Tour Tech: How do rubber compounds make a tire grip?

What made stage 1 of the 2020 Tour de France so slippery? Some reports pin the blame on a publicity caravan vehicle spraying soap bubbles on the course; others say it was simply the rain, which was the first major storm Nice had seen in quite a while. Either of those, coupled with road paint (think crosswalks) had the potential to make for ice-rink-slick conditions for riders. In such conditions, riders rely on the tire rubber compounds to keep them upright, but even the best tires can’t meet all conditions. So how do tire companies approach all those variables to create a tire that riders can rely on?

It’s all about the compound.

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“Grip is defined by a mechanical part and a chemical part,” says Samuele Bressan, head of product marketing for the bicycle division of Pirelli Tires. “The mechanical grip is coming from the capability of the tire (rubber + textile of the casing) to conform over the touching surface (the footprint) at a given frequency (the higher the speed, the higher the working frequency at macro level); the chemical grip is coming from the abrasion capability of the rubber itself with that surface for the sole time of the contact (the higher the load, the higher the working frequency at polymers level _micro level).”

Got all that? Let’s break it down.

Mechanical vs Chemical

As Bressan says, two distinct facets of the rubber are responsible for grip: the mechanical, and the chemical. The mechanical part means how well the tire adheres to the road based on its footprint. In other words, hysteresis (the lag between an impact and the tire’s shape change as a result) factors in here. The better a tire can conform around an impact, the better its mechanical grip will be. But that’s not the only factor.

“Simplifying it a lot,” says Bressan, “air inflation pressure, casing flexibility, rubber static flexibility, and the ability of the rider to load and push the tire onto the ground in relation to the asphalt conditions, are all parameters that heavily influence the mechanical grip.”

The other part is the chemical makeup of the tire. When a rubber compound is devised, there are several parameters that must be met, not just grip. That means there’s a balance between a tire’s grip capabilities (the abrasion capability Bressan mentions) and other factors like rolling resistance and durability.

Bressan says that “pure chemical know-how of formulation and desired rubber performances at given weather conditions, as well as the capability to manufacture such a polymer blend into a form of a bicycle tire tread, is instead the part which is totally due to the expertise of the tire maker, and proving the chemical grip.”

What happened during Stage 1?

To put it simply, a really crazy X-factor happened. When rubber compounds are developed, engineers simply can’t compute for every factor that will influence a tire’s grip. That’s an impossible task. It would be necessary to evaluate a specific oil or other factor and how it affects a rubber compound, then make an adjustment to accommodate. From there, you’d need to do the same thing for every type of chemical a rider may encounter on the road.

Whether it was soap on the road, motor oil, or just rain-slicked road paint, every rider’s tires during stage 1 encountered a condition that the mechanical and chemical grip could only counter up until a certain point. What is perhaps surprising is that riders met those conditions so frequently throughout the stage, pointing to some sort of condition not normally encountered during a race (hence the soap hypothesis).

Even variables riders face every day can’t always be factored in. “Different tarmac surfaces already vary so much in abrasion coefficient (which we normally track and measure as much as we can, from F1 circuits down to open roads for Rally Racing — and cycling is benefiting from that too) are in the range of hundreds,” says Bressan.

So Pirelli engineers hone a tire’s purpose and range of capabilities.

“We apply certain rules when our compounders develop rubber formulations, which are the results of literally a hundred years of experience in real world usage,” says Bressan. “For example, there are proportions among some chemical components and additives that you need to avoid (or preserve) if you want the crosslinks of the polymers to sustain sudden tire temperature changes (like what happens during a skid), or a quick change in point-pressure (such as when you hit a tiny slippery surface within a high-load situation). They are general criteria and rules…which are applied across the entire Pirelli products, differentiating them by the usage of the final application.”

What about tread?

Deep treads on mountain bike and gravel tires account for a lot of cornering grip. Road tires are an entirely different beast, and we’ve even heard from certain tire manufacturers that tread patterns on road tires don’t really factor into grip at all.

Bressan disagrees; he says that tread design can factor into the mechanical component of grip, and even help with the tire’s ability to deform at the contact patch to increase grip. He likens a road tire’s subtle treads to sipes in your car’s winter tires and even on many mountain bike tires. “They are there to allow the rubber to deform, and to either conform to the ground roughness better, or increase the working temperature while doing that. Deformation equals an increase in temperature.”

So perhaps tread does indeed factor into a tire’s ability to grip in corners, but that’s much further down the list of important components of a tire’s design, and it clearly didn’t save riders from sliding out during stage 1 of the Tour de France.

In layman’s terms

It sure was slippery out there.

The best rubber compounds in the world can only counter so many variables. Whatever was causing the roads to be as slick as they were during stage 1 certainly exists outside of the X-factors most manufacturers account for in their rubber compounds.