How to Fix Caster on a Car: A Comprehensive Guide

Caster is a critical angle in your car’s suspension system that significantly impacts steering feel, stability, and handling. Unlike camber or toe, caster isn’t about tire wear, but rather about the steering axis inclination towards the front or rear of the vehicle. Understanding and adjusting caster can dramatically improve your driving experience, whether you’re aiming for better track performance or enhanced daily driving comfort. This guide will walk you through how to measure, adjust, and understand the effects of caster on your car.

Measuring Caster Angle

Before making any adjustments, it’s crucial to know your current caster angle. Accurate measurement is the first step in effectively tuning your suspension. There are two primary methods for measuring caster: using turning plates with a gauge, and the manual method using suspension component points.

Using Turning Plates and a Caster Gauge

This method is favored for its speed and accuracy, especially in trackside adjustments. It requires specialized equipment but provides quick and reliable readings.

1. Setup: Position your car on a level surface. Place turning plates under the front wheels, ensuring they are set to 0 degrees.
2. Initial Reading: Attach a digital or bubble camber/caster gauge to the wheel hub or brake rotor. Set the caster gauge to 0 degrees.
3. Turning Outward: Turn the steering wheel outwards by 20 degrees using the turning plates.
4. Set to Zero: Without moving the turning plates, reset the caster gauge to 0 degrees again.
5. Turning Inward: Turn the steering wheel inward 20 degrees, back through the center, using the turning plates.
6. Final Reading: The reading on the caster gauge now indicates the caster angle for that wheel.
7. Repeat: Perform the same procedure for the other front wheel to measure its caster angle.

Measuring caster angle using turning plates and a digital gauge

Measuring Using Wishbone or MacPherson Strut Angles

This technique is more relevant in suspension design and can be used if specialized tools are unavailable. It involves a more manual approach using reference points on the suspension.

For Double Wishbone Suspension:

1. Side View: Visualize or draw a 2D side view of the front suspension.
2. Upper Mounting Point: Identify and mark the upper wishbone’s mounting point at the wheel hub.
3. Lower Mounting Point: Identify and mark the lower wishbone’s mounting point at the wheel hub.
4. Vertical Center Line: Draw a vertical line representing the wheel’s center line.
5. Connect Mounting Points: Draw a straight line connecting the upper and lower wishbone mounting points.
6. Measure Angle: The angle formed between the line connecting the wishbone mounting points and the vertical center line of the wheel is your caster angle.

Diagram illustrating caster angle measurement on a double wishbone suspension system

For MacPherson Strut Suspension:

1. Side View: Again, visualize or draw a 2D side view.
2. Strut Line: Draw a line along the length of the MacPherson strut, extending to the ground.
3. Vertical Center Line: Draw a vertical line representing the wheel’s center line.
4. Measure Angle: The angle between the strut line and the vertical wheel center line is the caster angle.

Adjusting Caster Angle

Factory cars rarely come with caster adjustments, making it a modification usually performed with aftermarket or motorsport parts. Here are common methods to adjust caster:

Adjustable Tension Rods or Arms

Primarily used in MacPherson strut systems, adjustable tension rods or arms allow for forward or backward movement of the lower control arm, directly influencing caster.

1. Location: Locate the tension rod, which typically connects to the lower control arm and the chassis.
2. Adjustment Mechanism: Identify the screw or adjustment point on the tension rod.
3. Forward/Backward Movement: Adjusting the screw changes the length of the rod, moving the lower arm forward or backward in the wheel arch. Moving it forward reduces positive caster; moving it backward increases positive caster.
4. Measure and Adjust: After each adjustment, remeasure the caster angle to ensure it’s within your desired specifications. Adjust both sides equally to maintain balance.

Adjustable tension rods for caster adjustment in a MacPherson strut suspension

Offset Caster Bushes

Offset bushes are a fixed but effective way to increase positive caster. They replace the standard bushes in the lower control arm mounts.

1. Bush Design: Offset bushes have their center bolt hole offset in a forward direction relative to the bush’s outer diameter.
2. Installation: When installed, the offset pushes the control arm rearward, effectively increasing positive caster.
3. Fixed Adjustment: Once installed, the caster increase is fixed and not adjustable without changing bushes again. This is a simpler, less expensive way to add caster but lacks adjustability.

