You know, tires are a lot like people. Tires have shoulders, toes, heels and soles in addition to belts and zippers. They need air to live and they can be bled. They can get hot, overheat, fatigue, get worn out, age, and wrinkle the same as you and I.
Tires can get cuts and blisters too. They are happiest when paired with the right mate but do experience separations occasionally.
They like to be well lubricated and are most appreciative of good mounting techniques. Tires are most unhappy when they are abused and neglected. And do you know anyone who isn't?
The approach of warmer weather and hot road temperatures means that the busiest season for tire changing is going to be quickly upon us.
This year you've probably got more contracts with your fleet customers to perform some aspect of their tire maintenance than ever before.
Usually, it's to perform their off-vehicle tire maintenance. That is, you're responsible for mounting/demounting, inspecting tires and wheels, determining their disposition (repair, retread, scrap, refinish, etc.) and holding tire inventory.
Some fleets may enlist your company to check their tires on vehicles parked in the yard over the weekend or in the evenings so that you can replace tires and wheels that are flat, worn out, damaged, need repair, etc.
Whether you're mounting tire/wheel assemblies in your service area and delivering them to the fleet or performing this service on the customer's yard, are you thinking about tire matching? You should be because you know that properly matched people as well as tires work best together.
There's a lot more to tire matching, or tire mating as some people call it, than just putting the same size tire on a dual position as the one next to it. A properly matched tire is the same tire size, has the same type tread pattern, same diameter, same construction and same air pressure as its mate.
Matchmaker, matchmaker
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 these perfectly matched couples break up and go their separate ways.
It's just like one to pick up a stray, skanky nail and run off, leaving its former mate to hope a suitable replacement can be found.
And that's your job. You are now a matchmaker!
Why is this such a big deal? Well, tires that have not been properly matched 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 which makes for a very messy divorce.
You may think I'm being melodramatic here, but in tests conducted by Bridgestone Americas a few years ago, it was found that tires with mismatched air pressures transferred the loads from the underinflated tires to the properly inflated tires.
When the pressure in the underinflated tire was reduced to 5 psi from 95 psi, the good tire ended up carrying 8,300 pounds when its maximum allowable load was 5,070 pounds!
Even an inflation mismatch of greater than 5 psi will result in two tires of a dual assembly being significantly different in circumference.
Testing has shown that a 5 psi difference creates a 5/16-inch difference in tire circumference in a steel radial tire.
Because they are bolted together, dual tires have to cover the same amount of road in a single revolution. So the large tire drags the smaller one. In a single mile, a 5/16-inch difference causes the smaller tire to be scuffed 13 feet.
If the tire runs 100,000 miles a year, it gets scuffed 246 miles! Because the footprint of the overloaded tire changes and since the underinflated tire scuffs as it runs, both tires with mismatched air pressures develop irregular wear.
The tire with the lower inflation pressure becomes bitter and wears very fast and irregularly. If allowed to run, eventually the overloaded tire will throw in the towel and the underinflated tire will run flat just to get even.
As a result, inflation pressures of dual tires should always be the same. Even minor differences in inflation pressure greater than 2 psi will create alienation and irregular wear patterns.
Pressure equalizers
That's why many truckers rave about tire pressure equalizers. This equipment ensures dual tire pressures are equal all the time and tires roll hand-in-hand down the road. While many truckers have bought this equipment for monitoring inflation pressure, they all swear that their irregular wear problems have disappeared, too. This is the reason why.
A situation similar to an underinflated dual tire occurs when a tire's diameter is smaller than its mate's. A difference of 1/4-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. Two tires with different diameters cannot cover the same distance in the same number of revolutions unless they are bolted together. In that case, this condition can lead to both rapid and irregular wear.
Since the larger tire has to flex severely to carry more than its share of the load, heat builds up within the tire, which leads to fatigue, 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 develops irregular wear that never evens out.
In the best scenario, both tires develop irregular wear. In the worse scenario, they kill each other.
As you can see, it is critical that tires in dual positions are matched by size, diameter and that their inflation pressures are the same.
But 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 rpm.
Unless the vehicle is equipped with an inter-axle differential, the tires on both of the tandem-drive axles must be closely matched to safeguard the differential and prevent excessive slip, loss of traction and uneven wear.
Inter-differential fight due to mismatched tire diameters will also 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 1/4-inch diameter (1/8-inch radius) 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 1/4-inch (1/8-inch radius) different from the average diameter on the other axle. (When measuring dual matching on the vehicle with a tire square, radius is being measured.)
Since a typical drive tire may lose as much as 41/2 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 1/4 inch in diameter. It's highly recommended that tires are rotated between the rear and forward drive axles to extend tire life and ensure all 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 1/4 inch and 1/2 inch across the axle.
In addition to matching tire diameters and inflation pressures on dual applications, it is very important that both tires have the same construction. 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. You can mix tubeless tires with tube-type tires, provided they are of equivalent sizes, have matched overall diameters and have the same inflation pressure.
I once visited a fleet that ran three or four brands of tires and went to the trouble of matching duals not only by size but by make and model. This degree of effort is not necessary.
It is acceptable to have multiple tread designs and brands across an axle as long as the dual matching tolerance is 1/4 inch in diameter. However, it is not a good idea to mix traction and rib designs since this is like pairing liberals with conservatives.
Many people think that if they mount a set of new radial retreads with the same tread design on a vehicle, that 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 and provide mounting/demounting services or can advise your fleet accounts of the following procedures when they mount their tires, matching tires by diameter (radius) size can be made quick and easy and saves the technician time when he installs a tire on a vehicle:
* 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;
* Mark the diameters on the treads so that they are visible when the tires are placed in the tire racks;
* Use tire calipers or dual tire maters to measure the diameter or radius of tires on the vehicle; and
* Select and install the appropriate replacement tire from the tire inventory.
To ensure inflation pressures are the same, always use air lines to inflate newly mounted tires as well as to adjust pressures in tires on vehicles that are attached to a regulator that cuts off pressure to tires when the designated pressure is reached.
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 1/4-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.
Finding the perfect match in life as well as in tire service work is essential to your happiness and long life just as it is for dual tires.
Taking your role as matchmaker seriously and taking the steps to ensure tires are mated properly will result in long tire life, better mileage and fewer headaches.