Ever looked at the footprint of a truck tire? The amount of rubber that actually hits the road at any given moment? It’s just a wee patch a few inches square. Multiply that by 18 or so and it doesn’t really change things much: you’ve still got a very small area of contact between your rig and the road. You’ve got 80,000 or 100,000 lb of dynamic weight connected to the asphalt by 18 little squares of rubber.
That’s about all there is between you and oblivion. Which makes a good case for being mighty careful about how you choose a tire and then keep it alive. And we haven’t talked yet about costs. That’s not a small thing, of course, with tires being a major expense.
It just doesn’t pay to skimp on any aspect of your tires, so let’s look at some of the issues more closely.
There’s rarely only one ‘best’ tire for your truck and its operating conditions, but there may well be one that’s better than what you’ve got now. If you know your needs, you can choose your tires well.
If you’re strictly a highway guy, for example, it just doesn’t make sense to go for a highly cut-resistant tread compound just because you like the tread design. There’s a huge difference between a tire meant for highway use and one built for garbage trucks. Consult your tire supplier and make sure you tell him exactly how you go trucking – terrain, speed, length of haul, gross weight, etc.
The tread design is important, for sure, but it’s a bit of a black art and there are many ways to achieve goals like good fuel economy, long life, or resistance to aquaplaning. You’re better off choosing tires on the basis of the manufacturer’s reputation for service than trying to find the perfect tread.
It’s worth noting that tread design for an over-the-road operation can affect fuel economy in a big way – you could see a gain between 6% and 14% with rib tread designs on all wheel positions vs. a combination of ribs with deep-lug treads on drive axles.
Unfortunately, you can’t have extremely long life and extremely good traction in the same tire. Like anything else, a tire is a deliberate compromise – if the rubber compound is soft or sticky, thus delivering good traction, it’s going to wear faster than a hard compound. The hard one will also deliver better fuel economy. Most tire makers will have models that are optimized for one or the other of these priorities, so it’s up to you to decide which is most important to you.
A key issue at the original purchase point is this: are you selecting tires with retreading in mind? A tire that costs $100 less than the premium product may give you a casing that can only handle one retread instead of two or even three. But retreading typically costs one-third to half the price of a new tire, with almost equivalent performance and longevity, so that initial saving on the lesser tire may be washed away. The key is to start with a quality tire that’s a good prospect for retreading.
In fact, retreading may not be for you, especially if you don’t own a trailer to which you’d send retreaded steer-axle or drive tires. Fleets have more flexibility in this regard simply because they have more wheel positions to play with. You won’t want retreads on your front axle, so if you’re just dealing with tractor tires, you’ve got your drives and that’s it. There’s no reason why you can’t retread those, but you might be a little reluctant to recycle a casing more than once at that position.
Retreading is done one of two ways: the ‘pre-cure’ process (also called ‘cold-capping’, where pre-cured tread rubber is applied to a prepared casing); or the ‘mould cure’ method where uncured tread rubber is baked onto the casing in a mould. Both methods use pressure chambers for baking, and each one is fine if properly done.
The casing used for either type of recap is first inspected carefully for hidden damage. In a good shop, the inspector will have a sophisticated inspection machine that uses ultrasound, magnetic imaging or other methods to find punctures, cable breaks or other flaws that can’t be seen. He’ll reject some casings that would appear to be OK on the surface. Further down the line is a guy who buffs off the old tread and another who makes any repairs required. Then the face of the casing is prepared with gum or some other material prior to application of the tread.
After the purchase of the right tire at the right price, you’ll be faced with protecting your investment. And while there are other factors here, it really comes down to inflation.
Underinflation will have nasty effects on vehicle handling and stability, but it will also reduce tire life. Tests have shown that a tire that’s 20% underinflated consistently will suffer about a 15% reduction in tread life. That translates to an unnecessary cost of maybe $50 per tire due to premature replacement. And if you had plans to retread that tire, you might lose again — prolonged operation in an underinflated state can kill the casing.
