Advice For Singles
Wide-base single tires aren’t for everybody, and that creates some real problems for those who do use them.
If fleets spec’d their equipment solely for their own needs, they wouldn’t worry about what subsequent owners thought of their tire choices. But sooner or later, almost all equipment is resold or traded, and users of wide-base single tires have to consider the resale value of their asset.
Since the market still seems to favor dual tires, demand for — or the value of — used trucks and trailers with wide-single tires is diminished.
To preserve the asset’s value at resale, fleets often spec suspensions and axles capable of accommodating both dual and wide-singles tires. Come trade-in time, the wide-singles — wheel, hub and all — can be switched out for duals.
For the most part, this works, but there’s a rub. Several actually.
Wide-single tires have different track widths and load lines than dual tires, and changing the tires and wheels can have quite an impact on the vehicle itself as well as on compliance with various dimensional and structural requirements.
A standard axle on a 102-in. wide trailer with dual wheels measures 77.5 in. from end to end, yielding a track width of about 101 in. Track width is the distance between the outside faces of the tires measured at any point above the lowest point of the rim. This measures the outside extremity of the rim itself, not the bulge in sidewall of the tire.
In Canada, the maximum track width is 2.6 meters (102.4 in.). Prior to 2009, the minimum track width for trailers with dual tires was 2.5 meters (98.4 in.), and 2.3 meters (90.55 in.) for wide-single tires.
Using wide-single tires on a 77.5-in. axle with 0-offset rims, your track width would be about 95 in. That met the minimum track width requirements in place at the time, but it’s a rather narrow track width for a 102-in. wide trailer.
When truck and trailer as well as axle and suspension makers look at vehicle stability, they consider the centerline of the contact patch of the tire. For dual tires, that would be the centerline between the two duals. On a wide single, the centerline is right down the middle of the tire.
This can get confusing because various suppliers use measurements like the distance between the inside sidewalls of the tire, the outside sidewalls of the tire, and the centerline of the tire. This measurement can also be referred to as the load line or the tire track width — and you thought this would be a walk in the park ….
From a stability point of view, an axle with wide-singles has a wider "tire track" dimension (centerline to centerline) than an axle with duals. Ergo, it has a wider stance and is more stable. But, the regulators focus on track width, and in 2009, a group of Canadian regulators sought to increase the minimum track width for trailers built from that year onward fitted with wide-single tires to 2.5 meters — the same as duals.
The obvious solution was to use an 83.5-in. axle, but that would have made it impossible to switch from wide-single tires back to duals for resale. Those axles are for committed believers in wide-single tires who do not intend ever to mount duals on their equipment.
In common use at the time and still to this day is the 2-in. offset wheel (sometimes referred to as a 2-in. outset wheel). With a standard 77.5-in. axle, these wheels give you an additional four inches of track width, bringing the axle out to about 97 inches — just 1.5 in. shy of the 2.5-meter minimum required under a proposed memorandum of understanding (MOU) set forth in April 2008.
While regulators seemed content to let this particular sleeping dog lie, industry made it clear that the minimum 2.5-meter track width could be met with additional axle spacers or larger wheel offsets. Either approach would work, but both would produce significant stress on wheels, hubs, and axle spindles.
In December 2009, regulators and industry compromised on a minimum track width of 2.45 meters (96.5 in.) for equipment manufactured in 2010 or later and fitted with wide-single tires, thus ending a potentially expensive standoff.
To date, all Canadian jurisdictions have agreed to this approach. However, as changes are required to various provincial regulation, permits are being made available until the regs are actually changed.
But it doesn’t end there.
Think of the 2-in. offset wheel as a cantilever of sorts. In shifting the load line outward by two inches, you are also shifting the centerline of the load on the wheel bearings.
According to Kurt Burmeister, ArvinMeritor’s general sales manager for North American field operations, switching between dual and single tires with 2-in. offset rims could significantly alter bearing loads and the service life of both the bearings and the hub.
