How Tires and Wheels Can Help Save Fuel
Tires keep you rolling, but they do not have to consume more than their fair share of fuel. The right tire choice and diligent maintenance will help keep the fuel bills down
Everything at the end of an axle has an impact on fuel economy. Tread patterns, tread compounding and depth, tire sidewall construction, wheel bearing tightness, mechanical deficiencies such as dragging brakes, and of course tire inflation pressure, each adds its own bit of resistance to the truck’s forward motion. The question is, can you do anything about it, or will you just drive yourself crazy chasing all those little demons?
Starting with the obvious: Are low-rolling-resistance tires a good investment? Fleets considering low-rolling-resistance tires often express concern that they give up more in tread life than they gain back in fuel savings. Is there still a case to be made for LRR tires at today’s modest fuel prices?
“There is less of a trade-off today in miles per 32nd of wear between low-rolling-resistance-tires and standard tires, due to improvements in tread compounding, tread designs, and tire construction,” says Peggy Fisher, president of TireStamp, and TMC tire guru. “However, since low-rolling-resistance tires usually have less tread depth than standard tires, they will still run fewer miles till wear-out.”
The big question is, will the fuel savings offset the loss in tread wear?
The answer, Fisher says, is maybe.
“When fuel is relatively inexpensive, like around $2-$3 a gallon, the savings in fuel may not balance out the loss in tire cost/mile. However, when fuel runs closer to $4.00 per gallon, it most likely does,” she says.
She says fleets need to run tire and fuel tests in combination on five to 10 vehicles for at least six months to determine what the loss in tread wear is and what the gain in fuel economy is.
This should be a comparison of a group of trucks with low-rolling-resistance tires versus a control group of trucks with standard tires. Fuel and tread wear records should be kept diligently during the test period. Keep in mind that fuel economy improves as tires wear, so the longer the test period, the better.
“Once the results are in, it is not hard to compute at what point the cost of fuel begins to outweigh the loss in tread wear,” she points out. “TMC’s RP208, Tire Cost Determination, shows fleets how they can determine their total tire cost, including factoring in the fuel savings from using low-rolling-resistance tires.”
In the meantime, tire manufacturers are always upping their game with new compounds and new tread and casing designs, all aimed at providing both performance and fuel savings.
“Michelin tire designers use a host of technology bricks to improve rolling resistance, such as FuelSaver rubber compounds or Dual Energy tread layers that maximize fuel efficiency of tires,” says Sharon Cowart, Michelin’s B2B product marketing director. “Through tire design, it is possible to utilize technologically advanced compounds and tire-construction techniques to minimize a tire’s rolling resistance. These tires have fuel-efficient compounds in the sidewall and bead area as well.”
Michelin recently announced it would offer the new Michelin X Line Energy D+ dual-drive tire to the replacement market. Three years in development, for the past year it has been installed exclusively on next-generation Freightliner Cascadias.
“The project yielded a product with the lowest rolling resistance ever offered by Michelin in a dual-fitment drive tire,” Cowart says.
SmartWay retreads offer improved fuel efficiency through a tire’s entire lifecycle.
Turn up the pressure
Maintaining proper tire air pressure is probably the single most important maintenance activity that a fleet can do to maximize its tire investment. Under-inflation is the biggest issue in the industry. It is the number one cause of premature tire removal. Underinflated tires build up excessive heat that can result in tire destruction and/or improper vehicle handling. In addition, it can result in irregular wear and poor fuel economy.
On the other hand, over-inflated tires increase the likelihood of crown cuts, impact breaks, punctures, and shock damage resulting from the decrease of sidewall flexing and an increase in firmness of the tread surface.
So, what’s the ideal pressure?
The tire people never commit to a number, because recommended pressures per given load can change with the tire size, load rating and design. Larger tires, such as an 11R24.5, require less pressure per given load than an 11R22.5 because of the greater volume of air in the tire, and it’s the air that supports the load, not the tire.
Still, the best you can usually get from the tire folks, at least for publication, is something like, “It is key for fleets to maintain proper air pressure for their loads to take advantage of the optimized footprint for fuel economy, traction and tread life. Both under and over inflation can affect the footprint shape, causing irregular wear and premature removal. Maintain all tires at the fleet target inflation pressure based on the manufacturers’ application data book for the particular axle load.”
Tires are at their most efficient when the tread is shallowest – but push this too far and you risk punctures and casing damage.