Inflation pressures should be checked as frequently as possible, weekly if you can manage it, most tire makers suggest. Truck tires lose 2 lb of air a month just by air permeating the inner liner and going through the tire. Air can also escape between the bead and wheel, as well as through improperly tightened valves, torn rubber valve grommets, or valve cores that have been blocked open by dirt and ice. And that’s not to mention the nails and other penetrating objects that tires seem to attract. Bottom line: the more frequent the checks, the more likely your tires will be correctly inflated and the longer they’ll live.
It’s a good idea to record what the inflation pressure should be, perhaps on a tire-inflation decal in a visible location on the truck. That will remind you, but it will also help to ensure that anyone else working on your truck can tell at a glance what pressure is called for.
Use an accurate inflation gauge, and don’t ignore the inside duals (inflate-thru valve caps make this chore easy). Clubs or ‘tire Billys’ are only useful to see if a tire’s flat, not to check air pressure. You need actual tire-pressure numbers to discover problems – like the tire that’s 10-15 psi below spec, which should be removed and inspected before it fails on the road.
Inflation pressures should be checked when the tires are cold, preferably early in the morning or before the vehicle is driven. Once the truck’s been out on the road, it may take three to four hours for the tires to cool down. Since heat increases air pressure (10-15 psi in normal operations), never bleed air from hot tires, as they’ll then be underinflated.
You’ll want to have a close look at your tires for other reasons too. Look for damage and separations, irregular wear patterns caused by axle misalignment or loose suspension components, and maybe even dual-tire mismatching. Use a tread-depth gauge and measure tread in the lowest spot on the tire. Be sure to check the tread all around the tire for low or brake-skidded spots. And note whether valve stems are accessible and valve caps are installed.
Effects of Speed
One of the biggest potential penalties of high road speed relates to tires. So it’s worth knowing that not many tires are suitable and approved for 75 mph. According to Tire and Rim Association standards, only the 11R24.5 in an ‘H’ load rating has the necessary capacity to sustain continued running at 75 mph on a 12,000-lb steering axle. Neither the 285/75R low profile or any 22.5-in. rubber loaded to 12,000 lb should be run at sustained speeds of more than 71 mph.
Bridgestone director of engineering Dave Laubie told a recent meeting of The Maintenance Council (part of the American Trucking Associations) that operators need to be aware of correct tire pressures and loading for higher speeds. At speeds above 66 mph, for example, radial tire pressures should be increased 5 psi. And the maximum loading on the tires should be reduced 4% between 66 and 70. Between 71 and 75 mph loading should be reduced 12% (see the accompanying chart).
At speeds up to 65 mph with standard pressures, a load-range ‘G’ tire in any of the popular 11R22.4, 11R24.5, 295/75R22.5 or 285/75R24.5 sizes can sustain a load of 12,000 lb all day. For heavily laden front ends over 12,000 lb – some axle-back cabovers are particularly prone to high front-axle loadings — a load range ‘H’ will allow the rig to run 65 mph with a 13,200-lb axle loading. But start running the truck over 65 and the problems start.
While this is potentially more dangerous on the steer axle, drive tires are not immune. A similar analysis from the same guidelines shows that the new generation 19.5-in. wheel and tire simply does not have the capacity to run 75 mph at a 17,000 pound-per-axle (34,000-lb tandem) loading.
Truckers who have cruised at elevated speeds for years with no severe problems may say this sounds alarmist. But Laubie says anyone driving at high speeds should know they’re stressing the tires to the limit and will suffer the consequences without recourse to the tire manufacturers through warranties or even litigation.
Those other consequences include greatly accelerated wear, irregular wear and casing degradation. Says Laubie, “Even if the tires don’t give immediate problems, casing durability suffers. Tires ride hotter and don’t last as long.”
Going from 65 to 70 mph or from 70 to 75 mph raises operating temp by about 20° F. Going from 65 to 75 mph may create as much as a 35° F spread. Tires also wear faster at elevated temperatures — testing shows a 45° F rise means around 15% faster wear.
Irregular wear happens because, as the tire spins faster, the footprint changes where the tire meets the road surface. As the tread flings outward in the center, the footprint elongates in the center area. When this happens, the shorter radius at the outside edge of the tires is forced to scuff against the road surface, leading to faster irregular wear.
At higher speeds, tires are also less resistant to impact, making it easier for them to fail in the casing or tread area.