"If you put a wide-base single on a conventional track housing, you have a tendency to load the outer bearing in the wheel-end system more than it is designed for. You’ll reduce the life of that bearing, and in the most severe state, you’ll see the bearing degrade, and possibly damage the spindle itself," he says.
This condition is particularly acute when using tapered spindles and bearings. The alternative is the straight spindle, sometimes called a parallel spindle or P-spindle. It’s a bit stouter, and both bearings are the same size, notes Bill Hicks Director of Product Development and Planning for Trailer Suspension at SAF Holland.
"Because of the perceived or actual problems with the tapered spindles when used with 2-in. offset wheels, most if not all of the axle manufacturers require that customers use straight spindles," he says.
"In some cases, suspension/axle manufacturers may actually derate the axle by up to 3,000 lb. For example, a standard 20,000-lb axle may be permitted only 17,000 or 18,000 lb loads when the offset wheels are used — depending on the extent of the offset."
As you might expect, the P-spindle is a little heavier and a little more expensive than a tapered spindle, but it’s likely preferable to a 2,000-lb derate.
While the issues with 2-in. offset wheels are well known and understood, they are by no means universal. Many fleets report exemplary service from such configurations. Still, Canadian regulators decided to err on the side of caution.
The December 2009 MOU requires that any trailer built in or after 2010 that is converted from dual to single tires must bear a certification label adjacent to the original compliance label identifying the company, or authorized dealer of a company that converted the trailer.
The label must also indicate the revised tire and wheel size designation and revised gross vehicle and axle weight ratings.
"When revising axle weight ratings, qualified companies are expected to account for the wheel offsets and the impact this may have on axle and bearing capacity," notes the MOU.
Regulators would seem to be on solid ground with this recommendation. Several axle and suspension makers, such as Hendrickson, warn that the use of offset wheels can impact axle ratings.
To provide an acceptable bearing service life, the rated load of the spindle bearings is typically reduced, says Jim Rushe, Program Manager, On-Highway Products at Hendrickson Trailer Suspension Systems.
"The rated load for an HP [i.e., straight] spindle bearing when used with a two-inch offset wheel is 20,000 lb. max, regardless of the original axle rating," he says. "Hendrickson does not approve of the use of the HN (i.e., tapered) spindle bearing with offset wheels."
The offset wheels can also damage some spindle designs, Rushe adds.
"The increased load on the outer bearing increases the load on the outer race of the spindle, which can lead to accelerated spindle wear, and possible contamination of the bearing lubrication."
And one more cautionary note is in order here. Just because you own the axle, the manufacturer still has some say in what you do with it.
Unauthorized changes to original OEM vehicle configurations could result in loss of warranty and less than desirable service life.
Steve Slesinski, director of product management, commercial vehicle products group, Dana Holding Corporation, points out that the appropriate axle type including track, spindle configuration, and axle-housing thickness are approved by the vehicle manufacturer based on the intended vocation, duty-cycle, and guidelines provided by the axle manufacturer.
"If an existing vehicle approved for use with dual wheels is converted to wide-based singles, the application approval for the chassis configuration must be approved through the vehicle manufacturer and or each component manufacturer," he cautions.
The other solution, of course, is to use axles designed specifically for wide-single tires. But as we’ve pointed out, their additional width will not allow for the retrofit of dual at trade-in.
Another compromise would be ArvinMeritor’s new 14X drive, axle.
The first designed for use with either wide-single or dual tires. We’re not aware of similar equipment for trailers, yet. Going forward, say in five to ten years, will industry acceptance of wide-single tires will be such that retrofitting will no longer be necessary?
This all hinges on the first owner’s demand that the vehicle have the best possible trade-in value. Over time, more fleets will spec wide-single tires, and as they gain market acceptance they might be considered an asset at trade-in.
Have your say
This is a moderated forum. Comments will no longer be published unless they are accompanied by a first and last name and a verifiable email address. (Today's Trucking will not publish or share the email address.) Profane language and content deemed to be libelous, racist, or threatening in nature will not be published under any circumstances.