What Data Is Needed For Four-wheel Alignment Adjustment?

For the front wheels responsible for vehicle steering, the positioning is relatively complex, with four positioning parameters: kingpin tilt backward, kingpin tilt inward, wheel tilt outward, and toe in, while the positioning parameters for the rear wheels are mainly toe in and toe out.
1. Main pin tilt backward
Function: Linear stability, steering return to normal
From the side of the vehicle, the kingpin axis (the center axis of the wheel steering) is not completely perpendicular to the ground, but slightly tilted backwards. The angle between the kingpin axis and the perpendicular is the kingpin caster angle. The presence of the kingpin tilt angle causes the left and right sides of the tire tread and tire sidewalls in contact with the road surface to undergo compression deformation when the wheel turns, producing a reverse force that causes the wheel to tend to self correct. The larger the kingpin tilt angle, the better the driving stability of the wheel, and the more obvious the correction effect. However, the corresponding turning of the steering wheel becomes more difficult.
On most vehicles, the caster angle does not exceed 3 °, but it varies with the vehicle's attitude and driving status, depending on the suspension's travel and the state of the wheels at that time. For example, during emergency braking, the vehicle's caster angle may approach zero or even negative. So many vehicles will find that their straight-line driving stability deteriorates (even with ABS) when emergency braking occurs during straight-line driving.
2. Main pin inclination
Function: Stability, steering return to normal
When viewed from the front (i.e. in the transverse plane), there is also a certain angle between the vehicle's main pin axis and the perpendicular, and the upper end tilts inward, which is called the main pin inclination. The angle between the main pin axis and the perpendicular is called the main pin inclination.
Due to the presence of the kingpin inclination angle, the trend of the wheels turning is for the wheels to move downwards as a whole. However, since the paved roads we drive daily are all hard roads, the effect presented is: when turning, the wheels will resist gravity and lift the front of the car, and when the steering force disappears, the wheels will automatically return to normal under the action of gravity. The larger the inclination angle of the main pin, the more obvious this correction effect, but an angle that is too large can also cause excessive tire wear, usually between 5-8 °.
The main pin tilt and the main pin tilt back are both designed to provide return force and driving stability, but the difference between the two is that the return force of the main pin tilt is independent of the driving speed, while the return effect of the main pin tilt back is more pronounced at higher speeds. Therefore, when driving at high speeds, the stability and return effect of the main pin tilt back play a dominant role, while when driving at low speeds, the effect of the main pin tilt back plays a dominant role.
3. Wheel Camber (Negative Camber)
Function: Increase the contact surface of the tire to counteract adverse effects
As the name suggests, wheel camber refers to the outward inclination of the wheel from the centerline, and the angle between the rotation plane of the wheel and the longitudinal vertical plane is the wheel camber angle.
If the vehicle maintains the wheels perpendicular to the road surface in an unloaded state, then when loaded or even fully loaded, due to the compression and deformation of the suspension stroke and the reduction of the clearance between the moving surfaces, the wheels will exhibit an "eight" shape "inward tilt, which increases tire wear and causes a significant burden on the wheel hub bearings. In order to reduce this impact, People have designed the advance of "wheel outward tilt" to counteract the occurrence of "inward tilt", so that after the vehicle is loaded, the wheels can make better contact with the road surface at a better angle, reducing the burden of eccentric wear and bearings. However, excessive camber angle can also lead to increased lateral wear of tires, and many car owners refer to "gnawing on the tire" as being related to the unreasonable setting of the original factory's wheel camber angle.
However, not all vehicles are designed with wheel camber. We can see that many high-performance cars, racing cars, and modified cars are set to have a "negative camber" (inward tilt) state in the shape of an eight, because high-performance vehicles are more concerned with the impact of centrifugal force on vehicles when turning at high speeds. When the wheels are driven at high speeds in curves, the deformation of the tires caused by centrifugal force leads to an outward tilt trend, Make sure that only the outer side of the tire can make contact with the road surface. Therefore, engineers use the initial "inward tilt" setting to offset this adverse effect, so that these high-performance vehicles can make more use of the middle or even inner tread of the tires in corners, increase the contact area, and improve the limit of cornering.
4. Toe in (front extension)
Function: To counteract the adverse effects caused by wheel outward tilt (inward tilt)
Although the outward tilt of the wheels counteracts some of the adverse effects of the vehicle's load, this design itself also has drawbacks: the outward tilt setting causes the wheels on both sides to spread outward, making it impossible to roll parallel and causing sliding while driving, which is why we have "toe in".
The distance between the front ends of the two front wheels is smaller than the rear end, which is called toe in. The function of toe in is to counteract the outward opening of the two wheels caused by outward tilt. The sliding of the two wheels towards the inside caused by toe in will also counteract the sliding caused by outward tilt, allowing the wheels to roll parallel forward in a non sliding manner. When the distance between the front end of the wheel is greater than that of the rear end, it is called negative toe in, or forward spread. This setting is to counteract the adverse effects of wheel inclination, and also to allow the wheel to roll forward in parallel.