For most of my childhood, I thought the most interesting thing about tires was that they had little whiskers of rubber sticking off of them (they are called “sprues,” by the way, and are a byproduct of the manufacturing process). As it turns out, tires are fascinating, and should be considered far more than an afterthought. They are the critical link between the car and the road.
Every ounce of force, whether propelling you off the line or helping you to clip an apex in a corner, is routed through these rubber wonders. What we focus on today is high-performance tires: How they are different from low-end tires, how vulcanization isn’t something to do with the Roman god of war, and how they can be, dollar for dollar, one of the best performance investments you can make.
Before we begin, let’s cover vulcanization, which happens to be one of my favorite automotive words. Vulcanization is the process of heating rubber and adding other materials to change its structure. Think of a three-legged race: The two people represent the polymer chains in rubber and the material that is added (often sulfur) is the tie that connects them. Without the connection between their legs, they could walk in different directions and never return. When they are tired, they can separate somewhat, but they will have to return to their original shape (mostly).
Vulcanization is an irreversible process that changes the rubber at a structural level. These changes are what allow tires to be developed to handle high speeds. To start our discussion, let’s look at the side (or sidewall) of a tire. The thing that I want you to focus on is the “Speed Rating.” It should look like this:
This rating corresponds to this chart from Tire Rack. The reason for this rating system is that performance tires experience extremely high stresses at speed. Exceeding the rated speed risks failure of the tire (in the case pictured, the maximum rated speed is 130 miles per hour).
And what’s the biggest bane of tires? Heat. Tires hate the heat more than an Orlando Magic fan. Increasing speed leads to increasing heat due to friction. There is friction between the tire and road, as well as friction that is internal to the tire as the rubber flexes. If the heat within the tire increases enough, the rubber, the grip, and the adhesion of the rubber to the steel belts can break down.
As the rubber heats up, it loses its ability to grip the road surface. “But, Owen!” you wonder. “Race cars intentionally heat their tires before races so how can that be true, you idiot?!” Well, when tires are heated, their material becomes sticky and flexible…up to a point. They have an optimal operating temperature range. Above (or below) that, the material will start to degrade during use, and grip will be affected.
Rubber is less capable of absorbing force when it is heated, and it is more likely to deform. As it starts to deform, wear increases and there will be more internal friction, which adds more heat and accelerates failure. Furthermore, as a tire is heated to extreme levels, the rubber will begin to change its shape and can lose its adhesion to its steel body plies, which provide the structure.
If the adhesion fails, the tire rubber will separate from the piles — this is the source of “road gators.” If you’ll notice, you see many more of them in the summer time due to the increased ambient and pavement temperatures. The pressure in the tire can increase (due to the ideal gas law) to the point that the structural integrity of the tire is affected by stressing the piles from the inside out. An increase of pressure will also reduce the contact patch of the tire, which further affects handling.
Performance tires combat the issues of heat by making tires wider. This increases the contact patch, therefore reducing sidewall height, which reduces flexing of the tire under load, and reducing tread depth, which minimizes the thermal mass that holds heat by reducing the total rubber in the tire. All of these factors combine to provide a more consistent and safe performance in high-speed situations.
Additionally, an increased grip and wider contact patch will help you apply all your torque effectively. Just like reducing the sidewall height limits tire flex, improving your tires means that improvements made to your car will have a more direct impact on its performance.
Given its top speed of 267 miles per hour, the Bugatti Veyron is an excellent example of advanced tire technology. Michelin developed special tires for the Veyron; they can last up to 10,000 miles, but they can only last for about 15 minutes at top speed. Luckily, you would run out of fuel (and road) before that happens, but it’s interesting to consider.
Also interesting is that each set of tires costs as much as a BMW i3 — roughly $42,000. Considering that they need to be able to withstand speeds in excess of 250 miles per hour while still providing excellent grip, ride, and durability, it’s no surprise that they are pricey. The world of tires is continually evolving and there are some interesting concepts on the horizon.