In this article, we will focus on what caster is and its effects on a vehicle’s performance.
Three common measurements make up the geometry of an automobile’s suspension. Caster is one of them, while the other two are camber and toe. You can adjust these variables to correct the suspension angle and solve various car problems.
Remember that the suspension system includes the tires and their pressure, shock absorbers, springs, and linkages—all the components that connect a car’s chassis to the wheels.
So, you can imagine how important these three measurements are.
What is Caster?
The caster angle, also called the castor angle, is the measured tilt from the top of the steering axis to the bottom of the wheel when the former is attached to the latter. This angle assesses the tilt of the axis in both directions.
Simply put, it is an alignment setting that occurs only on the front wheels when they turn. It is measured on the steering wheels when the vehicle is viewed from the side.
To visualize the angle, you can imagine the steering axis connecting the lower pivot point or the ball joint to their upper counterpart, respectively. This formation creates a gradient with the steep line passing directly through the lower pivot point.
The ease of steering the wheel and the vehicle’s stability depend on the caster. A zero or neutral caster will create a vertical shock absorber or a perfect spring.
Depending on the type of suspension, the wheel’s position will be south of the upper ball joint or the strut mount point while the axis will be vertical relative to the road.
Positive caster occurs when the wheel and the lower pivot point remain ahead of its upper counterpart.
Nowadays, vehicles apply this caster angle to prevent the vehicle from turning or inadvertently straying from the straight path when driving at high speed.
Negative caster occurs when the upper pivot point remains in front of the lower pivot point. When the vehicle drifts away from the straight line you want to head towards, it indicates the presence of negative caster.
Positive Caster vs. Negative Caster
We have already discussed what caster is and how it becomes positive or negative. Now let’s discover the effects of these angles.
The Effects of Positive Caster
As you already know, positive caster occurs when the steering axis tilts in front of the vehicle’s axis. It pushes the coil spring above the shock absorber towards the rear of the vehicle at the moment of driving.
A driver can distinctly feel the effects of positive caster when driving the car in a forward motion.
Due to the huge amount of self-aligning torque on the front tires and wheels created by the upper ball joint or the strut mount, the vehicle will remain stable in its lane when you drive in a straight line.
The effect of this caster angle generates tension that keeps the front wheels straight when driving at high speed.
Another effect of this torque is that the steering wheel returns to the 12 o’clock position in a turn. Otherwise, the car drifts from its straight path.
Positive caster is essential for improving a vehicle’s cornering performance and road holding. It increases traction by maximizing tire friction with the ground, which helps the vehicle stay grounded.
Increasing positive caster too much is not a good idea because it will make the steering heavier and harder to turn.
However, you don’t have to worry about it in modern vehicles because they have a power steering system to cancel out the heaviness of the steering wheel.
Another issue is that it can accelerate tire wear, even if the effect is not direct. Poor alignment of tires and wheels along with excessive positive caster can worsen the tire wear problem.
The Effects of Negative Caster
What is caster when it becomes negative? In this case, the steering axis tilts behind the vehicle’s vertical axis. Most older cars suffer from this problem due to the absence of a power steering system.
Setting a negative caster was the only way to make the steering wheel easier to turn in these cars.
Due to this negative caster, the steering wheel of older vehicles often feels too light, and it becomes difficult to keep the vehicle on the right track in a turn.
FAQ
1. What are the effects of positive caster on vehicle performance?
Positive caster tends to improve straight-line stability and steering returnability.
It ensures a self-centering effect, which makes the vehicle’s wheels naturally want to return to a straight position after a turn. This improves high-speed stability and helps maintain a straight path while driving.
2. Are there any disadvantages to having a high positive caster angle?
Although positive caster has its advantages, excessive positive caster can lead to increased steering effort at low speeds, making parking and maneuvering more difficult.
It can also increase the risk of “caster shimmy” or vibrations in the steering system if not properly balanced.
3. How does the caster angle affect tire wear?
A6: Incorrect caster angles can lead to uneven tire wear. Excessive positive caster can cause wear on the outer edges of the tires, while excessive negative caster can cause wear on the inner edges. Properly balanced caster helps ensure even tire wear and maximize their lifespan.
4. Can the caster angle be adjusted on a vehicle?
The caster angle can often be adjusted by a qualified mechanic or at an alignment shop. Adjusting the caster typically involves modifying the position of the upper or lower control arm mounting points, depending on the vehicle’s suspension design.
5. Is the caster angle the same for all vehicles?
No, caster angles vary from vehicle to vehicle and are determined by the vehicle’s design and intended use.
Different types of vehicles (e.g., sports cars, trucks, or sedans) may have different caster angles to optimize their handling characteristics.
Final Words
The power steering system is a revolutionary addition to modern vehicles. By using positive caster, this system increases road and driving safety by promoting stable driving.
Nowadays, cars are designed in such a way that their chassis and engine do not support negative caster.
If your vehicle pulls to one side while you are driving at highway speed or taking a turn, you need to diagnose a bad suspension alignment.