Sometimes, I actually think of something and am cheered up by the novelty of it. Back when Valentino Rossi was first on Bridgestones, there was a big hoopla about how, because of their big, heavy, stiff casings, these tires were extra-stable and needed to be heavily loaded by weight transfer to get the best out of them. And there was Colin Edwards, saying how amazed he was at how the tires just kept gripping the more load he’d put on them.
None of this makes sense because, in the past, the way things have worked is that a company makes its tire carcass softer so it then spreads out a bigger footprint, which engineers can then exploit by using softer tread rubber and so on. For example, each time Dunlop made its car-racing tire carcass softer—the first time was when it reduced the number of plies in the tire by switching from cotton to nylon—it got a bigger contact patch because the softer carcass could more easily spread out to make a big footprint.
It happened again in 2006 with Michelin’s response to Honda’s ’05 moaning for more traction. Michelin reduced inflation from 1.4 bar (20.3 psi) down to 0.9 to 1.1 (13 to 16 psi), and the result was a tire with such an exaggeratedly large footprint that its ensuing chatter knocked Rossi out of the early running in 2006.
What I think is actually happening is that the difference has nothing to do with the casing; it is the change in the basic nature of tread rubber. With Michelin’s or any conventional rubber, as you load the tire more and more, it grips more and more until some critical level is reached, after which further increases in load begin to produce less grip. This is called something like “stress saturation.” What’s going on is that the rubber gives more grip as it is pushed more intimately into the pavement texture, creating an increasingly larger area of true contact. But at some point, the limit of the rubber’s tensile strength is reached and grip starts to go away.
The tensile strength of conventional rubber derives partly from its vulcanization (sulfur bonds between rubber chains) and partly from short-range forces between rubber chains and the tiny particles of carbon black in the mixture. Now, along comes Bridgestone with a discovery of how to supplement the carbon black with ultrafine silica and of a method of making actual covalent chemical bonds between the rubber chains and the silica particles. With the extra strength this gives (but without making the rubber harder, as would happen with “harder vulcanization”), you can go on loading up the tire and getting more grip from it some distance past the point at which conventional rubber would be giving up in stress saturation. The bonding medium is the phrase I was reciting at the Indianapolis GP this year: “bi-functional silane coupling agents.”
Anyway, now I can stop pondering how making the tire carcass stiffer can improve traction. It doesn’t. It can’t. What’s doing the job here is B’stone’s unique rubber, which combines the softness to fill the pavement texture very intimately with rubber and the tensile strength to keep the rubber from breaking under the great stress of such high grip force.