Wide Wheel Issues
Single wides might not be compatible with trailers equipped with certain axles. TMC is investigating the issue.
Evan Lockridge
Contributing Editor
Single-wide tires may be the biggest development in truck tires since radials. Replacing dual tires with a single tire and wheel promises savings in weight and fuel consumption.
However, in some applications there could be a problem with running them on trailers equipped with certain axles.
The Technology and Maintenance Council of the American Trucking Assns. is looking into that issue. For a little more than a year, a task force has been studying the use of wide-based outset wheels on trailers equipped with axles using TN spindles.
Trailer axles are generally equipped with either TN spindles or TP spindles. TN spindles, the most common, have a large in-board bearing and a smaller out-board bearing. TP spindles have a large in-board bearing, but they also have a large out-board bearing. While this larger outboard bearing makes TP spindles more durable, it also adds weight.
The other part of this equation involves wheel offset. Wheel offset is calculated as the distance from the centerline of the wheel to the wheel-mounting surface. The wheel is outset when the centerline of the wheel is outboard of the wheel-mounting surface. The wheel is inset when the centerline of the wheel is inboard of the wheel-mounting surface. A 2-inch outset wheel is typically what you'll see with 14-inch wide-based single wheels.
Bringing this issue to the forefront were service bulletins from various hub, axle and trailer manufacturers advising against this configuration's use with TN spindles. Last year, the Truck Trailer Manufacturers Assn. also issued a technical bulletin advising that 2-inch outset wheels not be used with TN spindles. That association noted the bearing life of the P-spindle is "significantly reduced when used with a 2-inch outset wheel." The group advised when altering wheel end configurations, such as switching to single-wide tires, the original manufacturer should be contacted for the proper application.
Even Michelin, which has been touting the use of the single-wide tire more than any other tire maker, has expressed concern. In late 2003, it issued a technical bulletin stating "end users who have retrofitted vehicles with 2-inch outset wheels should contact their respective vehicle, axle or component manufacturers for specific application approvals or maintenance recommendations."
Both TMC and these component manufacturers are concerned about hub load ratings in these axles in relation to loads that run on the heavier side. A draft of the TMC Recommended Practice says when using single-wide wheels, "this outset condition increases the bending movement (load) on the hub and correspondingly lowers the hub rating as compared to dual wheel applications. In some cases, axle manufacturers may recommend heavy duty axle ends for single outset tire/wheel applications."
TMC's draft recommends hub de-rating guidelines for both TN and TP spindles. For instance, the draft says that if you're running standard ductile hubs with either a TN or TP axle with a load rating of 12,500 pounds on the hub, it would be de-rated to 11,000 with a 2-inch outset wheel.
The task force also must wrestle with what kind of Recommended Practice they will issue - "blue book" or a "red book." A "blue book" RP would have recommended engineering practices with technical details. In this case it would most likely feature charts recommending de-rating guidelines for axles based on their hub type, wheel outset and hub load rating.
A "red book" RP would be less technical in nature and center on maintenance practices while referring the fleet maintenance manager to the original equipment manufacturer for guidelines.
Regardless of which RP they decide on, the TMC task force is expected to note other important findings. One, that outset wheels may decrease the load rating for other components such as axles and bearings, and that manufacturers of those components should be consulted for approved load ratings. Two, that many manufacturers of axles and wheel end components recommend that the P-spindle be used in conjunction with outset wheels.
This does not mean you are imminently facing problems if you are running outset wheels on N-type axles. There are fleets that are running outset single wheels successfully, but at reduced load ratings. Rather, you need to talk with the manufacturer's representative and take to heart any recommendations to de-rate your axles.
