Summer is here and school is out, which means it's time for annual school-bus maintenance, including tires. But let's back up for a minute and talk about how school-bus tires are spec'd.
The type of road surface involved is the first thing taken into consideration. For paved surfaces, pure over-the-road tires are preferred; when routes include a significant portion of gravel or abrasive chip-and-seal surfaces, mixed-service tires (on-off road) are preferred. Since pure highway tread tires are often subject to tread chipping and stone damage when used on unpaved surfaces, the risk of casing damage and downtime outweighs the additional cost of mixed-service tires in school districts with rural routes.
The next thing to consider is tread design. With the exception of a few locations in the South, metro-service rib tires for steer axles and moderately aggressive traction drive treads for rear axles are preferred. Pure linehaul-type steer tires should generally be avoided, especially those with decoupling grooves and special shoulder designs to retard wear in high-speed service. These are susceptible to rib tearing and chipping in the high-side-scuff, heavy-wheel-cut conditions typical of school buses.
The more aggressive traction drive tires designed for P&D service usually perform better in school-bus applications than the deep-tread, high-mileage drive tires designed for tandem-axle linehaul service. Tires with a more aggressive traction provide superior handling in rain and snow, especially under varying load conditions.
Still spec'd with conventional sizes, buses have not kept up with the trend to newer low-profile designs. This may in part be explained by the fact that early low-pro tires — with their shorter, stiffer sidewalls creating more abrasive scuff in the tread area at high wheel-cut angles — did not always get good mileage.
But things have changed. Most tire makers now offer metro-service tires in low-pro sizes designed to overcome the mileage issue.
The layout of the chassis must also be taken into consideration. According to John Fay, director of school-bus marketing for International Truck & Engine Corp., about 28% of new full-size school buses are now flat-front (transit-style) configurations with rear-mounted pusher diesels and set-back steer axles. High steer axle loading means that many of these buses require higher load range tires. It's also important to spec these extra-ply tires for drive axles to keep things standardized and facilitate retreading programs. Be sure to keep these tires separate if your fleet has both conventional and transit-style school buses.
Maintenance recommendations for school-bus tires are similar to those for metro-service P&D vehicles, with special emphasis on steer-axle and inside-dual tire inflation pressures, and more frequent inspections for cuts, snags, bulges or other damage since school buses carry such precious cargo. Many operators pull tires at slightly higher remaining tread depths than is done in other applications. The long wheelbase typical of most buses can mean alignment problems that lead to severe tire wear.
However, in recent years manufacturers have corrected the Ackermann steering geometry problems that were typical of many earlier production buses, resulting in improved steer-tire wear. Fingertip diagnostics of steer tires is especially useful on buses to determine whether shoulder wear is due to improper toe settings or a drive-axle chassis thrust problem. Tire reps can help train your service personnel in this time-saving procedure.
In general, maintenance for school-bus tires is pretty basic, but it requires diligent execution to be effective. That's the lesson plan.