When the time comes to purchase bus tires, always consider two key factors — speed and load rating. The tire’s load rating is the most important factor, especially in fleets, because it determines the bus’ overall load-carrying capability. If you make the right choice, you’ll have a reliable tire that will last and perform to optimum efficiency. Make the wrong choice and the bus could suffer blowouts.
Tires — along with rims, axles, springs, and steering gear — determine a bus’ gross axle weight rating (GAWR). The GAWR is the maximum an axle is rated to carry by the manufacturer. It includes both the weight of the axle itself and that part of the vehicle’s weight supported by the axle.
But in practical terms, the bus’ GAWR is only as good as the lowest-rated component in its system. So, if the tires are the lowest-rated component, then the strength of the axles and springs means virtually nothing. You’re restricted by a weak set of tires.
That is an important point to remember when determining how much weight a vehicle can carry.
GAWR — along with frame strength and axle placement — ultimately determines the bus’ overall gross vehicle weight (GVW). GVW is the total weight of the vehicle and everything on board, including passengers and fuel. Therefore, as the potentially weakest link in the chain, tires can have a big impact on the vehicle’s overall GVW.
For example, a bus has a 12,000-pound capacity front axle and front suspension and a 13,220-pound combined capacity from the steel rims. However, if the front tires are only rated at a combined 10,300- pound capacity, then the front GAWR of the vehicle will still be only 10,300 pounds — almost a ton under its capability.
The same is true for the rear GAWR. Even with a 21,000-pound capacity rear axle and rear suspension and four steel rims (on a dual-rear-wheel model) with a combined 26,440-pound rating, the bus will still only have an 18,960-pound rear GAWR if that is the combined rating of its four rear tires.
With the 10,300-pound rating of the front GAWR and the 18,960-pound rating of the rear GAWR, the overall GVW of the vehicle is limited to 29,260 pounds because of the tires.
If you purchase larger front and rear tires, such as a set of 11R22.5, load range G, 14- ply tires (with an individual rating of 6,175 lbs. at 100 psi for the front and 5,675 lbs. at 100 psi for the rear), you could increase the vehicle’s overall GVW to 33,000 pounds. This translates to more passengers and more money for the operator. The bus’ load capability is now at the maximum allowed by its axle and suspension ratings.
Federal Motor Vehicle Safety Standard 120, which applies to tires and rims on trucks, buses, and trailers commonly found in fleets says the tires have to be at least equal to the GAWR load rating that a manufacturer posts on the vehicle certification label.
Some vehicles have a range of potential GAWRs, but the manufacturer establishes minimums, based on tire size. If there is no tire size designated for a particular axle on the label, then the lowest of the range of GAWRs determines the rating.