Ride Height:
Alignment geometry Is based on vehicle height and If height Is Incorrect, an alignment should not be attempted.

 Camber:
 Purpose
1 Maximize tire life
2 Isolate road shock
3 Enhance stability
4 Maintain bearing load
5 Reduce stress on suspension components

  Caster: 
Purpose
1.Obtain desired stability
2.Project vehicle load
3.Improve steering wheel return
4.Improve cornering
 

Toe:
Purpose
1 Bring about a running toe of zero
2 Reduce stress on steering components
3 Minimize tire wear
4 Centre steering wheel

 S.A.I.:
   Purpose
1. Keep wheels In straight ahead position
2. Help. wheel return from turning
3. Improve  directional  stability  reducing  the  need  for
additional positive caster
4. Helps In placing more load on larger Inner wheel bearing
5. Assist In maintaining straight line control when braking
On vehicles with long short control arms, the average S.A.I. Is 7 to 10 degrees.  On strut suspension, the average S.A.I. is from 10 to 17 degrees.  This may explain why some vehicles with a large amount of camber difference don't seem to pull.

QUICK CHECK
S.A.I. and Included Angle

*      Is a directional control angle
*      Is a built In angle

a)     *      On vehicles that have camber adjusters on bottom of the strut, S.A.I. angle does not change.
        *       If S.A.I.  angle  Is wrong  on  this  type  of front  end.  It would indicate a bent shock tower, bent lower control
arm or bent cross member, etc.
b)     *      On vehicles that have camber adjusters at the upper or lower control arm, or at the top of the struts, S.A.I. will
change, but Included angle will remain the same.  If  INCLUDED ANGLE Is Incorrect, a bent spindle or MacPherson strut Is Indicated.
 

  Included Angle:
Purpose
To give technician diagnostic Information to determine If the vehicle has bent parts.  Included angle can be changed on front wheel drive vehicles that have adjusters provided at the base of the strut housing. Also, Included angle can be changed on 4X4 when around tapered shim is placed behind the spindle and steering knuckle. To read a true included angle on these types of vehicles, you first have to adjust camber the same on both wheels.

Set Back
When one front wheel Is mounted further back on the body than the other.  It may occur from damage or It may be Intentional on some front wheel drive vehicles to help over come torque steer. Turning Angle or toe out on turns:  Purpose
1.     Reduce tire scuffing on turns
2.     Reduce tire squealing on turns
3.     Improve handling on turns

Scrub Radius:
   Purpose
1.     Provide the driver a feel for the road.
2.     Project vehicle load to larger Inner wheel bearing.
3.    Assist  Is   providing   stability   under   adverse   road conditions.
4.     Work In conjunction with static toe  settings to help bring about a running toe of zero.

Most rear wheel drive vehicles have positive scrub radius. This Is one reason most rear wheel drive vehicles specie static toe setting of toe in.
Some front wheel drive cars have positive scrub radius, thus, dictating the need of static toe out settings.
Other front wheel drive cars have a negative scrub radius. Therefore, dictating the need for a static toe-in setting.
 

Reduced Scrub Radius  -    easier steering

Any bump or cornering force that Is applied to the tire can exert a twisting force on the steering that Is proportional to the length of the scrub radius.
If the scrub radius Is zero, the twisting force will be zero.
Cars with zero scrub radius usually can be driven without assistance of power steering.

Factors that affect scrub radius are:
1.     wheel offset
2.     brake rotor width
3.     design of steering knuckle
4.     increased S.A.I. angle will able reduce scrub radius

Roll Centre
The roll centre height is determined by drawing a line from the tire contact print through the Instant centre of the control arms, which Is where we measured the swing arm length.  For a given swing arm length, the roll centre height can be raised or lowered by moving the intersection of the control arm lines up or down.  If we establish a given roll centre height, this will dictate the Instant centre height and that will determine the angle of control arms.
 

Tracking
If either set of wheels Is offset to one side. the rear wheels don't follow  the front wheels; the car DOG TRACKS. A more common problem has the rear toe out of adjustment, resulting In the rear wheels trying to drive the car sideways.  Despite going In a straight line, the car is pitched sideways.   The driver Is literally having to steer the car sideways In order to maintain a straight line. Besides loss of straight line stability and harder steering to one side than the other, dog tracking can cause severe tire wear problems.