rectangular sample of rubber is and how well it cushions jolts,’ said Persson. ‘In 2001, we first calculated ten new test components and then sent them off to be tested in practice. The results of theory and practice are in very good agreement.’

The decisive aspect for the success of the new theory is that Persson can calculate the area of contact between the rubber and the asphalt more precisely than has previously been the case.

One of the major effects to be calculated is the ability of the rubber to cushion jolts. In physical terms, this ability is based on the fact that the material is capable of storing energy.

If a soft material is pressed onto a hard uneven surface it is not uniformly loaded. Stones and elevations in the asphalt dent the tyres.

Wherever normal tyre material is dented, energy is accumulated and at a different location the material relaxes. Persson has found a mathematical description for the irregularities of the asphalt, which calculates them down to the atomic level. In addition to the irregularities of the road, the internal frictional forces of the material must also be taken into consideration.

A middle course is being sought between optimal adaptation to the road – a soft, clinging material – and the most durable possible tyre that doesn’t go to pieces when the brakes are applied.