Quick Answer
- Caster is the forward or rearward tilt of the steering axis seen from the side. Positive means the top of the axis tilts toward the rear.
- Mevotech says the caster on most vehicles is positive and set to 3 to 5 degrees. Positive caster helps the wheels return to center and improves straight line stability at high speeds.
- To calculate caster, read camber with the wheel steered 20 degrees out and 20 degrees in. A camber change of 2.4 degrees between the two gives about 3.5 degrees of caster.
Enter a caster angle to see whether it is positive, zero or negative and what that means.
Range: -5 to +15 degrees. Positive means the top of the axis tilts rearward.
Caster angle
+3.5°
The top of the steering axis tilts toward the rear
What Mevotech says
Mevotech says positive caster helps the wheels return to center when driving and improves straight line stability at high speeds.
Mevotech says the caster on most vehicles is positive and set to 3 to 5 degrees. Your vehicle's specification, by make, model, trim and tire size, is in the manufacturer's service information. Caster is not adjustable on all vehicles.
Get a plain-English explanation of your results and what they mean for your vehicle.
Positive, Zero and Negative Caster
Definitions and effects as described by Mevotech, a suspension parts manufacturer, in its guide to reading a wheel alignment report.
| Caster | Steering axis from the side | What Mevotech says |
|---|---|---|
| Positive caster | The top of the steering axis (spindle) tilts to the rear | Helps wheels return to center when driving and improves straight line stability at high speeds. |
| Zero caster | The steering axis is vertical when viewed from the side | Neither forward nor rearward tilt. |
| Negative caster | The top of the steering axis tilts to the front | Good for immediate turning, but unstable at high speeds, and may cause shimmy, wander or pulling to the side. |
Caster, camber, toe and thrust angle
| Angle | Seen from | What it is |
|---|---|---|
| Caster | From the side | Forward or rearward tilt of the steering axis (the spindle support arm). Measured in plus or minus degrees. |
| Camber | From the front | Inward or outward tilt of the tire at the top, in plus or minus degrees. Positive camber is when the top of the tire tilts outward. |
| Toe | From above | Compares the distances between the front and rear of a pair of tires, in plus or minus degrees or in mm or inches. |
| Thrust angle | From above | The direction of thrust set by the rear wheels compared with the center line of the vehicle. |
Source: Mevotech, How to Read and Understand a Wheel Alignment Report. See the camber calculator, the toe calculator and the wheel alignment guide.
How to Calculate Caster Angle from Camber
Take two camber readings
Steer the wheel the same angle (delta) away from and toward the vehicle center line and read the camber each time. Positive camber means the top of the wheel leans outward.
Find the camber change
Camber change = camber (steered out) - camber (steered in). With positive caster the wheel steered in is more negative, so the change is positive.
Calculate caster
Caster = asin( sin(camber change / 2) / sin(delta) ). For a 2.4 degree change at 20 degrees: asin( sin(1.2) / sin(20) ) = 3.51 degrees.
Quick estimate
For delta = 20 degrees, caster is close to camber change x 1.46, which is the change divided by 2 x sin(20). The formula above is exact for caster alone, and steering axis inclination cancels out in the difference.
Camber change for a given caster (calculated)
The camber change you should see between the two steered positions, for caster alone.
| Caster | Camber change at 20 degrees | Camber change at 15 degrees |
|---|---|---|
| 1.0° | 0.68° | 0.52° |
| 2.0° | 1.37° | 1.04° |
| 3.0° | 2.05° | 1.55° |
| 3.5° | 2.39° | 1.81° |
| 4.0° | 2.73° | 2.07° |
| 5.0° | 3.42° | 2.59° |
| 6.0° | 4.10° | 3.10° |
| 7.0° | 4.78° | 3.62° |
This is arithmetic from the formula above. We found no manufacturer publication of the camber swing method, so use it as a check, not as an alignment procedure.
Caster Specifications and Alignment Order
What Mevotech says about specifications, adjustment and preparation. NHTSA adds that a wheel alignment maximizes the life of your tires and prevents the car from veering.
| Topic | What the source says |
|---|---|
| Where the specification is | In the vehicle service manual. Alignment specifications are defined by the vehicle make, model, trim package and tire size. |
| Minimum and maximum | Specs list a preferred value with minimum and maximum limits. Mevotech says adjustments should split the difference between the min and max. |
| Adjustment order | In a front-end or four-wheel alignment, check the thrust line, then set caster, followed by camber and toe. |
| Adjustability | Caster may not be adjustable on all vehicles or may require the movement of the subframe. On a MacPherson strut system it may be adjusted with special adapters that move the upper strut mount, and some vehicles use a radius or strut rod. |
| Before an alignment | Check tires for damage and irregular wear, verify the correct sizes and pressures, examine steering and suspension parts for wear, check for excess weight in the vehicle, and verify the curb height. |
| When it is needed | After new tires, replaced steering or suspension parts, drift, pull or off-center steering, irregular tire wear, or an accident or hard impact. |
Caster is one of the angles that set where the steering axis meets the ground, which the scrub radius calculator shows for a wheel offset change.
How to Use the Caster Angle Calculator
- 1To read an angle, open Read a Caster Angle, type the angle in degrees and see whether it is positive, zero or negative and what Mevotech says about it.
- 2To calculate caster, open Calculate Caster from Camber, enter the camber with the wheel steered out and in, and the steering angle you used for both (20 degrees is common).
- 3Compare the result with your vehicle's specification in the manufacturer's service information, which gives the target and the minimum and maximum.
- 4Have caster checked and adjusted on a calibrated alignment machine, because it is not adjustable on all vehicles.
Frequently Asked Questions
Sources: Mevotech, How to Read and Understand a Wheel Alignment Report, for the definitions, effects, 3 to 5 degree statement and procedure; NHTSA for the wheel alignment statement. The camber swing formula is geometry. This tool does not replace a professional wheel alignment, and your vehicle's service information gives the specification.