ROCHESTER HILLS, Mich. (Sept. 12, 2005) — The summer heat's been especially horrific this year, so I'll bet you and your associates have been changing tires like pit crews at the Indianapolis 500.
Tread rubber is melting like butter and tires that are run underinflated are self-destructing in explosive magnificence.
While you're slapping tires on trucks as fast as you can, are you taking the time to match tires properly? Probably not if you're changing tires on the side of the road. You're just lucky you have a tire—the right size tire—and were able to get there quickly to get the truck back on the highway.
If you're on a fleet's yard, are you thinking about tire matching? You should be. You know, your job is not just that of a tire dealer or technician. You're really a “matchmaker.”
Searching for a mate
Tire matching—or tire mating as some people call it—is more than just putting the same size tire on a dual position as the one next to it. A properly matched tire is of the same tire size, same type tread pattern, same diameter, same construction and same air pressure as its mate.
Optimally, the best matched tires are two brand-new tires of the same make, model and size with exactly the same air pressure. However, in the real world, frequently it's not too long before this perfectly matched couple breaks up and goes their separate ways. It's just like one to pick up a nail and take off for parts unknown. But I digress.
You're faced with finding the right mate for the tire still on the vehicle to replace the tire that's been taken off. So why is this such a big deal? Well, tires that have not been matched properly by size and/or inflation have the same effect on tire life as low inflation or overload. An underinflated tire in a dual assembly shifts its share of the load to its mate, which then becomes overloaded, resentful and explodes.
To demonstrate this fact, Guy Walenga, engineering manager of North American Commercial Products with Bridgestone/Firestone, conducted tests on tires with mismatched air pressures. He measured the loads transferred to the properly inflated tires from the underinflated tires and found that tires inflated to 95 psi ended up being severely overloaded, carrying 8,300 pounds (their maximum allowable load at 95 psi is 5,070 pounds) when the pressure of its mate was reduced to 5 psi.
Because the footprint of tires that are overloaded changes and since the underinflated tires scuff as they run, both tires with mismatched air pressures develop irregular wear. If allowed to run, eventually the overloaded tires will call it quits and the underinflated tires will run flat just to get the last word in. This makes for one ugly scene. Air pressures of dual tires should always be the same.
Even minor differences in air pressure greater than 2 psi will create incompatibilities and irregular wear patterns. That's why many truckers rave about tire-pressure equalizers. This equipment ensures dual tire pressures are equal all the time. While many truckers have bought this equipment for monitoring tire pressure, they all say their tires' irregular wear problems have disappeared, too. Now you know why.
A situation similar to an underinflated dual tire occurs when one tire's diameter is smaller than its mate's. A difference of a quarter-inch in diameter may result in the larger tire carrying up to 600 pounds more than the smaller tire. This shift in load worsens as the difference in diameters increases. Since the larger tire has to flex severely to carry more than its share of the work, heat builds up within the tire, which leads to reduced strength, break down of the rubber components and eventual destruction of the tire.
The smaller tire, which must rotate at the same speed as the larger tire, is forced to scuff as it runs over the road to keep up, and it also develops irregular wear that never evens out. The overall result is abnormal and unequal treadwear for both tires in this very miserable, mismatched pairing.
As you can see, it is critical that tires in dual positions are matched by size and diameter and that their inflation pressures are the same.
Matching tires on drive axles is doubly important. Since tire diameter determines the revolutions per mile (rpm), the drive differentials may fail if the tires are fighting with each other to run at different rpms. Unless the vehicle is equipped with an interaxle differential, the tires on both of the tandem drive axles must be matched closely to safeguard the differential and prevent excessive slip, loss of traction and uneven wear. Inter-differential fight due to mismatched tire diameters also will lead to premature component failure. (This is like dragging the neighbors into a domestic disturbance.)
The four drive tires on a single-axle drive application should be matched within a quarter-inch or less across the axle. Twin-screw or tandem-drive axles require that all eight tires be matched so that the average diameter on one axle is no more than a quarter-inch different from the average diameter on the other axle.
Since a typical drive tire may lose as much as 4.5 inches in circumference due to normal wear and still be serviceable, it is possible that a wide difference in tire circumference may exist on the vehicle. Therefore, it is important that the tires on tandem driving axles be inspected and matched at regular intervals to ensure the tires remain matched within a quarter inch in diameter. Tire rotations between the rear and forward drive axles are extremely valuable to extend tire life and ensure these tires wear evenly.
It is also important to match tires on free rolling axles. Although a differential is not a consideration on trailers and dollies, maximizing tire life and extending treadwear still are. Therefore, tires on these axles should be mated within a quarter-inch side by side and a half-inch across the axle.
In addition to matching tire diameters and inflation pressures on dual applications, it is very important to mate like tire constructions. Never mix radials and bias-ply tires on the same axle since they have different load/deflection characteristics and don't like each other. Radial tires deflect more under a given load than bias tires. This shifts a greater share of the axle load to the bias tires, which then end up operating in an overloaded condition that results in reduced mileage and early tire failure.
Trucks with multiple drive axles should have tires of the same construction mounted on all drive positions.
It is acceptable to have multiple tread designs and brands across an axle as long as the dual-matching tolerance is a quarter-inch in diameter. However, it is not a good idea to mix traction and rib designs since this is like pairing Democrats with Republicans.
Size does matter
Many people think that if they mount a set of new radial retreads with the same tread design on a vehicle, they will be matched properly. Not true! Just because the retreads are all on the same type and size casing does not mean they all have the same overall diameter.
Due to the tires' prior service, the method of retreading used and the techniques employed during the retread process, newly retreaded tires with the same make, model casing and tread may have different overall diameters. Always check the dimensions of new retreads to ensure matrimonial bliss.
If you have control of a fleet's tire inventory or can advise your fleet accounts of the following procedures when they mount their tires, matching tires by size is quick and easy and saves the technician time when he installs a tire on a vehicle:
c Use a stationary tire meter mounted on the wall of the tire shop to measure the overall diameter of tires that have just been mounted.
c Mark the diameters on the treads so that they are visible when the tires are placed in the tire racks.
c Use tire calipers or dual tire maters to measure the diameter of tires on the vehicle.
c Select and install the appropriate replacement tire from the tire inventory.
The odds of having the exact diameter size tire to mate with an existing tire in a dual position are not always good. Therefore when mounting duals on a truck, place the larger tire (within the quarter-inch tolerance) on the outside. Since the outside tire wears faster than the inside tire, as it wears its diameter will approach that of the inside tire.
Also, if the smaller tire is on the inside, the crown of the road will help make up the size difference between the two tires as they run. So while the tires may not be perfectly matched, the conditions are right for at least a good friendship.
OK, so you may not be a Dolly Levi, the renowned matchmaker in New York from the musical “Hello, Dolly,” who wore big hats with feathers. But you can still be the matchmaker for truck tires and make many tires and fleets happy with your matchmaking services and expertise.
And guys…the big hat with feathers is optional.