Of all vehicle alignment adjustments, caster is probably the least known and understood, but you can rightly think of it as the unsung hero of proper alignment. If it weren’t for caster, vehicles of all varieties would be drastically less stable and predictable.
So what is there to know about caster, and how does it affect driving?
With a vehicle side view, picture a vertical axis (line) that runs straight through the center of your front wheels.
Caster, which is also referred to as caster angle, is the top to bottom angle of the steering axis and suspension components as they meet the wheel.
A zero/neutral caster setting would result in a perfectly vertical spring/shock with the center of the wheel positioned directly “south” of the strut mount or upper ball joint (depending upon the vehicle suspension type).
Caster is an alignment adjustment that occurs on the turning wheels only, i.e., the front wheels of a vehicle.
Positive caster positions the lower ball joint in front of the upper ball joint or strut mount. This results in the tire contact patch hitting ground in front of the steering system and suspension components.
The origins and numerous documented benefits of positive caster date all the way back to the late nineteenth century. (Discoveries of the French automotive engineering pioneer Arthur Constantin Krebs, for all you history buffs out there.) Positive caster creates two primary benefits:
You experience the benefits of positive caster and self-aligning torque every time the steering wheel naturally “unwinds” back to 12 o’clock when exiting a corner. And without positive caster and the resulting self-aligning torque, the vehicle would be very inclined to wander at highway speeds. (You’d feel that too! For a risk-free preview, send a shopping cart, which has negative caster at the front wheels, on its way. Just make sure there aren’t any cars or fellow shoppers around.)
Specifically, positive caster helps to create increased negative camber on the outside wheel during cornering. When cornering, the vehicle weight shift is met with a maximization of the tire contact patch on the outside front wheel that’s under maximum cornering load.
In performance driving circles a caster increase – sometimes to the maximum available setting that the vehicle will accommodate – is credited with improvements in vehicle handling, maximum traction, and steering responsiveness.
Negative caster positions the lower ball joint behind (rearward of) the upper ball joint or strut mount. Unlike in a positive caster arrangement, the wheel and tire contact patch hit the ground behind the steering system and suspension components.
Negative caster angle isn’t a setting used by modern road cars. The self-aligning torque and straight-line stability that is characteristic of positive caster are absent in a negative caster setting. In fact, a “loose” steering wheel and immense front wheel instability would result from a negative caster setup.
Caster settings are not a primary, direct factor in tire wear. However, excessive positive caster in association with other alignment maladjustments can exacerbate a tire wear issue. For example, a tire wear pattern called feathering can result from a combination of too much caster with incorrect toe settings. Tire feathering is characterized by a high-low, smooth-sharp variation in the wear pattern of the tread blocks.
Of all of the alignment settings, however, caster is the least likely to be a cause of uneven or abnormal tire wear. A holistic alignment approach involves verification of caster settings that are to your vehicle’s specification.
Caster might not get the headlines of the other alignment variables and adjustments, but it’s fundamental to vehicle performance and safety just the same. Next time you’re pushing a shopping cart around the store, note the unpredictability of the front end, and be thankful for Mr. Arthur Krebs and his positive caster concept!