Peter Weis, Chief Engineer, Polar Tank Trailer
Photo credit: Courtesy of Peter Weis
Bulk tank carriers specify their trailers to maximize payload, efficiency and safety. When the discussion turns to tires and wheels, they focus on stability, ride quality, fuel economy and weight.
Single wheel-and-tire assemblies can save hundreds of pounds of tare weight compared to duals, so they are especially appealing to bulk carriers. On a tractor-trailer combination, wide base tires and all-aluminum wheels can save upwards of 1,000 pounds compared to duals on steel rims. For a gasoline hauler, that equates to 165 additional gallons of fuel.
If you spec or service bulk tank trailers, it’s important to know how single wheels can affect maintenance of other components, as well as vehicle handling. Here are some tips that you can share with your shop personnel:
Standard Vs. Offset Rims
Wide-rimmed disc wheels have either a standard or an offset center. Whether one or the other is on the trailer will have a direct impact on how you inspect and maintain bearings, hubs and other wheel-end hardware.
A standard wide-rimmed wheel keeps the load directly above the hub, but results in a narrower “track” and less stability compared to duals. An offset wheel places the tire outboard of the hub to give the vehicle a wider track and greater stability. With a 77.5” axle, offset wheels provide an additional 2” of track width on each side for a total of about 97”.
However, even a 2” shift outward can stress the spindle and hub end, leading to reduced bearing life. Also, with an offset wheel, axle and suspension manufacturers may require specific spindle designs on their suspension systems. Without a fully engineered solution from the trailer manufacturer, you risk voiding your axle and suspension warranty.
Carriers can increase stability by spec’ing a longer axle. For example, on an 83.5” long axle, the axle and spring centers are six inches wider than on a 77.5” axle with no-offset rims. The result is a 102-inch track width, which can improve the ride dramatically.
A drawback to the longer axle is that it can’t easily be converted to duals later, which may impact trade-in or resale of the trailer.
Contact Area and Stability
Two 11R22.5 tires give an overall tire footprint of 124 square inches, while one 445/50R22.5 wide base tire has a footprint of 111 square inches. That’s 10 percent less contact area.
It may not seem like much, but if the trailer runs in applications other than over-the-road, the vehicle can be harder for drivers to control.
Also, because the wide base tire footprint is much wider than it is longer, the belt wires flex and twist more than on a conventional tire. This puts more stress on the tire’s belt edges near the shoulder groove. Any repairs near this area may potentially create a weak spot.
Wide base tires have a lower retreadability rate because they’re exposed to more twisting, turning and curbing. The rejection rate can be particularly high for tires used on non-offset or 2” offset axles.
Simply put, use a reputable retreader who’s qualified to work on your specific make and model of tire. Putting a narrow recap on a wide tire will reduce the contact patch and negate the performance characteristics the vehicle owner expects.
If you’re diligent about air pressure, wheel bearings and the wear condition of your wide base tires, you should be able to run them down to 4/32” to 6/32” before retreading.
Maintaining air inflation on any tire is critical. However, if you run underinflated with wide base tires, the risk of irregular wear, heat buildup and casing damage increases, much more so than with duals.
That’s why bulk carriers with wide base tires often consider central inflation systems a must-have item.
Weigh the Advantages
While duals continue to be the most common spec for bulk carriers, many find that the payload advantages of single tire-and-wheel assemblies offset the costs. Talk to your trailer manufacturer and tire suppliers about specs and maintenance practices that can reduce these costs while improving component life, miles-to-removal, inspections, and retreadability.
Peter Weis is chief engineer at Polar Tank Trailer, North America’s largest manufacturer of dry and liquid bulk tank trailers (www.polartank.com). It is a division of Polar Corporation (www.polarcorp.com) which also operates Polar Service Centers, a national parts, service and sales network for all makes of heavy duty vehicles and commercial tanks.