Offset caster bushes for increasing positive caster angle

Offset Top Mounts

For MacPherson strut systems, offset top mounts are another method, often combined with camber adjustment features.

1. Top Mount Design: These mounts have the damper (shock absorber) mounting point offset, typically rearward.
2. Caster Increase: The rearward offset increases positive caster by repositioning the top of the strut.
3. Fixed Position: Like offset bushes, the caster adjustment from top mounts is fixed once installed. Some adjustable top mounts offer a range of positions but are adjusted prior to final installation.

Utilize offset top mounts for MacPherson struts to adjust caster and camber, often with a fixed offset.

Adjustable Hubs with Shims

In high-end motorsport or custom suspension setups, adjustable hubs using shims provide incremental caster adjustments.

1. Shim Placement: Shims are placed between the wishbone (control arm) and the hub mounting points.
2. Rearward Spacing: Placing shims on the front side of the hub (between the hub and the forward wishbone mount) spaces the wishbone rearward.
3. Caster Adjustment: This rearward spacing increases positive caster in small, adjustable increments based on shim thickness.
4. Precision Tuning: This method allows for very fine-tuned caster adjustments, crucial in racing environments.

Effects of Caster Adjustment on Vehicle Dynamics

Adjusting caster significantly alters your car’s handling characteristics. Understanding these effects is key to making informed adjustments.

Benefits of Increasing Positive Caster

Increasing positive caster (tilting the steering axis towards the rear of the car) generally leads to:

• Enhanced Steering Feel: More feedback is transmitted through the steering wheel, giving the driver a better sense of connection with the road. Vague steering can often be improved with increased caster.
• Improved Steering Re-centering: The steering wheel returns to center more readily after cornering. This reduces the effort needed to straighten the wheel after turns, leading to smoother corner exits and less driver fatigue.
• Greater High-Speed Stability: Increased positive caster enhances straight-line stability, reducing steering wander and shakiness at higher speeds. This instills driver confidence and control, especially at track speeds.
• Increased Jacking Effect and Cornering Rotation: Positive caster increases the “jacking effect,” where the inside front wheel lifts slightly during cornering, transferring weight to the outside rear wheel. This aids in initiating rotation into corners and can reduce understeer.

Diagram illustrating the effects of positive caster on vehicle handling and steering

Drawbacks of Excessive Positive Caster and Reducing Caster

While beneficial, too much positive caster can introduce issues:

• Increased Steering Effort: Excessively high caster can make steering heavier, potentially leading to driver fatigue, especially in prolonged or demanding driving conditions like racing.
• Excessive Negative Camber Gain: High positive caster increases negative camber gain during cornering. While some camber gain is beneficial, too much can lead to understeer if the outside tire loses optimal contact patch. In such cases, reducing caster or static camber might be necessary.
• Rear Wheel Lift (Weight Jacking): The jacking effect from high caster can be so pronounced that it lifts the inside rear wheel, particularly in rear-wheel-drive cars. This can reduce traction, cause oversteer, and disrupt acceleration out of corners.

Reducing positive caster might be beneficial in specific scenarios:

• Reducing Steering Effort: If drivers experience arm fatigue, reducing caster can lighten the steering.
• Managing Understeer: If excessive negative camber gain is contributing to mid-corner understeer, reducing caster can help. However, adjusting static camber is often a better first step.
• Minimizing Rear Wheel Lift: In cases where weight jacking is causing rear wheel lift and loss of traction, reducing positive caster can help keep the rear wheels grounded, improving stability and traction.

It’s crucial to note that negative caster is generally detrimental, leading to unpredictable steering and instability, especially at high speeds, and should be avoided.

Counteracting Jacking Effects: King Pin Inclination (KPI) can be adjusted to counteract the jacking effect of caster. KPI creates a jacking effect in the opposite direction, allowing for high caster levels while mitigating excessive weight transfer.

Conclusion

Fixing caster on a car involves careful measurement, understanding different adjustment methods, and recognizing the effects on handling dynamics. Whether you aim to improve steering feel, enhance stability, or fine-tune your car for track performance, adjusting caster is a powerful tool in suspension tuning. Always make incremental adjustments, test the changes, and consider consulting with a professional for optimal results, especially in performance or motorsport applications.