Automotive Suspension Failures

Charles C. Roberts, Jr.

Automobile suspension systems are mechanical devices whose function is to support the vehicle body and other components above the wheels. There are a variety of designs including coil spring, longitudinal leaf, transverse leaf, torsion bar, MacPherson, Christy, and solid axle.

Figure 1 - MacPherson strut suspension

Figure 2 - Solid axle suspension

Figure 3 - Control arm suspension with coil springs

Figures 1 through 3 are drawings of typical suspension systems found on most vehicles on the road. Figure 1 is the classical MacPherson strut suspension, which is common on many front drive vehicles. The strut, which is also a shock damper, moves vertically while the control arm limits transverse and longitudinal movement. The system is compact, efficient and adapts easily to front and rear applications. Figure 2 is a view of an earlier design: the solid axle suspension with king pin. The solid axle beam is supported by springs and connects to a swiveling axle via the king pin. This suspension is often used on heavier vehicles such as trucks and on some older vehicles. Figure 3 depicts a control arm suspension with coil springs. This independent suspension system is used on many older and rear wheel drive vehicles. Automobile accident investigation may focus on a vehicle's suspension system, being guided by evidence of possible malfunction or statements from the insured driver or witnesses. Automotive suspension failure can be caused by a design defect, a manufacturing defect, poor maintenance or the accident.

Figure 4

Figure 4 is a view of a MacPherson front suspension on the right side of a compact car. Evidence suggests that the lower ball joint (arrow) failed, causing the vehicle to steer uncontrollably, which resulted in an accident. Figure 5 is a top view of the ball joint showing wear patterns from the drive shaft rotor just above the ball joint. The ball joint itself was dry and badly worn with no evidence of lubrication. The vehicle had over 100,000 miles on the odometer. The wear on the top of the ball joint suggests that for a period of time, the joint had failed and had moved vertically and rubbed against the axle rotor. The rotor was acting as a retainer of the joint, preventing it from separating from the suspension. This condition would result in excessive play in the steering, plus a loud noise that should have acted as a warning to the insured driver that a problem existed. The driver continued to operate the vehicle until the accident occurred. The failure of the ball joint was determined to be maintenance related with no evidence of a manufacturing defect.

Figure 5

Figure 6

Figure 6 is a view of a king pin assembly from a large road tractor. Figure 7 is a close-up of a crack in the king pin housing. A truck driver claimed loss of control while on a winding rural highway. Analysis of the housing fracture surface indicated that environmentally assisted cracking had caused the failure. What initiated the environmentally assisted cracking was severe wear from lack of lubrication, a maintenance related failure.

Figure 7

Figure 8

Figure 8 is a view of a front control arm suspension with a fractured tie rod end. Figure 9 is a close-up of the fracture surface. The driver indicated that the tie rod end suddenly failed, and an accident resulted. The lower arrow in Figure 9 points to a corrosion related crack that had formed through the tubing wall. Despite the corrosion damage to the tubing, the fracture surface (white area, upper arrow) is characteristic of a sudden overload, indicating that a sudden failure under normal conditions was unlikely. The likely cause would be an impact with a noncompliant object such as a curb or another vehicle.

Figure 9

Figure 10

Figure 10 is a view of a rear control arm bolt in a late model front drive automobile. The driver complained of loss of control, which resulted in a vehicle rollover and personal injury. The right end of the bolt had fractured. A close-up of the fracture surface is shown in Figure 11.

Figure 11

Metallurgical analysis of the part revealed improper heat treating of the bolt, which was the cause of the failure and was a manufacturing defect. When the bolt failed, the right rear control arm parted from the suspension, causing the right rear tire to point outward at an angle. The ensuing yaw motion terminated with a vehicle rollover.

Figure 12

Figure 12 depicts the right rear tire of a vehicle with a severe toe-in of about 30 degrees. The body damage is characteristic of having struck another vehicle. The control arm and tie rod end was badly bent, but not fractured, suggesting that this condition was most likely a result of the impact. Suspension systems are often blamed as a cause of an accident. Driver error can explain many of the accidents, while the remaining ones can be attributed to poor maintenance, design or manufacturing defects. Obviously, insurers are interested in causes of failure that suggest negligent behavior by some other party for subrogation purposes. If legal action is contemplated, then potential litigants should be placed on notice as to the existence of the evidence and a joint protocol developed before any destructive testing is performed.