Are Your Wheel-Ends Hurting Your Fuel Economy?
Any resistance to a wheel's free-rolling state on an axle can affect 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.”
Are Wheel Bearings Too Tight?
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.
Morrow 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 pre-adjusted bearing and hub assemblies that use a spacer to maintain bearing tolerance after a prescribed tightening procedure with predetermined torque values. He suspects 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.”
