Early beam-type axles with leaf springs have fixed camber. As the vehicle's frame and body lean on corners, the axle stays parallel with the ground. With this design, there is no provision to adjust the camber. Unless bent, the axle maintains its original camber setting.

By contrast, each front wheel on an IFS or twin I-beam suspension moves up or down independently. Short (upper) and long (lower) control arm "SLA" suspension compensates for body/frame lean on corners. As the vehicle corners, the body/frame rolls, causing the outside wheels to compress the springs. This moves the wheels upward into the fenderwells. The unloading inside wheels drop as the body rolls outward. (Functioning sway bars help minimize this roll.) On SLA suspension, tire track width remains relatively constant during this process. The arc of wheel travel is precisely engineered for maximum control and longer tire life.

SLA suspension is a true independent design. Ford's twin I-beam has been called IFS because each beam moves independently. However, these two solid beams pivot from fixed points. This means that on turns, the outer wheel compresses the coil spring and moves in a fixed arc upward. The inner wheel drops as the body rolls outward. Optimally, these arcs should cancel each other, keeping tire track width reasonably constant.

Twin-beam suspension has a drawback. If the chassis rises upward while the truck is on a straight road (like a raised railroad crossing taken at speed), the beams drop in fixed arcs, with each wheel shifting to positive degrees of camber. This narrows the tire contact patches and track. (Simulate this by lifting a twin I-beam truck on a frame hoist. Watch the front wheel drop angles.) When the chassis settles, the beams must force the tires outward to near vertical camber. Under certain driving conditions, lateral scuffing increases tire wear.

Twin I-beam is also sensitive to vehicle ride height. When installing a cab-over camper or a heavier V-8, the added weight requires frontend camber and toe-in adjustment. With SLA/IFS, wheel travel arcs have less effect on camber, and the steering toe is more controlled during suspension movement and changes in vehicle ride height.

Caster
Caster plays an important role in vehicle handling. Caster is the tilt of the kingpin or ball joint centerlines when viewed from the side. The forward or rearward tilt of this centerline, measured in degrees, is caster angle. Note that the kingpin or ball joint centerline tilts inward as well, but this is not caster. Inward tilt is kingpin inclination, a measurement that affects the steering axis inclination (SAI). When caster and camber are within specification, SAI should be as well. If not, suspect a bent or defective spindle/knuckle.

The typical front suspension calls for positive degrees of caster. Positive caster means that the upper end of the kingpin/ball joint centerline tilts toward the rear of the vehicle. Caster has a very important function: helping the wheels come back to center after turns. When there is no caster angle, the most notable symptom is the need to steer the vehicle back to center. Correct caster settings also help steer the vehicle straight ahead, despite the crown of the road. Caster angle helps prevent pull and reduces the risk of kingpin or ball joint shimmy.

Whether setting up a modified suspension or simply restoring a classic truck's steering, take caster, camber, and toe-in into account. Changes in spring length, dropped spindle kits, and lowering a truck may alter these adjustments. For improved steering control, safe handling, and minimizing tire wear, wheel alignment plays a vital role.