Caster is adjusted by physically moving a suspension pivot point forward or backward, typically at the upper control arm, strut mount, or lower control arm. This requires specific tools and often aftermarket parts, as most standard passenger cars lack factory adjustment. The process involves measuring the current angle, making precise adjustments, and then re-checking the full alignment, as caster changes will affect camber and toe settings.
For any adjustment, you must first measure your baseline. A professional-grade caster/camber gauge, mounted to the wheel hub, is essential. The standard procedure is to turn the steering wheel 20 degrees to the left, zero the gauge, then turn 20 degrees to the right and read the measurement. This angle, typically between 3 to 7 degrees of positive caster for most street cars, dictates your starting point.
The adjustment method depends entirely on your vehicle’s suspension design. Here are the most common approaches:
Double Wishbone Suspensions (using shims): This is a classic method on many trucks and older cars. The upper control arm is mounted to the chassis via a shaft with bolts. Adding or removing shims (thin washers) between the shaft and the frame changes the arm’s position. To increase positive caster, you remove shims from the front bolt or add shims to the rear bolt, effectively pulling the top of the wheel backward.
MacPherson Strut Suspensions (using plates): Most modern cars use this design, which rarely has factory caster adjustment. The solution is aftermarket adjustable camber/caster plates that replace the top strut mount. These plates allow you to slide the top of the strut assembly forward or backward in slots, directly altering the caster angle.
Adjustable Tension Rods (Strut Rods): Common in performance adjustments, replacing the fixed tension rod that connects the lower control arm to the frame with an adjustable, threaded unit. Shortening the rod pulls the lower control arm forward, increasing positive caster. Lengthening it pushes the arm back, decreasing caster.
Other Methods include using offset control arm bushings that reposition the arm’s pivot, or adjusting eccentric cams on the lower control arm mounts of some trucks and SUVs.
A critical industry practice is to perform a full wheel alignment after any caster adjustment. Changing caster directly shifts the camber and toe angles. Aligning caster first, then camber, and finally toe is the standard sequence to avoid chasing incorrect settings. Attempting alignment with worn ball joints, bushings, or tie rod ends is futile, as the suspension cannot hold a precise position. Replacement of worn components must precede any adjustment.
The table below summarizes the primary methods and their applications:
| Adjustment Method | Typical Vehicle Application | Key Action for More Positive Caster |
|---|
| Shim Addition/Removal | Double wishbone suspensions (trucks, classic cars) | Remove shims from front bolt or add to rear bolt of upper control arm shaft. |
| Adjustable Caster Plates | MacPherson strut suspensions (most modern cars) | Slide the top of the strut assembly rearward within the plate's slots. |
| Adjustable Tension Rods | Performance vehicles, aftermarket upgrades | Shorten the rod to pull the lower control arm forward. |
| Offset Caster Bushings | Various, as a bushing replacement solution | Install bushings that reposition the control arm pivot point. |
Positive caster improves high-speed stability and steering return but increases steering effort. Negative caster makes steering lighter but can compromise stability. For daily driving, a moderate positive setting is ideal. For performance, increased positive caster (up to 8 degrees or more) improves camber gain during cornering for better grip. Always consult your vehicle’s specifications or a trusted performance shop for target values.
