Trailer Report: Making Sense of Spread Tandems

There are good reasons to use 'em, and reasons not to. here's a way they'd work better or would they?

February 2008, - Department

by Tom Berg, Senior Contributing Editor - Also by this author

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Spread tandems - the widely spaced axle configurations found mostly on flatbeds, car haulers and refrigerated trailers - exist for good reasons, all related to weight. Under various weight laws, spread or "split" tandems can legally carry more pounds per axle than close-spaced tandems. The distance between the two axles can vary if the vehicle runs within a certain state whose weight law addresses axle spacings. Anything crossing state lines has to follow the federal weight rules, and they exactly define that distance.

Under the federal B formula, the axles must be 10 feet apart (though in practice they are placed 10 feet, 1 inch apart just to be safe for the times when a zealous enforcement officer measures that distance with a tape).With the "10-1" spread, each axle is considered independent, and can carry up to 20,000 pounds, versus 17,000 for a typical 5-foot tandem. The rig still may not exceed 80,000 pounds gross (but could go to 86,000 if folks lobbying for greater productivity succeed in liberalizing the laws).

For now, the extra allowable axle weight for the trailer gives more loading flexibility, and the driver doesn't have to worry as much about balancing the load between the tractor and trailer.

If the tractor's air-ride tandem suspension has a plumbed-in gauge, the driver can load his reefer trailer's front area until the gauge reads 50 psi, or whatever it does when it's carrying just about 34,000 pounds (which he recalls from scaling the rig). Then he can load the rest of the trailer as necessary, knowing that he can cram all the produce or whatever he usually carries into the rest of the trailer without overloading its spread tandem. Its air suspension automatically equalizes the weight among the four wheel positions.

With extra-dense cargo (potatoes, maybe), or if the trailer's a flatbed, he can bias the weight slightly toward the rear and be pretty sure that he's not exceeding axle weight. Even better, he can install an air gauge on the trailer tandem and learn to interpret it. (The best idea is to install on-board scales on the tractor and trailer, but that's another matter.) With a less-than-full load, an owner-operator can choose to save a bit of wear and tear on his tractor by shifting more weight to the company's trailer.

But when being pulled through a tight turn, a spread tandem suffers from huge lateral forces. This scuffs tires, and can break wheels and suspension parts. (I once watched a hot-headed young driver, apparently frustrated at not being able to find a parking slot at a truckstop, do a 90-degree right-hand turn, then two 180-degree U-turns, all within a minute, each time roughly dragging his loaded flatbed's spread tandem across the pavement as wheels and tires hopped and groaned. I was surprised that nothing seemed to break.) Damage from such abuse usually happens away from home and converts to an expensive road call.

Managers at big fleets are aware of those problems and figure they outweigh (so to speak) any loading advantages of a spread tandem. So they usually stick with the close-spaced tandem, spec'ing it with a slider that allows moving the tandem fore or aft to alter the weight it carries. That's fine if the driver knows how to properly use the slider, if he can break it loose from accumulated rust and crud in the rails when he needs to, and if he secures the pins after adjusting the slider so it doesn't run off the end of the trailer. (There are products that reduce those problems, and they include air-powered pin releases and tapered pin heads; trailer dealers should be aware of them.)

Why not use a steerable axle with a spread tandem? Steerable auxiliary axles are used on heavy construction trucks in so-called bridge-formula states. These axles are caster-steered, and their wheels automatically swing in the direction they're pulled. When the truck backs up, the axles automatically lift off the ground so their wheels don't go all askew. Lifting is actuated by the truck's backup-light circuit, which is energized when the driver shifts into reverse. (And of course they're also raised when the truck's empty.)

Wouldn't that work on a spread-tandem trailer? Putting a steerable axle at the forward position would let it swing through a turn as the trailer pivots on its stationary rear axle - no more hopping and scuffing. Putting a steerable axle at the rear would let the trailer pivot on its stationary forward axle while the rear one follows easily along. This would also shorten the trailer's effective wheelbase and make it more maneuverable. Any off-tracking of the trailer's rear end would be minimal, and in fact some trailers in Europe have steerable axles (though not with spread tandems).

Of course, most trailers need to be backed up to load and unload. The caster-steered axle would then have to be locked in the straight-ahead position. Then there'd be some tire scuffing, etc., if it's backed through a turn. However, there's another axle product that steers in reverse as well as forward. The reverse-steering auxiliary axle uses geometry that casters in either direction. A pair of common brake chambers push or pull the mechanism, changing the geometry as needed. As with steerable lift axles, the chambers are activated by the backup-light circuit. When the driver shifts out of reverse, the light circuit is de-energized and the chambers push the steering geometry into the forward position.

"Simple!" as former presidential candidate H. Ross Perot used to say. But he wasn't in the trailer business. Those who are in the business say a steerable axle is far more complex than a stationary one, so would add considerable upfront cost and require more maintenance, and that might offset any savings from longer tire and wheel life. Also, if the axle has dual wheels on each side (most of them do), there isn't enough room between the inner duals and the trailer's frame for any appreciable amount of wheel cut.

But, sez me, single-wide wheels and tires, especially if outboard mounted on wide-track axles, would have enough room for steerable wheels to cut, and most, if not all of the scuffing, would be eliminated. And tests have shown that compared to duals, single-wide tires will reduce fuel use by what - 3 to 5 percent? - due to lower rolling resistance.

Well, you can put single-wide tires and wheels on non-steerable axles and still get the fuel savings (not to mention about 400 pounds less tare weight), and some operators already do that. And, say trailer people, truckers who use spread tandems have learned to live with the tire scuffing. (Some use dump valves to bleed air out of one axle's bags and relieve it of weight during turns. That's illegal and overloads the one axle that then carries all the load, but that's the real world.)

Forget your steerable axle idea, Berg.


As Rosanne Rosanna Danna (remember her on the old "Saturday Night Live"?) used to say, "Never mind."


  1. 1. Dusty Rhodes [ October 19, 2017 @ 08:59PM ]

    In the early '60 I was hauling gasoline for Texaco out of the Lake Charles, La. barge terminal. We had 5 tractor tank trailer rigs and one of the tank trailers had a spread axle tandem with an air lockable rear swing axle.
    The trailer would track around a corner in the tractors tire marks. It was a dream to haul around. Got to where we didn't even check our mirrors when going around corners.
    Every time I see a spread axle trailer go by me on the highway I think back to that trailer and wonder why all these spread axle trailers aren't built that way.


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