When you look up the tables, however, recommended pressure can vary from a minimum of 80-85 psi up to 110 for maximum legal axle loads in drive and trailer tires. The message is really about consistency and attention. Keep a close eye on all the fleet’s chosen tire pressures and don’t be lax about it.
And of course, as you run your tires out, retreaders offer SmartWay retreads to help fleets maintain consistently good fuel efficiency through several lifecycles.
“The SmartWay program has resulted in most retread manufacturers coming up with new solutions, developing new products to meet the new standards, particularly for fleets operating in or doing business in California,” Cowart says. “Those fleets that travel into California are now required to run retreads that are SmartWay compliant, so some of this specific business has shifted to SmartWay-approved products.”
SmartWay publishes a list of new tires and retreads that are considered “verified” technology that improves vehicle fuel economy and reduces greenhouse gas emissions. To make this verified list, a tire must meet minimum rolling-resistance criteria when run on a laboratory dynamometer.
Are wheel-ends hurting your fuel economy?
Properly adjusted wheel bearings minimize irregular tire wear, reduce the potential for wheel separation, and may even help improve fuel economy. Image via SAF Holland
Any resistance to the free-rolling state of a wheel on an axle can be assumed to have some impact on fuel economy. There will always be some resistance to movement caused by friction between the moving parts in a wheel-end assembly, from the bearings rolling between the spindle and the hub, and even between the wheel seal and the spindle. If you dig deep enough, you’d find the viscosity of the lubricant can be a factor, too.
For a general application, the ideal bearing end-play/preload setting should be near zero to optimize life and binding of the bearing. The American Trucking Associations’ Technology & Maintenance Council’s Recommended Practice on wheel bearing adjustment (RP 618A) should be followed to get the correct setting, says Divjot Singh, an applications engineer at NTN Bearing.
“TMC recommends achieving a verifiable bearing end play of 0.001 to 0.005,” he says. “Excessive preload (tight setting) will cause excessive friction and the bearing will run hot. On the other hand, adjusting the bearing too loose causes excessive looseness and vibration in the system. Tightness should be verified by use of a dial indicator. Bower Heavy Duty Bearings by NTN also offers wheel-end clinics to go over this procedure step by step.”
However, Joel Morrow, head of research and development for fuel-economy leader Ploger Transportation, insists wheel bearing tightness in new trucks and trailers can be inconsistent, and he feels there’s a relationship between overly tight bearing adjustment and fuel efficiency. He claims that the difference between factory-tightened tractor and trailer wheel bearings and those adjusted to a slight preload condition in his shop with a special tool (Dr. Preload) produced a before-and-after improvement of 1.4 mpg in ad-hoc testing.
HDT is not aware of any fuel economy testing on wheel-bearing adjustment done to prescribed SAE or TMC standards.
“Most of the trucks we get from the factory have one or two wheel-ends that were too tight,” Morrow says. “Trailers are even worse than tractors.”
He says the condition is associated with the pre-adjusted bearing and hub assemblies that use a spacer to maintain bearing tolerance following a prescribed tightening procedure with pre-determined torque values. He suspects there may be some calibration issues with the tools used to install wheel hubs at the factory.
“Proper torque at the factory is critical to ensure the bearing adjustment is correct,” says Bryan Williams, ConMet vice president of engineering. “If the torque applied is too high, the bearings can be put into excessive pre-load, or if the torque is too low, the bearings can have excessive endplay. Both factors can affect fuel economy and the performance of the bearings over time. Typically, wheel bearings have very limited impact on rolling resistance, however many alternative factors, including but not limited to wheel seals, lube, environmental factors, and the installation process, have a much more significant impact on the rolling resistance.”
Properly installed wheel bearing/hub assemblies are not always “free rotating,” and this is not necessarily indicative of an issue that needs to be addressed, Williams adds. “You should be able to rotate the hub by hand, but it may require some effort and the hub may not continue to spin when released.
“The primary resistance to rotation on a newly installed hub typically comes from the wheel seal and not the bearings,” he explains. “Heavy truck wheel seals are usually two-piece designs, and upon installation, portions of the seal can be in contact, creating drag. Many seals have features built into them that are meant to reduce this drag after initial startup, either by reducing friction when the hub is turning, or by including shapes that are meant to wear quickly during initial operation, thereby creating clearance and reducing drag.”
Source: https://www.truckinginfo.com
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