The caster angle identifies the forward or backward slope of a line drawn through the upper and lower steering pivot points when viewed directly from the side of the vehicle. Caster is expressed in degrees and is measured by comparing a line running through the steering systems upper and lower pivot points (typically the upper and lower ball joints of an A-arm or wishbone suspension design, or the lower ball joint and the strut tower mount of a McPherson strut design) to a line drawn perpendicular to the ground. Caster is said to be positive if the line slopes towards the rear of the vehicle, and negative if the line slopes towards the front.
Caster angle settings allow the vehicle manufacturer to balance steering effort, high speed stability and front end cornering effectiveness.
Increasing the amount of positive caster will increase steering effort and straight line tracking, as well as improved high speed stability and cornering effectiveness. Positive caster also increases tyre "lean" when cornering (almost like having more negative camber) as the steering angle is increased.
What is the downside of positive caster? If the vehicle doesn't have power steering, a noticeable increase in steering effort will be felt as positive caster is increased. Other than that, the effects of positive caster are pretty much "positive", especially increasing the lean of the tyre when the vehicle is cornering while returning the tyre to a more upright position when driving straight ahead.
Most street car alignments call for the front camber and caster settings to be adjusted to slightly different specifications on the right side of the vehicle compared to the left. These slight side-to-side differences are called cross-camber and cross-caster.
For vehicles set up drive on the left side of the road, the left side is aligned with a little more camber (about 1/4-degree) & a little more positive caster (also about 1/4-degree) to help the vehicle resist the influence of crowned roads that would cause the vehicle to drift downhill to the left gutter. Since most roads are crowned, cross-camber & cross-caster are helpful the majority of the time, however, they will cause a vehicle to drift to the right on a perfectly flat road or a road that leans to the right.
The camber angle identifies how far the tyre slants away from the vertical when viewed directly from the front or back of the vehicle. Camber is expressed in degrees, and is said to be negative when the top of the tyre tilts inward towards the centre of the vehicle and positive when the top of the tyre leans away from the centre of the vehicle.
Since street suspensions cannot completely compensate for the outer tyre tipping towards the outside when the vehicle leans in a corner, there isn't a magical camber setting that will allow the tyres to remain vertical when travelling straight down the road (for more even wear), and remain perpendicular to the road during hard cornering. (for more generous grip) Different driving styles can influence the desired camber angle as well. An enthusiastic driver who corners faster than a reserved driver will receive more cornering grip and longer tyre life from a more negative cambered tyre.
However, with the aggressive negative camber, a reserved driver's lower cornering speeds would cause the inside edges of the tyres to wear faster than the outside edges.
Appropriate camber settings that take into account the vehicle and driver's aggressiveness will help balance tread wear with cornering performance. For street driven vehicles, this means that tyre wear and handling requirements must be balanced according to the drivers needs. However, the goal is to use enough negative camber to provide good cornering performance while not requiring the tyre to put too much of its load on the inner tread while traveling in a straight line. Less negative camber (until the tyre is perpendicular to the road at zero camber) typically will reduce the cornering ability,but results in more even tyre wear.
The thrust angle is an imaginary line drawn perpendicular to the axle's centre line. It compares the direction that the rear axle is aimed with the center line of the vehicle. It also confirms if the rear axle is parallel to its front axle then the wheelbase on both sides of the vehicle is the same.
If the thrust angle is not correct on a vehicle with a fixed rear axle, it often requires a trip to the frame / chassis straightening shop to correctly re-position the rear axle.
A vehicle with independent rear axles may have incorrect toe-in or toe-out on both sides of the axle, or may have toe-in on one side and toe-out on the other. The suspension on each side of the vehicle must be adjusted individually until it has reached the appropriate toe settings for its side of the vehicle.
An incorrect thrust angle is often caused by an out-of-position axle or incorrect toe settings. A collision may also cause this problem.So in addition to the handling quirks that are resultant of incorrect toe settings, thrust angles can also cause the vehicle to handle differently when turning one direction vs. the other.
A rear-wheel drive vehicle "pushes" the front axle's tyres as they roll along the road. Tyre rolling resistance causes a little drag resulting in rearward movement of the suspension arms against their bushings. Because of this, most rear-wheel drive vehicles use positive toe-in to compensate for the movement, enabling the tyres to run parallel to each other at speed.
Conversely, a front-wheel drive vehicle "pulls" the vehicle through the front axle, resulting in forward movement of the suspension arms against their bushings. Therefore most front-wheel drive vehicles use some negative toe-out to compensate for the movement, again enabling the tyres to run parallel to each other at speed.
Toe can also be used to alter a vehicles handling traits. Increased toe-in will typically result in reduced over-steer, help steady the vehicle and enhance stability. Increased toe-out will typically result in reduced under-steer, helping free up the car, especially during initial turn-in while entering a corner.
WARNING: Before adjusting toe outside the motor manufacturers recommended settings to manipulate handling, be aware that toe settings can and will influence wet weather handling and tyre wear as well.
Excessive toe settings often bring with them drive-ability problems, especially during heavy rain. This is because the daily pounding of horse and trailers (heavy vehicles) on many highways leave ruts that fill with water. Since excessive toe means each tyre is pointed in a direction other than straight ahead when the vehicle enters a pool/puddle that causes only one tyre to lose some of its grip, the other tyres toe setting will push (excessive toe-in) or pull (excessive toe-out) the vehicle to the side. This may make the vehicle feel "unsettled" and very "nervous". The driver may have the same feelings!!!
Additionally the vehicle's toe is ONE of the most critical alignment settings relative to tyre wear. A toe setting that is marginally out can make a huge difference to tyre wear. Consider that if the toe setting is just 1/16-inch off its appropriate setting, each tyre on that axle will scrub almost 2.5 meters sideways every kilometre! Extend that out and you will discover that rather than the tyres running parallel to each other, the front tyres will scrub 1/4-km sideways during every 100 km of driving. Incorrect toe settings will destroy your tyres very rapidly.
Wheel alignment should be conducted / checked every 8,000 to 10,000 kilometers and or checked after travelling on severe road conditions. Should the vehicle be subjected to a violent impact with potholes or object, alignment must be checked and re-set.
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