Want a quick and easy way to get the most hours or miles out of the tires on your equipment and trucks? Keep the right amount of air in them.
“The biggest threat to the life of a tire is incorrect air pressure,” says Gary Nash, director of sales, Yokohama Tire Corp. off-the-road products for a broad range of construction equipment. “Tire pressure has to be just right. Not too high and not too low.”
By their choices of rubber compounds, tread patterns and construc-tion methods, tire manufacturers can make tires to stand up to cuts, shocks, heat and other stresses. But, they can’t make tires that perform and last when not inflated properly.
Besides the money spent to replace tires that fail prematurely due to increased wear, internal damage or even a blowout or other sudden tire failure, improper inflation can cost you in other ways. For example, reduced traction and flotation can lower productivity and increase rolling resistance, hurting fuel economy.
The cost of under-inflation
Most on- and off-road tires with improper air pressure have too little air. Over time, tires lose pressure at about 1 psi per month as air escapes between the tire bead and where it fits against the wheel. Pressure can also change about 1 to 2 psi for every 10 degrees F change in ambient temperature. Tire pressure also rises as tires warm up when you operate your machine.
As Nash points out, under-inflation can reduce payload capacity and cause early tire failure in several ways:
Uneven wear. Unlike a properly inflated tire in which the entire width of the tread contacts the ground, under-inflation causes the tire to run more on the outer edges of the tread, leading to more wear there than in the center and reducing gripping action.
Overheating. Under-inflation causes excessive deflection of the tread, allowing the lugs of the tread to move back and forth across the ground surface. That increases heat-generating friction. Failure of tire-reinforcing cords is another possibility as the cords and weakened rubber rub together.
Low tire pressure on dual-mounted tires can also lead to overheating, if they are deflated enough so that adjacent sidewalls sag and rub against each other.
“Once a tire overheats, the rubber can melt and revert back to its natural state,” Nash explains. “In addition to causing the tread and plies to separate, overheating causes cracks in the innerliner of tubeless tires. As a result, air can leak into the casing to create knots and other weak spots in the sidewall.”
Rim displacement. While this condition is not very common, the O-ring of an under-inflated tire can deteriorate from heat buildup and crack. This weakens the seal between the wheel flange and the tire bead, allowing air inside a tubeless tire to leak out.
The price of over-inflation
Tire life and performance can also suffer from higher-than-recommended air pressure.
Uneven wear. Too much air in a tire increases the ground contact pressure at the center of the tire, causing more wear there than on the outer areas of the tread and reducing overall traction.
More penetration damage. “By stretching the rubber, too much air pressure reduces the ability of the tire to close around or envelop a sharp-edged object,” Nash says. “This can increase the explosive force and the resulting damage should something penetrate through the tire.”
More cord damage. An over-inflated tire reduces protection of the cords from uneven surfaces, leaving them more vulnerable to rupture from cuts or shocks.
Bead problems. Too much air pressure can loosen the rubber that encases the steel wires of the bead, causing it to fail as the wires rub against the wheel flange.
Check your pressure
The correct inflation pressure depends on type and size of tire, as well as the machine type, ground conditions, load, speed and other factors. So, when inflating tires, follow your equipment operator’s manual. The information there reflects the manufacturer’s recommendations for your specific make and model.
How often should you check the pressure? “Much depends on how frequently you use your machine,” Nash says. “If you operate it daily and the tires are set up properly, pressure won’t fluctuate much from day to day. So, you may not need to check it every day. However, if you use your machine only periodically, check tire pressure every time before starting work. Extreme temperatures and poor working conditions, such as unusually rough surfaces, call for more frequent tire pressure checks.”
To make sure your tire gauge is registering correctly, Nash recommends checking its accuracy periodically using a master gauge and recalibrating it as needed. To determine if your tires are inflated properly, Nash suggests checking the air pressure twice — once when the tire is cold and again at the end of the work shift.
The manufacturer’s recommended tire pressure is based on cold air pressure and is designed to account for the normal increase in pressure during operation. How-ever, he notes, checking pressure the second time can reveal any problems due to the tire or how you are operating the machine.
“To prevent problems, the operating pressure should be no more than 5 percent higher than the cold tire pressure,” Nash says. “If it is, look for the cause and correct it. Maybe you’re overloading the machine or your ground speed is too fast for the tire.” Wait until the tire is cold before bleeding off any excess air. That prevents the tire from becoming under-inflated when it’s cold.
Checking for excessive wear
It’s also a good idea to measure the outside diameter of your tires to see if the tread of a tire on one side of your machine is worn more than on the other side. The tire with the larger diameter will carry most of the load and will be more prone to damage.
“If the difference in outer diameters is extremely large, the smaller-diameter tire will slip and scrape along the ground, causing the center of the tire to wear quickly,” Nash says. “The larger tire will carry more of the load and is more apt to generate excessive heat from overloading. The difference in diameter could also damage the differential, gear box or other drive train components of your machine.”
The same goes for the duals on your truck or trailer. You can use a steel tape to measure the circumference of each tire. Or, you can lay a 1 x 2 across the top of the two tires and use the vertical distance between the 1 x 2 and the tread to measure any difference in height between the tires.
For example, for a radial tire with a section width of 9 to 14 inches, the maximum allowable difference in diameter between the two tires is 0.3 inch, and the maximum difference in circumference is 1 inch.
Tables showing the maximum allowable differences for various sizes of tires and how these differences affect load distribution are on page 66 of the Yokohama Off-the Road Tires Handbook (available at www.yokohamatire.com). Click “Tires” and scroll down to “Off-the-Road” and click.
“In no case, should you try to correct any difference in diameters by adjusting the inflation pressure,” Nash says.
Greg Northcutt is a freelance writer based in Port Orchard, Wash. He can be reached by e-mailing this publication at editor@onsiteinstaller.com.
