Mismatched tires on dual wheel positions cause many problems not the least is that their retreadability can be impaired.
Mismatched duals have the same effect on the life of tires as low inflation or overload. An underinflated tire on a dual assembly shifts its share of the load to its mate, which then becomes overloaded and frequently fails permanently. A difference of 15 psi inflation may result in the less inflated tire supporting 500 pounds less than the tire with the inflation. A similar action occurs when one tire’s diameter is smaller than its mate. A difference of 1/4 inch in diameter may result in the larger tire carrying 600 pounds more than the smaller. The shift in load becomes more prevalent as the difference in diameters or inflation becomes greater.
Improperly matched duals are subject to rapid tread wear because the larger tire carries more load and will wear fast and unevenly. Although the mismatched duals have different diameters, they must rotate at the same speed. The smaller tire then also wears unevenly because it is forced to scuff over the road. The overall result is abnormal and unequal tread wear for both tires.
Improperly matched duals may also lead to blowouts as a result of one tire being required to flex severely in doing more than its share of the work.
In addition to matching diameters and inflation pressures on dual installations, it is very important not to mix radials and bias ply tires on the same axle due to different load/deflection characteristics of these two types of tires. Radial tires deflect more under a given load than bias ply tires. If radial and bias ply tires are mixed in dual installations on the same axle, the bias ply tires will bear the greater part of the axle load and may operate in an overload condition that will lead to reduced mileage and early failure.
Radial tire overall diameter will govern the revolutions per mile obtained from a given tire. It is necessary to closely match tire revolutions per mile with tandem drive axle units coupled directly together, as when an interaxle differential does not exist or is locked out. Otherwise, the drive transmission may freeze up or fail in some way, and/or excessive slip on one of the sets of tires will lead to a loss in traction and uneven wear.
Twin screw, tandem drive rear axles require the eight tires to be matched so that the average tire diameter on one axle is no more than 1/4 inch different from the average tire diameter on the other axle. This rule holds true for all tire sizes used on over-the-road trucks. Equal tire pressures should also be maintained.
It is important that the tires of tandem driving axles be inspected and matched at regular periods, as determined by the type of service.
The difference in dimensions of the tires on a dual assembly should never exceed the figures shown in the chart below.
Dual Matching Tolerances
Diameter (Inches) Radius (Inches) Circumference (Ins)
8.25R20 and under 0 to 1/4 0 to 3/4 0 to 1/8
9.00R20 and up 0 to 1/2 0 to 1-1/2 0 to 1/4
Twin screw (all sizes) 0 to 1/4 0 to 3/4 0 to 1/8
The measurement and mating of duals is very important when mounting a new set of radial retreads. Just because the retreads are all on the same type and size of casing does not mean they all have the same overall diameter. The service they were subjected to prior to retreading may have an effect on the size of the retreaded tire.
When mounting duals on a truck, there will generally be some difference in the diameter of the two tires (within the limits described above). The outside tire wears faster than the inside tire. As it wears its diameter will approach that of the inside tire. Additionally, any crown on the road will favor the placement of the smaller diameter tire on the inside.
At the time of mounting duals on a vehicle, locate the two valves diametrically opposite (180 degrees apart) for accessibility. Hand holes on disc wheels must be located so that the inside valve is accessible.
By properly matching duals, you will greatly reduce the probability of tire problems and will find that your worn tires are more retreadable, thereby reducing your fleets tire costs, and that’s the name of the game!
This article originally appeared in a slightly different form in a Goodyear Tire & Rubber Company publication. Reprinted with permission.