World Superbike: The Payne Effect

Pirelli tire engineer talks about “the step.”

Pirelli Tires Promotional Shot shot

How many times have I carefully prepared questions for tire engineers and received only information-free replies that would do credit to any presidential hopeful?

My experience with Pirelli has been entirely different. A long relationship with motorcycle race-tire manager Giorgio Barbier has greatly boosted my understanding of tire technology, and recent conversations with compound engineer Fabio Meni have provided wonderful specific information. Last year I asked Meni what determines the temperature range in which a given compound operates best. He replied that it is the mix of compounding oils and waxes added to the rubber.

Early in the rubber business, engineers found it almost impossible to mix the necessary reinforcing carbon black into uncured rubber by itself, but the addition of so-called mixing aids or “extender oils” greatly eased mixing. Why not just mix the ingredients harder? Mixers are already driven by electric motors with thousands of horsepower, and the harder you mix rubber, the hotter it becomes and starts to vulcanize right in the mixer.

Friction between surfaces is actually generated by zillions of tiny “welds” that form between the two. As sliding occurs, each weld forms, is stretched until it breaks, snaps back, and is remade farther along in a cyclic process that takes place at extremely high frequency. If the rubber is too immobile (extender oil viscosity too high), it cannot make, break, and remake enough bonds rapidly enough to generate maximum friction. This is why rubber lacks grip on lap one. Increasing the rubber’s mobility by use of less viscous extender oil moves the point of peak performance downward in temperature.

Sylvain Guintoli race action shot

Riders constantly talk about how their tires “take a step down” after a few laps, so I asked Meni what physical process in the rubber is responsible for this perceived drop in properties. “This has a name,” he began. “It is called the Payne Effect.” A.R. Payne was a British rubber researcher whose work on this subject was first published in 1962.

“In the compound,” Meni continued, “the carbon-black particles are not present as separate entities but exist as aggregates—clusters of particles. As the tire is put into service, the high strains to which it is subjected have the effect of breaking up these aggregates over time, and this alters the rubber’s properties.”

Meni went on to say that it is not so much that the tire loses grip as it feels different to the rider. This is “the step” that the rider feels after a few laps, after which the tire’s properties may change little through the rest of the race.