Everything you need to know about switching from dual wheels to wide-base singles you can learn at a playground. Picture a teeter-totter with a big kid on one end, a little kid on the other. The big kid is on the ground, while the little guy's legs are dangling in the air. The load is unevenly distributed. If you moved the fulcrum closer to the big kid, the beam would eventually balance.
The thing about wide single tires is the length of the axle and the distance between the tires. In most cases, users of dual wheels switching to wide-single tires will need to use a wheel with as much as a 2-inch outset. There's no problem with that provided you have beefier bearing on the axle spindles.
With the tapered "R" spindle axles used on tractors, there are two bearings on the axle spindle; the inner bearing is larger than the outer bearing. By installing a wheel with a 2-inch outset -- as is a common practice when switching from dual tires to wide-singles to maintain a suitable track width -- you shift a larger portion of the load to the smaller outer bearing. That's why we hear of wheel-end and bearing failures resulting from the use of offset wheels when switching from dual tires to wide-base single tires.
It's important to note that those failures are not universal, nor are the tires, the wheels or the hubs singularly to blame. More often, it's a combination of factors including a lack of understanding of the impact of the switch, incorrect components, lack of engineering oversight and poor maintenance practices.
Some fleets have seen remarkable success using wide-single tires with 2-inch offset wheels, Con-way Truckload among them. Con-way Truckload's former vice president of maintenance, Bruce Stockton, heeded the warnings about wheel bearing problems and managed to get 10 years or more out of trailer axles using the 2-inch offset wheels.
"Back then we were retrofitting 10-year-old trailers with half a million miles on them, and they are still running today with the 2-inch offset wheels," Stockton says. "It's not a problem when it's done right."
There's a clue in Stockton's statement. He was retrofitting trailers, not drive axles. Trailers usually use parallel or "P" spindles, which use the same size bearing at the inner and outer position. The larger outer bearing on a "P" spindle is more robust and better able to manage the increased load caused by shifting the load line outward through the use of the 2-inch offset wheel.
In other words, there may be more factors at play in a certain situations than meets the eye. In other words, what works for your competitor for various reasons may not work for you for various other reasons.
What we're trying to say here is don't take for granted that switching from duals to wide-base 2-inch outset wheels is going to happen without consequence.
In January 2010, Meritor went on record saying the company will no longer approve applications with wide-base single tires and 2-inch outset wheels on tapered-spindle drive axles with standard-width housings. Nor will Meritor approve aftermarket vehicle modifications to install wide-base single tires and 2-inch outset wheels.
"In in-service fleet analysis and in computer modeling we found the service life of the outer bearing on tapered spindles could be compromised by the additional loads imposed on the outer bearings by the 2-inch outset wheels," says Meritor's general manager for North American Technical Service, Charlie Allen.
In 2011, Meritor published a White Paper "Understanding the Impact of Wide Base Single Tires on Axle and Wheel-end Systems" outlining the engineering concerns involved with 2-inch outset wheels and tapered "R" spindle.
In the paper, Meritor explains that the load line position on the bearings, particularly the outer bearing, depends on several factors, all of which vary: the cup-to-hub-flange dimension, the offset of the wheel, the thickness of the wheel (steel vs. aluminum) and the thickness of the brake drum flange. Combined, these can push the load line outboard beyond acceptable limits.
It should be required reading for any fleet considering a transition from duals to wide-singles on standard-width axles.
Dana, on the other hand, says in an axle and driveline service bulletin, ABIB-0511R2, 2-inch outset wheels are approved on its DS404, DST40, DSP40, & D40-145 [Pro40] axles provided the tire track width, as measured from center to center of the tires, does not exceed 73.3 inches.
Not a Universal Problem
"There are a lot of wheel-end variable to consider," says Steve Slesinski, Director of Global Product Planning at Dana Holding Corporation Commercial Vehicle Products. "It's not a case of here's the axle, here's the problem. Industry people tell me there are issues with poor quality wheel bearings, maybe some poor maintenance practices and concerns about various other components at the wheel end. And oh, by the way, some of those trucks also happen to run wide-base tires."
Slesinski says many of his customers continue to report positive results with the 2-inch outset wheels.
"There are so many variables here that I can't point to a single issue that is a consistent source of problems," he notes. "I can say that our pre-set LMS hubs help with bearing adjustment, which has been seen as a weakness in some fleet maintenance practices."
Maintenance has to be seen as key factor in getting the most out a system using standard-width axles and 2-inch outset wheels, but it's worth noting that even well run maintenance shops like those of Ryder System have seen early bearing failures with such setups.
"We have experienced early bearing failure in some applications using the wide-base tires with standard axles," says Ryder's vice-president of Supply Management, Scott Perry. "We have also seen accelerated wear of wide base products, especially when the tractor-trailer combination is used in a heavy outbound/light or empty return duty cycle."
Perry says it's standard practice at Ryder when ordering new vehicles with wide-base tires to use wide track axles, but they have done some conversions.
"In some instances we have allowed for the conversion of an in-use tractor to wide-base tires using a standard axle under very specific conditions," he notes.
Meritor's Allen stresses that you can't just slap a set of wide-single tires on a truck like you did with the mag wheels you bought at Princess Auto for your '72 Camaro. There's more here to worry about than the bolt pattern.
"When you increase the offset, you extend the load line outward, and that applies excessive load to the outer bearing," Allen says. "We have tracked bearing life relative to load and adjustment, and we see degradation in both cases on the order of 60 to 80% in severe circumstances."
It would take far too much space to describe all the possible damage conditions, but suffice to say, much grief can be avoided by following manufacturer's recommendations -- especially for bearing selection and adjustment.
"The standard 0.005 end play adjustment is really the outer limit in my mind," Allen says. "You shouldn't ever be outside that. The closer you are to .001/.002 the better. If you can't get the adjustment down that fine, then using a preset hub may be the best option in assuring longer bearing life."
Wide-base single tires certainly offer some compelling advantages, namely savings of about 400 pounds per vehicle and fuel savings too great to ignore in many applications. Lingering doubts about the acceptance of the tires leave many fleets reluctant to commit to wide-track axles. The offset wheel offers an alternative, but perhaps the intermediate track axle is worth considering. That's uses a shorter outset, and is reportedly easier on the wheels bearings.
In any case, don't simply assume because Fleet A gets away with using 2-inch outset wheels that your fleet can too. There are just too many variables in there to make such an assumption. Always consult the original equipment manufacturer before making substantial changes to the vehicle.