And if you paid more for a tire with lower rolling resistance, you can kiss the fuel economy advantage goodbye at high speed. According to Laubie, any advantage gained by going to a shallow skid-depth tire is lost when the tire is pushed faster. Expect an 11% to 12% drop in fuel economy.
If there’s one single, simple thing that anyone can do to improve tire wear and minimize costs, it’s this: install cheap little metal valve caps to help maintain proper inflation, says John McErlane. Former national fleet services manager for Bandag, he sees truckers waste millions of dollars worth of tire casings a year.
“Usually,” he told me a few years ago, “it’s because they run them soft until the plies start to separate, or they don’t repair damage when they get the chance.
“A spring-loaded valve core was never meant to be a sealing device, and it’s not. The valve can hold air until you put a cap on, but it takes a valve cap to hold air. And the only one I trust is a metal cap with a rubber grommet inside it. I don’t trust a plastic cap to hold air.
“A lot of truckers don’t believe that because they can’t see the air that escapes through a valve,” McErlane says, “but you can prove it with a bike because there’s not much air in a bike tire and leaks show up fast.
“Pump up the tires on a bike and put a proper valve cap on just one of them, then check the pressures a week later. Even if both tires are good, the one without the valve cap will have lost air. If you don’t believe that, try it again with the valve cap on the other tire.”
The High Cost of Sloppy Alignment
A properly aligned truck can extend tire mileage significantly, while also helping in a big way with handling and fuel economy. And most alignment problems are due to axles that aren’t situated quite right. On a properly aligned vehicle, all the axles should be perpendicular to the frame and parallel to each other, and all the wheels should track the front ones.
Tandem drive axles that aren’t parallel or axles that aren’t perpendicular to the chassis centerline have a definite effect on steer-tire wear. Rear tandems that aren’t perpendicular to the frame but are parallel to each other create a ‘thrust angle’ that tends to push the vehicle off course. You’ll feel the vehicle pulling in the direction the drive axles are angled and you’ll be forced to steer in the opposite direction. So steer-axle tires are constantly subjected to ‘scrubbing’ or side forces as they correct the direction of travel, which results in fast and irregular wear. You’ll see irregular wear on the drive tires as well.
An out-of-parallel condition on trailers, known as ‘dog tracking’, is something we’ve all seen – and maybe experienced. With the trailer traveling at an angle to the tractor, you’ll feel the rig wandering and you’ll have to make constant steering corrections to keep it going straight. Tires on a trailer with this condition are dragged sideways a few feet for every mile of operation, which can add up to thousands of miles in severe conditions over the course of a few months.
Tandem skew happens when tandem axles aren’t parallel to one another or perpendicular to the chassis centerline, meaning drive axles will fight each other and the driver. Trailer axles with tandem skew will be dragged in different directions. This condition causes excessive and rapid tread wear on all tires of a vehicle. The steer tire on the same side of the truck on which the drive tires are closest together will wear into an out-of-round condition.
Drive-axle misalignment will create irregular wear on drive tires as well as steer tires and is the most common cause of alignment-related irregular wear. Trailer axle misalignment will produce irregular wear on trailer, drive and steer tires and is the second leading cause of alignment-related irregular wear. Steer axles are least susceptible to alignment-related irregular wear.
Tandem axles should be parallel to each other within 1/8 inch and all axles should be perpendicular to the chassis centerline within 1/8 inch.
Axle parallelism, otherwise known as tracking, is one of the most common causes of fast tire wear and also one the easiest to correct. Failure of the wheels to track is usually due to the following:
* Uneven axle spacing or the lack of parallelism between axles.
* Broken spring leaf.
* Worn springs.
* Loose U-bolts.
* Bent frame.
* Improperly adjusted torque or locating rods.
* Excessively worn torque or locating rod bushings.
Note that axle alignment isn’t the only cause of irregular tire wear. It can also be due to out-of-balance wheel/tire assemblies, improper tire inflation, worn shock absorbers, or worn bearings, among other causes. Really, the entire truck needs to be in good condition for “alignment” to be good. — Peggy Fisher, Tire & Wheel Editor, Heavy Duty Trucking
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