As tire engineers, understanding how rolling resistance is measured is crucial for design, development, and benchmarking. While both SAE J2452 and ISO 28580 aim to quantify this critical performance characteristic, they approach it from different angles, making one more suitable than the other depending on your objective. This Q&A article with Technical Director, Cliff Hodges, will break down the key differences, ideal use cases, and advantages of each standard.

What's the fundamental difference between SAE J2452 and ISO 28580?

The core distinction lies in their testing methodologies and the conditions under which measurements are taken:

• SAE J2452 (Coast-Down Method): This is a dynamic, transient test. Imagine a vehicle coasting down a hill – the tire isn't at a constant speed or temperature. J2452 mimics this, measuring rolling resistance as the tire's speed decreases. The tire is not in thermal equilibrium during the measurement, though the rapid nature of the test means it operates under roughly isothermal conditions. Its primary intent is to provide data for vehicle simulations and fuel economy modeling.

For both passenger car and light truck tires, SAE J2452 specifies a matrix of load and inflation pressure conditions to be tested:

• Passenger Car Tires: Involves four test steps, varying the load from 30% to 90% of max, and adjusting inflation pressure relative to a base pressure (e.g., +10 kPa, -40 kPa, +60 kPa, -40 kPa).
• Light Truck Tires: Involves five test steps, with loads ranging from 20% to 100% of max, and specific inflation pressures given as a percentage of max (e.g., 110%, 50%, 100%, 60%, 100%).

These multiple points capture a range of real-world operating conditions and also challenge various parts of the tire (e.g. sidewall, tread, reinforcements, etc.) to provide.

• ISO 28580 (Single-Point Test Method): This is a steady-state test. The tire is run at a specific, constant set of conditions (load, pressure, speed) until it reaches thermal and structural equilibrium. Only then is the rolling resistance measured. It's a "single-point" test because it provides one definitive measurement at a precise operating condition. This standard is heavily focused on comparability and international standardization.

Why does thermal and structural equilibrium matter, and how do these standards handle it?

Tire temperature significantly influences rolling resistance. A hotter tire generally exhibits lower rolling resistance due to changes in material properties. Also, the tire's construction break-in is critical for true energy loss measurement. We see structural break-in in longitudinal force (Fx).

• SAE J2452: Because it's a dynamic, quick test, the tire doesn't fully stabilize thermally. While this makes it relevant for transient vehicle behaviors, it means the result isn't a "fixed" value at a specific temperature.
• ISO 28580: This standard specifically requires the tire to reach thermal equilibrium before measurements begin. This ensures that the measured rolling resistance is a consistent and repeatable value at a stabilized temperature, making it ideal for comparisons.

What are the main use cases for each standard?

Your project's goals will dictate which standard is more appropriate:

• Use Cases for ISO 28580:
  • Tire Labeling and Regulatory Compliance: If you're designing tires for markets with rolling resistance labeling requirements (like the EU), ISO 28580 is the standard to follow. Its focus on comparability ensures fair and consistent ratings across different manufacturers.
  • Inter-Laboratory Comparisons: When you need to confidently compare rolling resistance data between your lab and a supplier's lab, or between different testing facilities, ISO 28580's correlation procedures (often using Standard Reference Test Tires - SRTTs) are helpful.
  • Benchmarking and Competitive Analysis: For understanding where your tire stands against competitors in terms of energy efficiency under a defined, repeatable test condition.
  • Quality Control: To ensure consistent rolling resistance performance during tire production.
• Use Cases for SAE J2452:
  • Vehicle Performance Modeling and Simulation: If you're a vehicle OEM or a tire engineer working closely with vehicle dynamics, J2452 data is directly applicable to simulating vehicle fuel economy, range for electric vehicles (EVs), and coast-down characteristics.
  • Understanding Dynamic Tire Behavior: For research into how rolling resistance changes with speed during transient events, providing a more "real-world" dynamic picture than a single steady-state point.
  • Tire Contribution to Vehicle Force: When you need to quantify the tire's specific contribution to the overall resistive forces acting on a vehicle.
  • Early Design Stages: To inform tire design parameters based on their expected dynamic rolling resistance performance in a vehicle.

What other variables could be considered for research studies?

Given the difference in range performance for EVs at cold temperatures, one of the natural questions for tire engineers is to try to understand how rolling resistance changes in colder temperatures as well. Smithers has introduced a cold rolling resistance machine designed to understand these effects on tires tested to either SAE J2452 or ISO 28580. In initial studies, tire rolling resistance was found to be more sensitive to cold temperatures and did not perform as well compared with higher temperatures.
Another variable that could be introduced into custom studies is a different surface on the drum. While standard tests call for a smooth, steel surface, different grits and coatings could be used on the drum to attempt to mimic various road surfaces in an attempt to understand any specific differences in tire performance.

While both SAE J2452 and ISO 28580 measure rolling resistance, they serve distinct purposes. If your goal is standardized, comparable, and globally recognized ratings (especially for labeling and regulatory compliance), ISO 28580 is your best choice. If you need to understand the dynamic behavior of rolling resistance for vehicle performance simulation and design optimization, SAE J2452 provides more relevant data.

Our experts can work with you and your team to better understand your testing and development goals and choose the right program for your needs. Contact us today.