A vehicles alignment is its suspension geometry. It decides how the tires contact the road, the angle of the suspension system as well as the angle of the wheels as the vehicle travels down the road. These are all factors in vehicle handling, tire wear and how straight the vehicle drives down the road. If suspension work is done to the vehicle, an alignment will most likely be needed to fine tune the suspensions geometry. Even if a vehicle drives straight down the road, it may be causing serious tire wear if not aligned correctly. To align a vehicle, an alignment rack is needed. This is not something that can be “eye-balled.”
Caster is the forward or rearward angle of the suspension system, as viewed from the side of the vehicle and is measured in degrees. Positive caster indicates that the lower suspension member is more forward on the vehicle than the upper member. Almost all suspension systems are designed with positive caster (typically 2° to 4°). This offers good directional stability and good steering wheel return to centre, although is does cause a slight increase to required steering effort. Negative caster could result in a shimmy at high speeds. Caster will pull the vehicle to the side with the least (positive) caster. Caster is not considered a tire wearing angle, however excessive positive caster can reduce the suspension systems ability to absorb bumps in the road and can cause tire scuffing if left this way. To measure caster, the wheels must be turned. This is why rear caster will not show up on an alignment sheet.
Camber is the inward or outward angle of the wheel as viewed from the front or rear of the vehicle and is measured in degrees. Positive camber indicates that the top of the wheel sticks out farther than the bottom of the wheel. Negative camber indicates that the bottom of the wheel sticks out farther than the top of the wheel. Different suspension systems will have a different specification for either positive or negative camber. Slight positive camber will result in less tire wear, less steering effort and slightly less road shock when driving over bumps but will not allow the vehicle to handle as well as it could. Negative camber allows the vehicle to handle better but will result in increased tire wear on the inside of the tire. A typical specification for camber could be -0.25° to +0.25° for an average road car. Performance vehicles will have more negative camber because handling is more important than ride quality or tire wear in that application. Different technicians will be bias to either positive or negative camber based on their own opinion and most drivers are not able to tell the difference anyway. If you want your vehicle to be set-up a certain way, (if you are reading this, most likely with negative camber) you can give your request to the technician but they may have their own opinion about camber.
Note: Most factory suspension systems will not let you adjust camber very far, but some car owners install after-market camber plates or other camber adjusting devices that will allow you to adjust camber much farther negative than the factory intended. I just wanted to point out that there is a point where negative camber will result in less handling ability in exchange for looks. When the outside of the tire has less contact with the road, this results in less tire to road contact. Negative camber is only a handling benefit to a point. This point is different from vehicle to vehicle and should be taken into consideration when looking into after-market camber adjusting devices.
Camber and caster are what is called lead angles. They control which way the vehicle pulls when the driver lets go of the steering wheel. You may think that vehicles should be aligned with zero lead left or right. This is not the case. Roads have what is called “road crown” built into them. This means that each side of the road is built on a slight angle to direct water off the road when it rains. In North America, we drive on the right side of the road which means that the side of the road we drive on is always leaning to the right. This must be made up for to get a vehicle to drive straight down the road. Caster pulls to the side with the smaller degree value while camber pulls to the side with the bigger degree value (keep in mind that negative numbers are of less value than positive numbers (don’t laugh, it is an easy mistake to make)). A typical technician will set-up the vehicle with 0.25° lead to the left in the front only to compensate for road crown.
Toe-in/positive toe is when the leading edges of two tires on the same axle are closer than the trailing edges with the steering wheel straight. Toe-out/negative toe is the opposite of this, with the trailing edges of the tires closer to each other than the leading edges. Toe is not a leading angle which means that it will not pull the car to either side, but it is potentially the most tire wearing angle. It also controls the steering wheel centre position. This will be adjusted with the tie-rods. If toe is set incorrectly, the vehicle may drive straight down the road but the steering wheel may rest on an angle and/or the tires could wear down very quickly because of the constant skidding action that occurs with too much toe-in or toe-out. Toe-in is considered positive and toe-out is considered negative. Toe can be measured in degrees or inches.
The thrust angle refers to the angle that the rear wheels are pulling. An ideal thrust angle on any vehicle would be zero. A slight thrust angle can be caused by just one rear wheel being out of alignment. Each vehicle will have a specification for how much of a thrust angle is acceptable. When a vehicle has an excessive thrust angle, it will cause the rear end of the vehicle to be pulled out to the side when the vehicle drives down the road. When this happens, the rear wheels do not follow the front wheels. This is called “dog tracking” and can cause tire wear as well as the driver needing to counter-steer to keep the vehicle driving straight down the road. A thrust angle to the left is a negative thrust angle and a thrust angle to the right is a positive thrust angle.
SAI and IA
SAI stands for steering axis inclination. It is the angle of the suspension system as viewed from the front or back, compared to true vertical. The angle of the suspension system is the line between the two ball joints on an SLA type of suspension system or the line between the upper strut mount and the lower ball joint on a MacPherson strut suspension system. It is considered non-adjustable and is mainly used to check if something is bent or damaged. IA stands for included angle and is SAI compared to camber instead of true vertical. It is also mainly used to check if something is bent or damaged. SAI and IA are measured in degrees and will have an individual specification as well as an allowable variation from side to side. If SAI or IA are out of specification, something is bent in the suspension system.
Wheelbase and Setback
Wheelbase is the distance between the centre of the front wheel and the centre of the rear wheels. This distance should be the same from side to side. If one side has a shorter wheelbase than the other, it is referred to as setback. Setback indicates that something is bent or a suspension component is severely worn out.
The difference between the centre point of tire to road contact and where the SAI intersects the road is the scrub radius. If the SAI intersects the centre point of tire to road contact, the scrub radius is zero. If the SAI intersects the road to the inside, the scrub radius is positive. If this happens on the outside, the scrub radius is negative. The point where SAI intersects the road is important because it determines how the wheel moves while turning. Having zero scrub radius causes the wheel to simply pivot, but a positive or negative scrub radius will cause the wheel to “swing” on the SAI. Too much positive or negative scrub radius will affect the vehicles toe during acceleration and braking. A scrub radius of zero causes increased steering effort because both sides of the tire try to fight each other. This is called “squirm” and can cause a vehicle to feel twitchy going through corners as the inner and outer areas of each front tire try to spin at slightly different speeds. Most modern systems will be designed with slightly positive or negative scrub radius, depending on the application.