A-Arm Suspension Calculator

Double-wishbone IFS — camber gain, roll center, motion ratio, scrub, KPI & bump steer

3D view — drag to orbit; use the travel slider to cycle the suspension

lower arm upper arm upright shock tie rod roll center

Front view — both wheels (drag the white hardpoints at ride)

Kinematic curves vs wheel travel

How the A-arm calculator works

A double-wishbone front end is a planar four-bar in the front view: the lower arm swings the lower ball joint, the upper ball joint follows from the upper arm and the upright, and the wheel rides with the upright. Sweeping the travel gives every curve below. Origin = vehicle centerline at the ground, on the front axle line, at ride height; y = lateral, z = up.

What it computes

  • Camber gain — how camber changes through travel; set by the arm lengths and angles (the front-view instant center).
  • Roll center & FVSA — height of the roll center on the centerline and the front-view swing-arm length (low/long = low jacking).
  • Track change — lateral scrub of the contact patch through travel; long center-mount arms minimize it.
  • Motion ratio — shock travel per inch of wheel travel; sets spring rate and shock stroke.
  • KPI & scrub radius — steering-axis geometry that sets steering effort and feel.
  • Bump steer — toe change through travel; zero when the tie rod is concurrent with the arms at the instant center (verify with a gauge).

Long-travel desert target bands

  • Camber gain — ~−0.25 to −0.6°/in; enough to stay planted without big track change.
  • Track change — keep within ~2.5" at full bump; long arms help.
  • Roll center — near the ground for low jacking, small migration.
  • Motion ratio — ~0.5–0.66; balance shock stroke vs load.
  • KPI / caster / scrub — ~9° / 6–7° / 2–3" for off-road.
  • Bump steer — tie rod within ~2° of pointing at the instant center.

Presets

  • Center-mount long-travel (18") — long lower arms off a central pivot; flat camber, near-ground roll center, low bump steer.
  • Desert mid-travel (14") — a bulkhead-mount beam-delete Baja Bug layout.

Steering and suspension are life-safety systems. This is a geometry starting point — mock up on a jig, verify bump steer and clearances on the car, and have welds done/inspected by someone certified.