Screenshot: Harry P. Ness (Facebook)

Have you ever driven down a straight road and noticed that the car in front of you appeared to be driving sideways, with all four tires visible from directly behind? If so, you’ve been witnessing “Dog Tracking.” Here’s why it happens.

Making the rounds on Facebook is this video showing a second-generation Ford Explorer appearing to drive sideways down a straight road. The entire right side of the vehicle, as well as all four tires, are visible from directly behind:

I’ve seen such cars a number of times on Michigan roads, and it’s just a bizarre sight. So what’s going on?

To find out, I called up Woody Rogers from Tire Rack, who knows a thing or two about alignment, and I rang up Ken Lehto, a former Ford chassis engineer who worked on a bunch of leaf-spring suspension systems, including the one under the rear of the second-generation Explorer shown in the video. (Full disclosure: Ken is also the brother of onetime Jalopnik contributor Steve Lehto).

Rogers told me that Dog Tracking happens often in straight axle vehicles: The rear gets out of whack, and the solution is often just to adjust the front end to make the car drive in a straight line. “Often times, it comes after some kind of accident,” Rogers told me. “It’s a symptom of probably a much bigger problem.”

Lehto agrees, beginning by describing what’s going on. Essentially, he told me the rear axle is not lined up perpendicular to the axis of the car, and because the rear solid axle always travels essentially perpendicular to its own axis, that’s going to cause some issues.

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“The rear axle is trying to steer the rear of the truck to the left,” he wrote me in an email. “If the driver were to hold the steering wheel straight, the rear of the truck would want to come around from the left and cause the truck to make a right turn - hence the driver must compensate by turning the wheel to the left.”

What Lehto is describing here is a concept called “Thrust angle,” which Tire Rack defines on its website as “an imaginary line drawn perpendicular to the rear axle’s centerline” that “compares the direction that the rear axle is aimed with the centerline of the vehicle.”

In other words, what’s happening with the Explorer is that there’s a large negative thrust angle, which means the rear axle’s heading is markedly different than the chassis’ heading. Here’s a more thorough description from Hunter Engineering, a company that’s been manufacturing car alignment devices for decades:

Thrust angle is something that can be measured on the alignment rack, with tire service company Les Schwab showing an example of a typical alignment sheet in the image below. Notice thrust angle there on the bottom :

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Keeping the thrust angle as close to zero as possible reduces chances of “dog tracking” like what’s shown in the top video. But, especially with a solid axle, fixing the problem with an alignment isn’t always possible.

That brings us to the question: How can an axle be pointed that far in the wrong direction in the first place? Before we get into that, let’s have a look at the Explorer’s basic suspension setup, which is known as a “Hotchkiss” design, employing two longitudinal leaf packs to act as the springs and to locate the solid rear axle.

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As shown above under a random Explorer that I slid under in Kentucky (yes, I risked being shot to take these pictures), four leafs together make up a leaf pack, which is fastened to the underside of a solid axle.

The ends of the leaf spring (which contain bushings) are held in a frame-mounted shackle on the rear of the truck and directly onto the frame at the front.

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The way this all works is this: as the vehicle goes over a bump, the wheel transmits forces to the axle, which pulls up on the leaf spring, bowing it upward. As the spring flexes, it straightens, and its fore-aft length gets longer, so the rear shackle rotates about its mounting point on the frame to accommodate.

With that out of the way, let’s talk about how the axle is located under the vehicle. We’ll start with the center bolt, represented by the red dotted line above. Also called a center pin, it goes through a hole drilled through all four leafs. The pin has a round head on one end that goes into a hole in the leaf spring perch on the axle, and a nut on the other end, which sticks through a hole in the leaf spring mounting plate.

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The leafs are squeezed together and against the perch on the axle via two U-shaped bolts appropriately called “U-Bolts,” which wrap around the axle, drop down on either side of the leaf pack, and are put in tension via four nuts on the underside side of a mounting plate.

Looking at the image below (which shows my Jeep Cherokee’s rear suspension, which is set up similarly to the Explorers, except inverted), you can see that fore-aft and lateral movement of the axle is eliminated, because the head of the center pin prevents it from shifting relative to the frame-mounted leaf springs.

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“In the mounting pad on the axle [labeled leaf spring perch above],” Lehto told me, “there’s a hole. When you install the spring, you make sure that the center bolt sticks through that hole, and that kinda keeps [the axle] from shifting fore and aft.”

Lehto described some possible causes of the Explorer’s strange Dog Tracking condition in an email. For one, he said, one of the two leaf spring center bolts could have broken or fallen out of the locating hole in the axle, allowing one side of that axle to shift fore-aft along the leaf spring.

If the axle slides forward on one side, it will cause both rear tires to toe in one direction, yielding a nonzero thrust angle, and thus making the rear of the car want to drive in a direction other than where the nose of the car is pointed.

In addition, Lehto says the front leaf spring frame mount on the left side could have failed, allowing the spring to move rearward, or the frame mount on the right side could have failed, allowing the entire spring to shift forward.

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He also says there could have been a failure in a leaf or in a U-bolt mounting plate. In addition, he told me, a U-bolt could simply have loosened, allowing the center pin to come out of its locating hole in the axle-mounted spring perch, and thus letting the axle shift on one side.

A bent frame could also cause such a condition, as could a large disparity in leaf spring stiffness, as a flat leaf spring on one end and a highly-bowed spring on the other would cause the axle to shift rearward on the saggy side.

On vehicles with independent rear suspension, the rear toe could possibly be adjusted out, and the dog tracking reduced. But this phenomenon is more common with solid axle vehicles with leaf springs, and for them, the strange handling associated with this weirdness, also called “crab walking,” usually means fixing a bent or broken bit of hardware.

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That said, there’s apparently a device called a “thrust alignment plate,” which actually offsets the leaf spring’s center pin, and allows it to be moved relative to the axle’s spring perch hole to fix thrust angle issues rather simply. It doesn’t seem like the best way to fix what’s almost certainly a structural issue, but it’s interesting nonetheless:

So if you see a car that looks to be traveling sideways down the road, it’s probably because the rear axle is pointed in the wrong direction due to an issue with the frame, the springs, or the way the leaf springs are mounted to the axle. In any case, it means the car is deeply screwed up, and possibly structurally compromised. So maybe give them a bit of space.