Introduction

The topic of rolling resistance is important for understanding its impact on fuel economy and exploring potential avenues for improvement. A study by a leading vehicle manufacturer revealed that tires contribute to 17% of a vehicle's fuel consumption. This figure, based on the EPA test for determining fuel efficiency, encompasses several factors: rolling resistance, weight contribution towards accelerating and decelerating the vehicle, the angular inertia involved in these processes, and the aerodynamic effects of spinning tires.

While 17% is significant, the potential for meaningful improvement is limited. Even a substantial 20% reduction in rolling resistance would only yield a 3.4% overall reduction in fuel consumption—a change that is barely detectable through conventional methods of tracking odometer readings and fuel usage.

Example Energy Flows

For a late-model midsize passenger car, energy flows during urban and highway driving are significant. According to the U.S Department of Energy, most rolling resistance is caused by the internal friction of the rubber as it moves through the footprint area, which is measurable in the form of heat generation. However, accurately quantifying this heat is challenging. Therefore, special test fixtures, such as dynamometers used in EPA fuel economy tests, measure the energy consumed as a tire rotates.

Example energy flows for a late-model midsize passenger car: urban driving, highway driving. Source: U.S Department of Energy

Various methods exist for testing rolling resistance, all yielding similar results in terms of relative values between tires, though they produce different numerical results. Until a common, standardized test method is established, tire manufacturers will not publish rolling resistance values. 

Factors Influencing Rolling Resistance

Rolling resistance is primarily caused by the internal friction of the tire. Several conclusions can be drawn from this:

• Tire Deflection: The more a tire deflects, the higher the rolling resistance. This relationship is largely influenced by the load versus inflation pressure. Increasing inflation pressure reduces deflection, but the effect diminishes at higher pressures. For example, adding 1 psi over the recommended placard pressure has a more significant impact than increasing from 5 psi to 6 psi over the placard pressure.

• Tire Material: The amount of material, especially in the tread area, affects rolling resistance. New tires typically have higher rolling resistance than worn-out ones due to the additional material. Consequently, when purchasing new tires, a temporary decrease in fuel economy should be expected. Additionally, all-terrain, winter, mud and snow, and off-road tires have more rolling resistance compared to comparable all-season tires. Higher speed-rated tires also exhibit higher rolling resistance due to their additional cap plies.

• Tread Compound: The tread compound's internal resistance to movement impacts rolling resistance. There are three interrelated properties—tread wear, traction, and rolling resistance. Optimizing one of these properties usually involves sacrificing at least one of the others. Tires with high traction compounds often sacrifice tread wear or rolling resistance. Similarly, tires with high UTQG treadwear ratings may compromise on traction or rolling resistance.

Effect of Inflation Pressure on Rolling Resistance

Vehicle Manufacturer Priorities

Vehicle manufacturers are aware of the "triangle" effect in tire design. Since the EPA fuel economy rating displayed on the vehicle window doesn't account for traction and tread wear, manufacturers often prioritize low rolling resistance to achieve better fuel economy ratings. This typically results in moderate traction and poor tread wear, and occasionally poor traction as well.

Poor traction and tread wear are often cited as evidence that original equipment (OE) tires are of poor quality. However, this perspective overlooks the manufacturer's optimization strategy aimed at balancing fuel efficiency and performance. This balancing act by manufacturers highlights the importance of the materials used in tires. One critical component in this regard is the tread compound.

The Role of Tread Compound in Rolling Resistance

Rubber, in its natural state, is not black. The black color comes from carbon black, a primary reinforcing agent in rubber. Carbon black is essentially pure carbon, similar to coal but without impurities. As the largest component in the rubber mix aside from the rubber itself, carbon black is important due to its chemical compatibility with hydrocarbons.

Carbon black has been used for hundreds of years and is integral in various applications beyond tires. The specific type of carbon black used can significantly influence the tire's properties. For instance, fine-grained carbon black is ideal for ply skims, while large-grained, highly structured carbon black enhances tread wear but increases internal friction.

Silica as an Alternative

Silica, an engineered form of sand, is sometimes used as a partial substitute for carbon black. Unlike ordinary beach sand, this highly engineered silica can improve rolling resistance without compromising tread wear or traction, provided the compound is adjusted accordingly. These adjustments can also enhance traction and tread wear further.

Silica's benefits stem from its ability to reduce internal friction within the tire, leading to lower rolling resistance. This innovative material allows tire manufacturers to create tires that are both fuel-efficient and durable, making it a popular choice in modern tire design.

Practical Considerations for Consumers

When shopping for tires with good rolling resistance to improve fuel economy, the focus should be on overall value. A tire with excellent tread wear offers better long-term savings as fewer replacements are needed. Maintaining optimal rolling resistance is best achieved through proper inflation. Regularly checking tire pressure and maintaining it slightly above the placard specification—3 to 5 psi—is recommended. This approach improves fuel economy without significantly compromising traction or safety. However, it is crucial not to exceed the maximum inflation pressure indicated on the tire's sidewall.

It is important to clarify that the recommendation to slightly increase tire pressure is not an endorsement of excessively high pressures. A car with slightly higher tire pressure may offer crisper steering and improved fuel economy, but the pressures should always stay within safe limits. Specifically, they should not exceed the maximum pressure indicated on the tire sidewall, except for certain tires with specific ratings. With these practical tips in mind, let's address some of the most frequently asked questions about tire rolling resistance.

FAQs

Most frequently asked questions about tire rolling resistance:

• Q: How do seasonal tire changes impact rolling resistance?
  A: Seasonal tires are designed with specific conditions in mind. Winter tires, with their deeper treads and softer rubber compounds, generally have higher rolling resistance compared to summer tires. This is because they are engineered to provide better grip in cold conditions. Switching to appropriate tires for the season not only ensures safety but also helps manage rolling resistance and fuel efficiency effectively.
• Q:  Can tire rolling resistance vary between identical models under different conditions?
  A: Absolutely! Even tires of the same model can exhibit different rolling resistance depending on external factors. For instance, road texture plays a crucial role—smooth asphalt will yield lower rolling resistance compared to rough, uneven surfaces. Additionally, ambient temperature affects tire flexibility; colder conditions can make tires stiffer, increasing resistance, while warmer temperatures may lower it. These variations underline the importance of context in tire performance.
• Q:  How do eco-friendly tires achieve low rolling resistance?
  A: Eco-friendly tires achieve low rolling resistance through innovative design and materials. They often feature advanced silica compounds that reduce energy loss. Additionally, these tires have optimized tread patterns that decrease friction while maintaining safety and performance. Some eco-friendly tires are designed with lower weight, which further reduces rolling resistance. These green innovations contribute to better fuel economy and lower carbon emissions.
• Q:  Can tire rolling resistance vary with altitude?
  A: Absolutely! Altitude affects air pressure, and this can influence tire rolling resistance. At higher altitudes, the lower air pressure means less air resistance, but the tires might need to be adjusted to maintain optimal performance. Interestingly, the tire's interaction with the road can also change, making altitude a surprising factor in rolling resistance.
• Q:  How does the concept of airless tires fit into the future of rolling resistance?
  A: Airless tires, also known as non-pneumatic tires, represent a futuristic approach to minimizing rolling resistance. These tires use flexible structures to maintain shape without air pressure, eliminating the risk of punctures and reducing maintenance. The design inherently reduces rolling resistance by maintaining consistent contact with the road and improving energy efficiency. As technology advances, airless tires could revolutionize both passenger and commercial vehicles.

We've only begun to scratch the surface of car safety in this discussion. For a more thorough professional exploration of automotive maintenance and safety, our website houses an extensive array of informative articles.