Ever wonder what gives your steering that nice weight and straight-line stability? It’s probably your vehicle’s caster angle. Caster angle plays an important role in steering and handling, and yet it is often misunderstood.
In this article, we will discuss what caster is (in simple terms), the caster’s effect on steering, and the difference between positive and negative caster.
What is a Caster On a Car?
A vehicle’s suspension geometry can be measured by three common methods, which can be varied, called caster, camber, and toe.
Caster is measured on the steering wheels (i.e., the front wheels) as an angle that measures the forward or backward tilt of the steering axis when viewed from the side.
That steering axis connects the lower pivot point (usually the lower ball joint) with the upper pivot point (usually the upper ball joint), and it forms an angle with a vertical line drawn through the lower pivot point.
A positive caster indicates that the vehicle’s wheel (i.e., lower pivot point) is in front of the vehicle’s upper pivot point, a negative caster indicates the lower pivot point is behind the upper pivot point, and a zero caster indicates that the steering axis is perpendicular to the ground.
Simply put, caster contributes to the ease with which a vehicle steers as well as the extent to which a vehicle will tend to go straight ahead on a line, once committed to that path, without a skilled driver providing input at the steering wheel.
Today’s vehicles use a positive caster to help it resist unintentional turning of the vehicle, or wandering/straying off the intended line, while traveling forward at a speed.
A vehicle that pulls to one side while traveling in a straight line may have a negative caster. A car with power steering should have a positive caster, so this vehicle should have a mechanic check the suspension and realign it.
Caster vs Camber
Camber is an angular measurement in the suspension that measures a different axis of tilt from caster. Camber is how much the wheels “lean” in towards the car or out away from the car, when looking straight on at the front of the car into the grill.
This affects the area of the tire that contacts the ground while executing numerous driving manoeuvres. You may have seen or heard of cars that are “slammed” or have “stance” – these are slang terms for excessive negative camber.
Negative camber refers to the top of the wheels pointing toward the car body and the bottoms of the wheels pointing away from the car body, relative to a vertical line going through the centre of the wheels, with no camber becomes the new vertical.
Negative camber increases grip while turning/cornering, but decreases grip while accelerating or braking in a straight line and too much negative camber will wear the tires out rapidly.
Positive Caster Effects
Positive caster occurs when the lower pivot point is in front of the upper pivot point when viewed from the side of the vehicle. This results in heavier steering, or in the case of no power steering, more effort on the steering wheel (you can’t really make a huge difference in steering effort with power steering).
Modern vehicles readily take advantage of positive caster, because they have power steering and don’t have the heavier steering effort. Positive caster aids in straight-line than lateral stability because of tire self-aligning torque.
Self-aligning torque is generated on the vehicle’s front wheels when the lower pivot point is farther forward than the upper pivot point, and the tires want to be steered back to neutral on turn-in by force pressure applied to the front wheels.
When the tires want to return to straight ahead, that self-aligning torque pressure will be replicated back into the suspension geometry after the turn is completed and the vehicle is traveling forward in a straight line. Additional self-aligning torque is created when some of the weight of the vehicle is behind the tire, which would help with tire location straight ahead.
However, too much positive caster can lead to severe understeer, which is why most modern vehicles do not have comparable caster greater than 3 to 5 degrees. This is the best range with positive caster for maximizing straight line stability while not sacrificing comfort while turning.
Because of the geometry of the suspension components, positive caster also increases negative camber when the wheels are being turned. This is good for cornering to maximize the amount of tire contact and contact area to the ground on the outside front wheel, which is under the greatest load in a turn.
Bicycles utilize positive caster, as well, because the front wheel steering axis is angled forward for more stability in a straight line.
Negative Caster Effects
Negative caster is a holdover from old car and tire technology and is only found in older vehicles due to outdated vehicle dynamics and tire designs.
Negative caster allows for light and easy steering, which results in less directional stability during straight-line travel and less handling performance than vehicles with positive caster.
Negative caster is often present where there is a sloppy or loose feeling steering wheel (this is much less common than a wheel that is hard to turn), and the loose feel can be a safety hazard because the vehicle may not turn as expected when the steering wheel is turned.
If there is a large difference in how the steering wheel rotates and how the vehicle turns, the wheels can lead to a collision, assuming that the driver has their attention on driving.
Negative caster is uncommon in modern vehicles. If you suspect negative caster in your vehicle, it should be checked at an alignment shop or by a qualified service professional.
Non-symmetric caster on the car should be corrected as a driving safety issue, in that the car will wander towards the side of the vehicle with the lesser caster, causing excessive wear on the tire and suspension parts.
How is Caster Adjusted?
To adjust the caster, the ride height, caster, camber, and toe must all be measured because one affects the others.
There are specific tools that can do this – the bubble gauge and turn plates. The more advanced the equipment, the more accurate the measurements, so if you have the option of having this done at an auto alignment shop, expect that your measurements will be the most accurate by having it done there.
With the front wheels off the ground, the wheels are turned in and out by 20 degrees. In this case, a bubble gauge attached to the hub of the wheel measures caster.
To make adjustments, you add or remove shims to the front of the upper control arm or rear mounting points.
When the caster is set to your satisfaction, check the camber, toe, and ride height measurements again and make those adjustments so you can confirm that all measurements are in spec for your vehicle.