What caster is and why you adjust it
Caster is the angle of your steering axis when viewed from the side of the vehicle. It measures how far forward or backward that axis tilts compared to a vertical line. Most vehicles have positive caster — the steering axis tilts backward — which helps the wheels return to center after a turn and keeps the vehicle stable at highway speeds.
You adjust caster when your vehicle pulls to one side, the steering feels heavy or unresponsive, or when a wheel alignment shows caster is out of specification. Unlike toe and camber, which wear out and drift over time, caster usually only needs adjustment after suspension work, collision damage, or if the vehicle was built with a caster angle outside the manufacturer's range.
Caster adjustment is more involved than other alignment angles because it typically requires moving suspension components — usually the upper control arm or strut tower — rather than straightforward turning an adjustment bolt. The exact method depends on your vehicle's suspension design.
Key Takeaways
- Caster is measured in degrees and describes how far the steering axis tilts backward (positive) or forward (negative) when viewed from the vehicle's side.
- Most vehicles specify a caster range of 2 to 4 degrees positive, and adjustment is needed when the vehicle pulls to one side or steering feels abnormal.
- Adjustment methods vary by suspension type: shims behind the upper control arm, eccentric bolts on the strut, or adjustable upper control arm brackets are the most common.
- You will need a caster/camber gauge, jack stands, basic hand tools, and access to the vehicle's alignment specifications from the manufacturer or a service manual.
- If your vehicle has MacPherson struts or a complex suspension design, professional alignment equipment may be necessary to adjust caster safely and accurately.
Measuring caster before you adjust
Before making any adjustment, measure the current caster angle on both the left and right sides. Use a caster/camber gauge — a magnetic protractor that mounts on the wheel hub and reads the angle directly. Place the vehicle on a level surface, remove the wheel, and attach the gauge to the hub with the magnetic base.
Rotate the steering wheel fully left, then fully right, and record the gauge reading at each extreme. The difference between the two readings, divided by two, gives you the caster angle. Compare this to the manufacturer's specification, which you can find in the service manual or alignment specification sheet for your vehicle's year, make, and model.
Write down the caster angle on both sides. If one side is significantly different from the other, that imbalance is often the cause of pulling or steering issues. Most manufacturers allow a difference of no more than 0.5 degrees between left and right.
Adjusting caster with shims on the upper control arm
On many older vehicles and some trucks, the upper control arm bolts to the frame through a bracket. Shims — thin metal spacers — sit between the bracket and the frame. Adding shims behind the upper control arm (toward the rear of the vehicle) increases positive caster; removing shims decreases it.
Raise the vehicle on jack stands and remove the wheel. Locate the upper control arm bolts where they attach to the frame. You may need to remove the brake caliper and rotor to access them fully. Loosen the bolts but do not remove them completely.
Slide shims in or out as needed. Each shim typically changes caster by 0.25 to 0.5 degrees, depending on thickness and the distance from the pivot point. Tighten the bolts firmly, lower the vehicle, and remeasure caster with the gauge. Repeat until the angle matches the specification.
Adjusting caster with eccentric bolts on strut suspensions
MacPherson strut suspensions use eccentric (off-center) bolts at the upper strut mount or on the lower control arm. Rotating these bolts shifts the strut position and changes both caster and camber. The bolt has a round head with a flat edge; rotating it moves the wheel in a specific direction.
Raise the vehicle and remove the wheel. Locate the eccentric bolt — usually at the top of the strut tower or on the lower control arm bracket. Loosen it slightly (do not remove it). Use a wrench to rotate the bolt head; each full rotation typically changes caster by 0.5 to 1 degree, depending on the vehicle.
Rotate the bolt in small increments, lower the vehicle, and remeasure caster. Because eccentric bolts affect both caster and camber, you may need to adjust camber afterward to keep both angles in specification. This is why professional alignment shops use specialized equipment for strut vehicles — the two angles interact.
Adjusting caster with adjustable upper control arm brackets
Some modern vehicles have adjustable upper control arm brackets with slotted holes. Loosening the bolts allows the bracket to slide, which changes the caster angle. This method is less common than shims or eccentric bolts but offers fine adjustment without removing components.
Raise the vehicle and remove the wheel. Loosen the bolts on the upper control arm bracket — do not remove them. The bracket can now slide forward or backward in the slots. Moving it backward (toward the rear of the vehicle) increases positive caster; moving it forward decreases it.
Slide the bracket slightly, tighten the bolts, lower the vehicle, and remeasure. Make small adjustments — usually 0.25 degrees at a time — until the caster angle matches the specification. This method is often the easiest for DIY work because it does not require removing bolts or installing shims.
When to stop and call a professional
If your vehicle has a double-wishbone suspension with no visible adjustment points, or if the caster specification is very tight (less than 0.5 degrees tolerance), professional alignment is the safer choice. Some vehicles require removing and repositioning the entire subframe, which demands a lift and specialized tools.
If you adjust caster and the vehicle still pulls or the steering feels off, the problem may not be caster alone. Uneven tire pressure, worn suspension bushings, or bent components can mimic caster problems. A professional alignment shop can diagnose the root cause and confirm whether caster adjustment will actually solve the issue.
After any caster adjustment, have the vehicle's toe and camber checked and adjusted if needed. Caster, camber, and toe are interdependent on some suspension designs, and changing one can throw the others out of specification.
Tools and materials you will need
A caster/camber gauge is essential — expect to spend $30 to $100 for a basic magnetic version. You will also need a jack and jack stands, a wrench set, a socket set, and the vehicle's service manual or alignment specification sheet. If you are using shims, you may need to purchase a shim kit specific to your vehicle, typically $15 to $40.
Have a helper on hand if possible — measuring caster requires rotating the steering wheel fully in both directions while reading the gauge, which is easier with two people. Wear safety glasses and work gloves, and never work under a vehicle supported only by a jack; always use jack stands.
Frequently Asked Questions
How much does caster typically change after an accident or suspension work?
Collision damage can shift caster by 1 to 3 degrees or more, depending on the impact location. After suspension work like replacing struts or control arms, caster often drifts 0.5 to 1.5 degrees from the original specification. Always measure caster after major suspension repairs.
Can I adjust caster without removing the wheel?
No. You need to mount the caster gauge on the wheel hub, which requires removing the wheel and brake components. The gauge reads the angle of the hub itself, not the wheel rim, so the wheel must be off.
What happens if caster is too negative or too positive?
Excessive negative caster (tilting forward) makes steering light but causes the vehicle to wander and pull. Excessive positive caster (tilting backward) makes steering heavy and can cause shimmy at highway speeds. Both extremes reduce tire life and handling predictability.
Do I need to adjust caster on both sides?
Yes. Caster should be within 0.5 degrees on both sides. If one side is significantly different, the vehicle will pull toward the side with less positive caster. Always measure and adjust both sides to match the manufacturer's specification.
Can I adjust caster without a caster gauge?
Not accurately. A caster gauge is the only reliable way to measure the steering axis angle. Attempting to estimate caster by feel or by measuring ride height will lead to incorrect adjustments and ongoing handling problems.