Wrench Tech – February 2022
About 20 years ago I was cruising up Interstate 65 northwest of Indianapolis and came upon a tractor-trailer moving at a normal speed but tracking rather abnormally.
The dry-freight trailer was “dog-tracking,” also called “crab-walking.” Its right-rear corner was tracking a foot or more to the right of the tractor’s path. If the truck driver was aware of it, he kept going anyway because all four wheels on the trailer were parallel and rolling freely. He could report the problem when he got to his destination, assuming it wasn’t a consignee’s dock, where another load might be shoved aboard the crippled van.
The trailer was lettered for Ryder System, and I happened to know the guy in charge of maintenance for the huge fleet’s vehicles, Blaine Johnson, a former mechanic who had risen to be a vice president. I noted the trailer’s unit number and, when I found a place to pull off, I pulled out my cellphone and called him at his office down in Miami.
“Hey, Blaine,” I said, “I’m out here on I-65 following one of your rigs, and you’ve got a trailer crab-walking its way toward Chicago.”
I gave him the approximate location of the trailer and its number, and after chuckling and thanking me, he said he’d get in touch with shop people at the location where the trailer was assigned.
A few days later he called me back to relay how the shop foreman had reacted to the report.
“How would you know about that?” the surprised foreman asked.
“Oh, I’ve got eyes all over the place,’” Blaine said he’d told him. He added that the trailer had been brought in for some work.
What could’ve caused both axles to go out of whack?
Probably the trailer’s wheels hitting a tall curb, or being yanked through a large, deep pothole, or who knows what, but it must’ve been at something other than low speed. That means the suspension also might have been damaged.
“From the standpoint of spring suspensions, impact to the tires or suspension are a common cause of misalignment,” said Steve McDonald, applications engineer at Hutchens Industries, which makes suspensions. “This can damage the radius rods or displace the hangers enough to shift the axle. Worn components can contribute to axle misalignment as well. Maintaining proper torque on the fasteners will go a long way to prevent component wear.”
There might not be much visible damage, he said, but if the axle moves just a quarter inch it’s enough to throw it out of alignment. It’s no longer parallel with the following axle, and tires of the axle are scrubbing against the pavement. That causes abnormal tread wear, one of the symptoms of misalignment, and one of the things drivers should look for during pre- and post-trip inspections.
If you’re an owner-operator who pulls somebody else’s trailer, you might not care too much, except to report it at the first opportunity. But dragging tires create friction that must be overcome by your tractor’s engine, and that costs you fuel. So you should trade that trailer for a good one as soon as you can. If you own the trailer, you’ll want to fix it quickly.
The first step is parking it on a level surface and taking some simple measurements to determine the extent of the misalignment condition. Proper height of the kingpin, as listed on the trailer builder’s data plate, should set by adjusting the landing gear up or down.
The Technology & Maintenance Council has a recommended practice that involves measuring the distances between the trailer’s kingpin and the axle ends.
The two distances should be identical, and if they’re not, you’ll know that axle is misaligned, and by how much. RP 708D lists the tools and equipment needed to do the measuring, which include these items:
- A 50-foot (minimum) steel tape measure, marked in 1/32-inch or millimeter increments.
- A tensioning device, like a fish or engineering scale, to keep the tape taut during measuring, and something to secure the device to the tape measure.
- A kingpin adapter (an oval device that mounts to the kingpin) or short pole to extend the king pin’s location downward, and a level to plumb the extension and to ensure the upper coupler platform is level.
- Wheel end (or spindle) extenders, each with a leveling bubble. The measurements will be from the center of the kingpin to the centers of the axle hub, as determined by a plumb bob to the tape laying on the pavement. Additional measurements include distances between the two hubs on each side of the tandem. A shop that does a lot of this work will probably use laser or computerized electronic alignment equipment.
The Truck Trailer Manufacturers Association also has recommendations.
One passage says: “Before taking axle alignment measurements, make sure the trailer is unloaded and free the suspension of any binds by pushing the trailer backward and then pulling the trailer forward. While pulling the trailer forward on a level floor, apply the brakes and release. This will ensure that an adjustable undercarriage is in its rearmost locked position. Trailers with air-ride suspensions shall be aligned with the suspension at its nominal ride height. The trailer must be level from side to side as well as from front to rear. Note: Neither service nor parking brakes shall be applied during the measurement procedure.”
With the trailer’s landing gear down, the tractor decouples from the trailer to allow access to the kingpin. Then inspect that and many other parts. Among them are the suspension and slider (if the trailer has one, it should be at the rearmost position), hubs and spindles, and bearing settings. Then measurements begin (see illustration), and axles are realigned as needed by using adjusting mechanisms on the suspension.
If you do not have a facility and tools for this kind of work, and you’ve done little or none of it, now is a good time to look for a shop with the equipment and experienced people to do the axle alignment on your trailer. And next time you’re on the road, be sure the images of the trailer’s sides are the same in each mirror. If you see little of one side and a lot of the other, you just might be crab-walkin’ to Chicago. LL