Tires are one of the most critical components that need to be managed by vehicle fleet operators today. Not only can tires represent one of the largest maintenance issues, they also can have a significant effect on vehicle performance.
Many existing recommendations and procedures are rooted in decades-old experiences and technologies. Additionally, it is important to review all procedures from the standpoint of individual fleet makeup, duty cycle and operator/driver experience. What may be good for one fleet may not be optimal for another.
Sifting through federal and state (provincial) regulatory requirements will provide only minimal performance standards. Although those standards need to be met, they don’t paint an adequate picture of the numerous considerations that need to be managed in order to provide the best balance of performance objectives.
One of the major considerations a truck fleet faces on a day-to-day basis is where to position replacement tires when the time or need presents itself.
U-Haul operates the largest truck fleet in North America, with a total at any one time of close to 100,000 trucks. Of those, approximately 25 percent are four-wheel light duty trucks with gross vehicle weight ratings (GVWR) of less than 10,000 pounds. The remaining 75 percent are six-wheel medium duty trucks with a GVWR of up to 20,000 pounds.
These trucks are rented to individuals with diverse demographic backgrounds, not only from an experience level, but also from physical-operating environments.
U-Haul utilizes one of the most comprehensive and dynamic maintenance systems of any fleet operator today. Procedures and policies are updated on virtually a daily basis. Tire monitoring and replacement procedures are just one of the issues included in the overall process, and they are continually reviewed for optimal performance and safety.
A key question is whether to place the newest tire(s) on the front or on the rear of a vehicle. A survey of other U.S. fleet operators failed to indicate a clear and consistent policy. In many cases, this is attributable to diverse fleet composition, duty cycle, driver experience and personal preference.
Through an analysis, it was determined that from a vehicle dynamics standpoint, different policies were required for four-wheel and six-wheel trucks.
Vehicle braking, cornering and traction are directly affected by tire condition. However, there appears to be no difference in requirements for four- or six-wheel trucks.
Additionally, it is felt that drivers can quickly adapt to individual vehicle capabilities within this category. Once drivers have familiarized themselves to a specific vehicle, they are able to use good judgment and safely operate.
Rapid air loss (RAL) presents one of the most significant conditions which must be considered when replacing tires on a vehicle. It is commonly understood that tires with reduced tread depth are more susceptible to punctures and major damage-inflicting road hazards which can cause rapid air loss including, but not limited to, what is commonly referred to as “blowouts.”
When a tire experiences RAL, not only can the vehicle “kneel down” at the corner which suffered the RAL, but the tire/wheel combination itself also experiences increased resistance to rolling. Both of these factors cause a force vector which, combined with the forward-momentum vector, will cause the vehicle to deviate from its desired path. The effect of this deviation can differ significantly if the RAL occurs on the steer axle or the drive axle. (See illustration.)
For both four- and six-wheel vehicles, RAL on the steer axle can cause a moderate to high level of understeer. This is the tendency of a vehicle to steer less than the driver’s input would suggest.
Although levels of understeer vary from vehicle to vehicle, it tends to introduce a stabilizing effect on the vehicle’s dynamic performance. Many times this characteristic is referred to as “pushing” or “plowing.”