Caster Angle Explained

Positive and negative caster

Positive caster tilts the steering axis rearward at the top. This creates a trail effect that helps the wheels return toward straight ahead and improves directional stability. Negative caster tilts the axis forward and generally reduces returnability. Most modern vehicles use positive caster, often with values that would have been considered high on older manual-steering vehicles.

Caster is not the visible tilt of the wheel. It belongs to the steering axis formed by the upper and lower pivot points, or by the strut mount and lower ball joint on a strut suspension.

How caster is measured

Imaging aligners calculate caster by observing camber change as the wheels are turned through a controlled angle. The software must know the actual steering angle and track the targets without interruption. The vehicle should rest freely on centered turn plates, and the steering should move smoothly without brake application or plate binding.

Cross caster and directional behavior

Cross caster is the left-to-right difference. A vehicle may tend toward the side with less positive caster, but tire force, camber split, road crown and power-steering behavior can alter the result. Use the vehicle specification instead of applying the same road-crown offset to every vehicle.

Caster adjustment

Caster may be adjusted through control-arm cams, strut rods, shims, movable subframes or adjustable links. Because these components also position the wheel fore-and-aft and sideways, caster adjustment can change camber, setback and toe. Follow the manufacturer sequence and monitor all values live.

1. Verify ride height and component condition.
2. Center the adjusters within a usable range where practical.
3. Adjust caster and camber together according to the suspension design.
4. Tighten with the suspension at the required position.
5. Repeat the caster sweep after tightening.
6. Complete front toe last.

Caster as a diagnostic value

Caster outside range on a non-adjustable vehicle can indicate a shifted subframe, bent strut, damaged control arm, incorrect bushing position or ride-height problem. Compare caster with setback and SAI to identify whether the wheel has moved fore-and-aft or whether the steering pivot itself is displaced.

Best practices

• Center the turn plates before the vehicle is positioned.
• Remove locking pins only when the vehicle is secure.
• Do not press the brake unless the procedure requires it.
• Use the requested sweep angle and follow on-screen prompts.
• Repeat the sweep after major caster or camber changes.
• Investigate poor repeatability before adjustment.

Frequently asked questions