Vital to a tire’s creation, it transforms polymers and additives into a resilient product able to withstand torturous miles of highway driving.
Destructive to a tire’s lifecycle, it breaks down rubber compounds and affects vehicle handling.
Heat. A tire can’t live without it, but can be destroyed by it.
The typical vulcanization method for tire production uses heat to produce a chemical reaction between materials including natural and synthetic rubber, sulfur and other chemicals. Considered irreversible for commercial purposes, the process cross-links long polymer chains to prevent them from moving independently in the chemical matrix. The result is a durable tire that distorts when subjected to a force and returns to its original shape when the force is removed.
That’s the good news.
As a tire rotates under the weight of a vehicle, it repeatedly deforms and recovers, generating lost energy called hysteresis. As the energy dissipates, heat builds up in a tire. If a tire is subjected to an increased level of distortion or the deformation occurs at a greater frequency, the heat buildup will escalate and begin tearing apart what it helped create.
Tire companies often demonstrate the effect of high- and low-hysteresis rubber compounds by using two rubber balls that appear to be identical. The looks are deceptive, which becomes apparent when the balls are dropped from identical heights at the same time.
The first ball bounces high, having a very elastic, low-hysteresis rubber compound reminiscent of a super ball. After striking the ground, it rebounds and relinquishes a majority of the energy it absorbed. The second ball, with its high-hysteresis rubber compound, comically refuses to bounce, absorbing much of the energy, which indistinguishably heats up the rubber.
Synthetic rubber compounds tend to possess high-hysteresis values, while natural rubber has a low-hysteresis value. In tires, high-hysteresis compounds generate increased friction for improved grip and traction, while low-hysteresis compounds provide cooler operating temperatures. Race tires contain more synthetic rubber; truck tires have more natural rubber.
Designed for Heat
Heat generation within a truck tire is controlled mainly by the hysteretic loss characteristics of materials used in the tire, according to Paul Grosskopf, technical director for Michelin North Americas’ truck tire unit.
“Just like other tire performance characteristics such as wear, traction and rolling resistance, temperature buildup within a tire is part of a tiremaker’s design specifications that are checked and controlled,” says Grosskopf.
“By optimizing various design parameters under normal operating conditions, excessive heat buildup is avoided, which helps to ensure the long-term integrity of a casing throughout its entire life, including retreading.”
He says there are more than a dozen different compounds used in the construction of a truck tire.
“Each compound used in a tire is formulated to optimize the overall tire performance, including heat generation,” he says. “For example, a sidewall compound needs to have good abrasion, tear and long-term cracking resistance, as well as low hysteretic loss. As a tire deflects, the hysteretic loss generated by compounds is the main source of heat generated.”
A tire’s tread also can play a role in heat buildup.
Grosskopf says the choice of tread patterns and compounds influences many critical tire performance considerations. “Wear, traction, rolling resistance and heat buildup in the crown of a truck tire are all impacted by these design parameters,” he says. “An appropriate tread pattern and compound need to be developed for each specific application.”
Construction and fabrication variables in a tire’s design need to be managed in order to ensure that heat buildup is controlled to an acceptable level under normal operating conditions, he adds. “This includes ensuring that correct materials are used and a truck tire’s size, shape and weight are created within precise tolerances.”
Controlling heat buildup in tires goes beyond the design and manufacturing processes. Fleet operators need to take responsibility for proper care and maintenance of their vehicles’ tires, which opens the door for tire dealers to further educate customers.
“Critical parameters regarding heat buildup in tire casings also include driving conditions and maintenance,” cautions Grosskopf. “The more a tire deflects, the higher the hysteretic loss and the higher the heat buildup. This means that a tire that is overloaded or underinflated for the load it’s carrying will generate more heat. In addition, running at higher speeds or at higher ambient temperature will also increase heat buildup.”
Increased tire temperatures are the principal cause of rubber degradation and can lead to fatigue cracking, belt separation, tread block tearing and chunking. It can affect a tire’s air permeability, which can cause underinflation. At the same time, it’s been discovered that air permeating into a tire’s rubber compound can accelerate oxidative degradation.
Ultimately, excessive heat buildup can damage a truck tire to the point that it is no longer a viable candidate for retreading, but even a retreaded truck tire must be capable of handling the heat.
Tom Brennan, vice president and director of Michelin Retread Technologies, says the ability of properly retreaded tires, even new generation wide single retreads, to run at cooler temperatures can enhance their performance and enable them to be considered for retreading multiple times.
“Choosing the right retread provider ensures that heat buildup will be addressed in the retread process and the products’ engineering and design,” says Brennan.
He said Michelin Retread Technologies utilizes co-extrusion technology to allow multiple rubber compounds to be used in his company’s retread products. A retread’s top compound is engineered to provide precise performance characteristics required by a tread design’s specific application. The bottom compound is engineered to maintain a cool tread running temperature, therefore lowering the operating temperature of a casing.
According to Brennan, his company is now able to design and provide retreads with greater tread depths. “Normally, heat would increase significantly with deeper treads,” he says, “but by using co-extrusion technology, the multiple-compound design allows our treads to deliver exceptional performance and wear characteristics, while maintaining a cool running temperature.”
He explains that Michelin’s use of more natural rubber, combined with the way the natural rubber is mixed to make finished rubber compounds, has a large impact on a tread’s ability to resist heat buildup, while maintaining expected traction and wear characteristics.
“Our retread process ensures that a casing is returned, as closely as possible, to its original architecture during the buffing process,” he says. “This eliminates stresses when a new tread is applied to the casing.”
He says any unnecessary stresses on a casing and its new tread can lead to higher running temperatures. “We ensure that the appropriate amount of under-tread is applied on each casing by measuring it during the process with a patented device.”