Tire Rolling Resistance Part 2: Defining Rolling Resistance(Lea en español)
Tire rolling resistance is defined as the force required to maintain the forward movement of a loaded pneumatic tire in a straight line at a constant speed. And just like the laws of physics and forces of nature, it is an obstacle every vehicle has to overcome to transport passengers and cargo to their destinations.
Tire rolling resistance is caused by the natural viscoelastic properties of rubber along with the tire's internal components constantly bending, stretching and recovering as they cycle between their loaded (where the tread footprint flattens against the road) and unloaded states. The final contributor to tire rolling resistance is the tread's interaction with the road.
"Tire rolling resistance has an impact on vehicle fuel consumption estimated to range from about 4% during urban driving to 7% during highway driving."
The tread area represents a new tire's single largest and heaviest region and is the greatest contributor to tire rolling resistance. The tread and its underlying plies typically account for about 2/3 of a new tire's rolling resistance, while the sidewall and bead area represent the remaining 1/3.
Larger tires require more rubber and longer reinforcing cords than smaller tires. Therefore within a single tire model line, there is typically a relationship between tire size, weight and the resulting rolling resistance force where larger tires have more rolling resistance than smaller tires.
The most common laboratory test measures the force required to rotate a tire at 50 mph against a large diameter steel drum. Multiple samples of each tire size/model are tested to establish an average rolling resistance value. And since tire rolling resistance typically declines moderately as tire temperatures rise from cold to normal operating conditions during the first 30 minutes of driving every time the vehicle is used, values are recorded in the laboratory after operating temperatures and rolling resistance values stabilize.
Tire Rolling Resistance Force is measured in pounds or kilograms of resistance. Comparing Rolling Resistance Force provides a direct way to compare tires of the same size, as well as offers an accurate means of comparing differently sized tires to one another.
Tire Rolling Resistance Coefficient is calculated by dividing the measured rolling resistance force by the tire size's prescribed load during the test. Comparing Rolling Resistance Coefficients only allows comparing tires within a single size. Tire Rolling Resistance Coefficient does not allow comparing different sized tires.
As noted earlier, larger tires generate higher Rolling Resistance Forces than smaller tires. However the larger tire's greater overall diameter, circumference and rolling radius allows its tread area to bend, stretch and recover more easily as it cycles in and out of contact with the road. Larger diameter tires also revolve fewer times per mile and cycle at a slower rate than shorter tires for any given speed. While this essentially reduces Rolling Resistance Coefficient (larger tires will often have a lower Rolling Resistance Coefficient than smaller tires), it still does not change the fact that a larger tire actually generates more Rolling Resistance Force that the vehicle's engine has to overcome.
Unfortunately comparing tire Rolling Resistance Coefficients is somewhat like comparing the fuel efficiency of an 8 passenger, 15 mpg sport utility vehicle to a 4 passenger, 30 mpg car based on miles per gallon per passenger when fully occupied. While both vehicles offer the same miles per gallon per passenger fuel efficiency when fully occupied, the sport utility vehicle will always use more fuel than the car (as well as more fuel per passenger anytime the sport utility vehicle is driven below its maximum passenger capacity).
Regardless of their calculated Rolling Resistance Coefficients, a large tire generating 30 pounds of Rolling Resistance Force will require more energy to roll than a small tire generating 15 pounds of Rolling Resistance Force.
Tire rolling resistance has an impact on vehicle fuel consumption estimated to range from about 4% during urban driving to 7% during highway driving. The engine and driveline is estimated to consume 80% of the fuel, while the remainder is used to overcome inertia, wind resistance, converted into heat by the brakes or consumed when the vehicle is idling. The automotive industry estimates a 10% reduction in tire rolling resistance will result in a one to two percent improvement in vehicle fuel economy. While that might not seem like a lot, it can reduce fuel consumption by a couple of tanks per year and make the purchase of lower rolling resistance tires a better value over their lifetime.