Double Wishbone and Short-Long Arm Suspension

Double wishbone suspension — also called short-long arm or SLA — uses two wishbone-shaped control arms at each corner. One upper arm and one lower arm connect the steering knuckle to the vehicle frame or subframe. The geometry of these two arms controls how the wheel moves as the suspension travels.

The steering knuckle is attached to the outer end of each control arm through a ball joint. The inner ends of the control arms attach to the frame or subframe through bushings. As the wheel moves up over a bump, both control arms pivot on their inner bushings while the ball joints allow the knuckle to rotate. The spring — separate from the shock absorber in most double wishbone designs — sits on the lower control arm and controls ride height.

Double wishbone suspension allows very precise control over how the wheel moves during suspension travel. By making the upper arm shorter than the lower arm — which is why it is also called short-long arm — engineers can tune the suspension so that the wheel gains negative camber as it moves upward. This keeps the tire contact patch more evenly on the pavement during hard cornering. Performance vehicles and luxury vehicles often use this design because it provides better handling characteristics than MacPherson.

Double wishbone designs have both an upper and lower ball joint at each corner. Inspection requires the vehicle to be on a drive-on rack with suspension loaded. Grasp the tire and check for play in both directions — lateral movement at 9 and 3 o'clock for tie rods, vertical movement at 12 and 6 o'clock for ball joints. Upper and lower ball joints each have their own specification. Do not assume the lower is good because the upper is okay — check both independently.