Automotive Linkage Failure | PropertyCasualty360


Figure 1. Right rear link. (Photo: C. Roberts)

Drivers frequently claim that their vehicle suddenly tipped to one side, resulting in an accident. Occasionally, the cause may not be the fault of the driver. This case study illustrates this point. According to the insured, she was driving at the speed limit when her vehicle suddenly swerved, causing loss of control and hitting a wall head-on.

During an inspection in the body shop, there was serious damage to the front of the vehicle but no damage to the rear. It was discovered that the right rear link which controls the alignment of the right rear wheel, as shown in Figure 1, had separated from its normal position (arrow). There was no sign of impact causing the tie rod to detach, which controls the right rear wheel toe-in adjustment. This can lead to severe misalignment and loss of control that matches that reported by the driver.

Tie correctly installed. Figure 2: View of a rear wheel tie rod correctly installed on the same vehicle. (Photo: C. Roberts)

Figure 3 shows the threaded end of the tie rod that has come loose. Close inspection of the threads on the right shows that the crest of the threads is distorted to the right. This suggests that excessive force was pushing the adjusting nut to the right. The locknut has a chisel groove, suggesting that someone was trying to loosen the locknut by turning it counterclockwise. This nut and screw have a left-hand thread, which means that by turning counterclockwise, the jam nut is tight against the adjusting nut.

Deformed wires. Figure 3: Chisel (left arrow) and deformed threads (right arrow). (Photo: C. Roberts)
Normal and distorted threads. Figure 4: Normal and distorted threads.

Figure 4 is a drawing of the nature of the damage to the threads. The upper drawing shows the ridge and root of a normal threaded screw. The lower drawing shows the damaged wires with the deformed crest to the right, which corresponds to a force to the right.

Apparently, excessive torque was applied to the locknut, which damaged the threads under the adjusting nut. Due to the normal load of the tie rod due to the traction of the drive wheels and variations in the road, the tie rod eventually separated from the threaded connection, causing severe misalignment of the right wheel and loss of vehicle control.

The main cause is excessive torque applied to the locknut, which damaged the threads in the adjusting nut area. This is an incorrect setting procedure. The vehicle’s suspension had recently been realigned at a local store.

Charles C. Roberts, Jr., Ph.D., PE, ([email protected]) is President of C. Roberts Consulting Engineers, Inc., which provides professional engineering services in the areas of accident reconstruction, failure analysis, fire causation, explosion analysis and biomechanics.


Leave A Reply

Your email address will not be published.