Through the 1950s, all domestic trucks featured a ladder frame with leaf springs front and rear. These semi-elliptic springs arch below the framerails. The steering knuckles pivot on kingpins at each end of a solid axle beam. A truck's load capacity reflected its frame strength, axle beam size, steering gear, linkage stamina, spring load rates, and brake capacity.

Use of a beam front axle with a torque tube or Hotchkiss rear axle was common. Ford pickups through 1941 had a transverse leaf spring front and rear with a torque tube rear axle. (Radius rods in wishbone form lend support to the front axle beam and torque tube rear axle.) GM used semi-elliptic leaf springs front and rear with a closed, torque tube rear axle on 1/2-ton trucks through first-series 1955 models. When second-series 1955 GM models rolled from the assembly lines, the solid Hotchkiss rear axle with an open driveshaft became a mainstay for all American trucks.

By the end of the 1950s, the emphasis on ride quality and faster highway speeds demanded better handling. Automobiles had independent front suspension; rear link-and-coil suspension appeared on GM cars. In a bold initiative, the 1960 Chevrolet and GMC light trucks offered rear link-and-coil suspension and independent front suspension (IFS). Unlike GM passenger cars, the double A-arm front suspension used torsion bars instead of coil springs.

The use of torsion bars was not widely accepted. At the time, only Chrysler's cars used torsion bars with IFS. By 1963, GM light trucks adopted double A-arm front suspension with coil springs. Eventually, rear link-and-coil suspension would yield to a traditional leaf-spring layout. A front stabilizer bar, spindles with ball joints, a rear track or panhard bar and other passenger car features were present in GM's 1960 truck suspension-in a far more rugged form, of course!

Ford, Dodge, I-H, and Studebaker light trucks maintained their one-piece beam front axles for several more years. Ford broke ground in 1965 with twin I-beam front suspension. The twin half-axle system closely maintains the stamina and proven worth of beam axles.

These half-axles are offset, and their pivot points overlap. Radius arms support the beams fore-and-aft. Coil springs near each axle end replace conventional leaf springs for longer travel and improved ride quality. Maintaining conventional kingpins and spindles, twin I-beam suspension qualifies as independent front suspension but falls far short of the ride quality and efficient suspension geometry found in GM's unequal length A-arm truck suspension.

Suspension Wear
When inspecting a truck's suspension system, check out all pivots and moving parts. On vintage models, the axle beam must be true, not bent or damaged. Make sure the kingpins and spindles fit properly before checking the beam for straightness. Loose spindle bushings will distort the caster and camber readings. A bent or worn kingpin will also distort these measurements. Camber and caster angles are the best indicator of a beam axle's condition. (Learn more about caster and camber in next month's lesson.)

On kingpin suspension with leaf springs, the spring bushings and shackles are a common wear point. This also applies to rear leaf springs. Later trucks use rubberized spring bushings, which wear and deteriorate over time. Early chassis use bronze bushings at the leaf-spring anchor eyes and the shackle ends. Play, looseness, or deterioration is a sign of failure.

Check for kingpin play by jacking the truck up and placing the beam axle on safety stands. Grip the wheel and tire at 6 and 12 o'clock. Rock the wheel while looking inside at the spindle and kingpin area. (This usually takes an assistant.) Movement between the spindle and kingpin is an indication of play. Anything more than the slightest movement is too much. New pins and new, ream-fitted bushings should fit together with no perceptible play.