In a recent article for Commercial Carrier Journal, William Estupinan – Vice President of Technical Service for Giti Tire USA – engaged in an in-depth discussion on the topic of low rolling resistance tires. Rolling resistance is defined as the force that resists the motion of a tire as it rolls on a surface. High rolling resistance increases drag, which can have a negative impact on a tire’s fuels efficiency. In the article below, Estupinan elaborates on the science and technology behind the development of low rolling resistance tires:
by William Estupinan – Vice President of Technology Service for Giti Tire USA
The largest expense for any trucking company is probably the cost of fuel. It accounts for 39% of all expenses. A single commercial truck can use as much as $70,000 worth of diesel, which is around 20,500 gallons. Therefore, a 20 truck fleet can save as much as $42,000 in a year by using SmartWay-verified tires.
To be informed buyers, it is important for fleets to first understand the basic technology behind fuel-efficient tires. Three main factors impact the rolling resistance in a tire: tread compounding, tread pattern design and tire structure. Tests show that over 50% of the rolling resistance of a tire is generated from the tread and belt package.
Tire companies develop tread compounding techniques to reduce the energy absorption and consequent heat generation within the tread and belt package that, at the same time, do not compromise other important factors such as durability. Low energy absorbing materials are sometimes referred to as reduced hysteresis materials in this context.
The tread pattern design is also an important consideration when trying to improve the fuel efficiency of a tire – streamlined ribs, blocks and lugs; good balance between cap and base compounds; shallower tread depth; and stiffening the belt package are all-important considerations.
Typical tread depths vary depending on the axle and from one manufacturer to another. However, as a rule of thumb, you will expect that modern fuel efficient tires for steer, drive and trailers will have reduced tread depth because the deeper the tread the higher the rolling resistance; it is just a basic law of physics.
Today, the typical tread depth of modern fuel-efficient tires is as follows (in 32nds of an inch):
Steer: 18-19. Some exceptions are: 15, 16 and 20.
Drive: most of them are 26, some 28 and very few 30
Trailer: 11, 12, 13
As a tire rolls on the highway, deep treads tend to deform and squirm more and generate more heat than shallow treads. Therefore, a shallow tread tire, like a trailer tire, typically has lower rolling resistance than a deeper drive pattern with blocks and lugs. Another advantage of a shallow tread is that the initiation and expansion of irregular wear are less likely to occur, as results of the reduced squirm.
Similarly, a worn tire tends to have lower rolling resistance than a new tire of exactly the same tread pattern.
It is also important to understand the dynamics of rolling resistance. Like air resistance, rolling resistance increases with speed. In fact, part of tire rolling resistance is the aerodynamic resistance of the tire as it moves. Even though rolling resistance doesn’t increase as fast as air resistance with an increase in speed, rolling resistance is present—and a major factor—at lower speeds. Just as with air resistance, the actual amount of rolling resistance is influenced by many factors, including load, speed, inflation pressure, tread pattern and tire design. Therefore, although fuel economy has a greater impact in high-speed long haul operations, it plays a role even in high load, on-off operations.
On a tractor-trailer combination, the steer tires contribute 15-20 % to fuel economy, drive tires 30-40 % and trailer tires about 40-50 %. Considering that trailer tires have the biggest impact on fuel economy and that trailer tires represent the biggest population in the average fleet in America, selecting the proper trailer tires for the fleet will have the biggest impact on miles-per-gallon consumed than any other wheel position. Therefore, the first priority for a fleet interested in saving a significant amount of money is to start moving into Fuel Efficient tires (SmartWay verified) starting with the trailer axles and continuing on to steer and drive tires.
Fleets need to remember that having low rolling resistance tires, which will have a direct impact on their fuel economy, is just part of the equation. Having a full set of SmartWay verified tires at 80 or 90% of the recommended air pressure is like having the tank full, with a leak at the bottom of the tank. All the theoretical savings provided by the tires will be wasted by not having the proper air pressure calibration…or rotating the tires every 20,000 miles…or not fixing the mechanical problems that cause irregular wear and reduce the ideal foot-print, etc.
Another consideration is that having a low rolling resistance tire that will not secure a minimum number of retreads is also a waste of time and money. A conscientious fleet manager or maintenance manager needs to look beyond the initial investment to find a product that provides a full array of benefits besides the theoretical low fuel consumption in order to really achieve the very best cost per mile from their tires.
In summary, a good low rolling resistance tire should deliver a good balance of traction/cornering/braking, competitive tread life and a tread pattern and compounds which will allow the tire to roll more easily than regular tires.
