How do you adjust the caster on a McPherson strut?
Adjusting the caster on a McPherson strut is a crucial maintenance task for ensuring proper vehicle handling and tire wear. The caster angle, which is the angle of the steering axis relative to the vertical axis, plays a significant role in the vehicle's steering characteristics and stability. This article will provide a detailed, step-by-step guide on how to adjust the caster on a McPherson strut.
What is Caster Adjustment?
Caster adjustment refers to the process of aligning the steering axis of a vehicle's front suspension system. The caster angle is the angle between the steering axis and the vertical axis, as viewed from the side of the vehicle. A positive caster angle, where the top of the steering axis is tilted rearward, provides stability and straight-line tracking, while a negative caster angle can lead to poor steering feel and reduced high-speed stability.
Tools and Materials Needed
- Automotive repair manual for your vehicle
- Caster adjustment tool or shims
- Torque wrench
- Chalk or marker
- Wheel chocks
Before beginning the caster adjustment process, ensure that your vehicle is parked on a level surface and the wheels are chocked to prevent the vehicle from moving.
Steps to Adjust Caster on a McPherson Strut
Locate the caster adjustment components. These are typically found at the top of the McPherson strut, where the strut connects to the vehicle's chassis.
Loosen the caster adjustment bolts or nuts, but do not remove them completely. This will allow you to adjust the caster angle.
Use a caster adjustment tool or shims to adjust the caster angle. Refer to your vehicle's repair manual for the recommended caster angle range and adjustment procedure.
Once the caster angle is set, tighten the adjustment bolts or nuts to the specified torque value using a torque wrench.
Recheck the caster angle to ensure it is within the recommended range. Repeat the adjustment process if necessary.
After the caster adjustment is complete, have the vehicle's alignment checked and adjusted by a professional, if necessary.
Proper caster adjustment is essential for maintaining optimal vehicle handling and tire wear. By following the steps outlined in this guide, you can ensure that your McPherson strut is properly aligned and your vehicle is performing at its best.
How do you adjust the caster angle?
Caster angle is adjusted by altering the position of the suspension's pivot points, typically by moving the upper or lower control arms forward or backward using adjustable arms, shims, or eccentric bolts. Increasing positive caster involves moving the upper ball joint rearward or the lower ball joint forward, while decreasing it involves moving them forward or backward respectively. Be aware that caster adjustments also affect camber, so the camber angle must be re-checked and adjusted as needed to maintain proper alignment.
This video demonstrates how to adjust caster using an adjustable control arm: 1mRMB Racing IncYouTube · Dec 7, 2020
Methods for adjusting caster
- Adjustable Control Arms: These are a common method, especially on performance or modified vehicles. You can loosen the control arm, adjust its length or position to move the pivot points, and then tighten it down.
- Shims: For certain suspensions, shims are placed at the front or rear of the upper control arm's attachment points. Adding shims to the rear and removing them from the front will increase positive caster.
- Eccentric Bolts/Cams: Some vehicles use eccentric bolts or cams, often on the lower control arm. Turning these eccentric components moves the ball joint or pivot point, which adjusts the caster angle.
- Adjustable Strut Rods: Vehicles with MacPherson strut systems may use an adjustable strut rod connected to the lower control arm. Adjusting this rod by moving it forward or backward will change the caster.
- Caster Wedges: For solid axle vehicles with leaf springs, caster wedges are used. These are placed between the leaf spring and the axle to change the angle of the axle and set the caster.
This video shows how to adjust caster using shims: 58sFast Monty's GarageYouTube · Jan 31, 2022
Important considerations
- Vehicle compatibility: Not all vehicles have adjustable caster. Many front-wheel drive cars, for example, do not have an adjustable caster angle from the factory.
- Camber change: Adjusting caster will almost always affect the camber angle. You must check and re-adjust the camber after every caster adjustment.
- Stability and handling: Positive caster improves straight-line stability and self-centering, while negative caster makes the steering more responsive but less stable at high speeds.
- Accuracy: Adjustments must be made equally on both sides of the vehicle for proper alignment. A maximum difference of one-half degree between sides is generally recommended.
How do you adjust the caster on a MacPherson strut?
One way to adjust caster is with an adjustable arm. In the case of a MacPherson strut system this can be in the form of an adjustable tension rod that holds the lower arm forwards or backwards in the wheel arch. The screw can be adjusted to move the wheel forwards or backwards adjusting the caster angle.
What happens when you have too much positive caster?
Too much positive caster makes steering heavy and difficult at low speeds, and can cause the steering to feel "snappy" or "twitchy". While it improves high-speed stability, it can lead to increased steering effort, potential oversteer due to the "jacking effect," and may increase steering system vibrations or "caster shimmy".
Negative effects of too much positive caster
- Heavier steering: The steering wheel becomes significantly harder to turn, especially at low speeds like when parking.
- Snappy or twitchy steering: The strong self-aligning torque can cause the steering wheel to snap back to the center very quickly, making the vehicle feel "twitchy".
- Increased steering effort: Without power steering, excessive positive caster requires a lot of force to turn. Even with power steering, it can put more stress on the system.
- Potential oversteer: A high caster angle can cause the inner front wheel to lift during a turn, a phenomenon known as "jacking". This puts more weight on the outer rear wheel, which can lead to oversteer.
- Caster shimmy: Excessive caster can lead to vibrations or "caster shimmy" in the steering system.
- Increased tire wear: Excessive positive caster can exacerbate tire wear issues, particularly when combined with incorrect toe settings, leading to "feathering" of the tread blocks.
Balancing caster settings
- Excessive caster is a problem because it makes the steering heavy and less responsive at low speeds, even though it improves high-speed stability.
- A proper alignment should find a balance that suits the vehicle's intended use. For performance applications, it is sometimes set higher to improve cornering stability, but this requires power steering to compensate for the extra effort.
- The goal is often to find the sweet spot where stability is achieved without making the steering uncomfortably heavy or causing undesirable effects like oversteer or tire wear issues.
What are the symptoms of a misaligned caster?
Signs Your Caster Angle Is Off
- Vehicle pulling to one side, especially at higher speeds.
- Uneven steering effort—feels harder to steer in one direction.
- Steering wheel doesn't automatically return to center.
- Tires wearing unevenly despite regular rotations.
- Driver fatigue from constant correction.
