Suspension Systems Diagnosis and Repair
Spring and beam suspensions
Leaf spring suspensions (multi-leaf, taper-leaf, parabolic) locate the axle through spring eyes, shackles, hangers, U-bolts, and center bolts; tandem tractors often add an equalizing beam. Every rubber bushing in that chain is a wear point, and a worn shackle or spring-eye bushing lets the axle steer, producing wander and a pull that changes with load. Torque U-bolts with the vehicle at ride height and weight on the springs, never with the axle hanging, and retorque after the first few hundred miles. A broken center bolt or a shifted leaf clamp lets the axle walk.
Air suspensions
Air ride systems add air springs, a height control valve, and linkage. Ride height is the master measurement: it must be set to the OEM dimension before any alignment or driveline diagnosis, because height changes caster, camber, and U-joint angles. Check the valve linkage for bending, the bags for chafing and leaks, and remember a bag can leak only at certain heights. Shocks damp motion; they never carry weight, so a sagging truck is not fixed with shocks.
Rule of thumb
If a tractor leans to one side, measure ride height left and right with the air system at normal pressure. A difference beyond OEM tolerance sends you to the bag, the valve, or a bent link, not to the springs. To confirm a bad height valve, disconnect its link and move the arm by hand; the frame or cab should rise and fall smoothly.