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Join Date: Feb 2002
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Car: 97 Accord
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Repost: Suspension tuning 101: alignment concepts
Origionally posted by Texan 4-21-01
CAMBER: Camber is the angle the wheel deviates from perfectly vertical when looked at from straight ahead. Positive camber would have the top of the wheel inclined outwards, away from vehicle center, while negative camber has the top of the wheel leaning inwards to vehicle center. Contrary to popular belief, any and all camber angles hurt tire adhesion to the road, and for one obvious reason. Tires create the most grip when they put the biggest footprint onto the pavement possible, and any significant camber angles shrink the all important contact patch. The reason people associate negative cmaber with good handling is because as body roll occurs in a corner, positive camber is naturally imparted to the outside wheels. The suspension's camber angle at static ride height (plus it's camber curve, see below) will determine whether the wheel goes into positive camber during body roll, or simply balances out to zero camber. So just know that ideally we want zero camber at all times, but like most things automotive a compromise must be struck: dial in a bit of negative camber at static ride height for the least amount of positive camber possible at maximum effort cornering.
CAMBER CURVE: A camber curve is created by most suspensions because camber constantly changes as the suspension is compressed and expanded. Generally speaking, any independent suspension will increase negative wheel camber as it compresses, and increase positive wheel camber as it expands. Hence, the camber curve lets us see what camber angle the wheel will be at with the suspension at a given amount of compression or expansion.
CASTER: Caster is the angle of inclination between the mounting point of the spindle at hub center to the upper A arm (in cars with upper A arms anyways), when veiwed from the side of the car. If you drew a vertical line through the hub center, then another from this point to the spindle's mounting point on the upper A arm, you would get rearward biased angle on any car (called positive caster angle). This design concept is critical for high speed directional vehicle stability, camber gain during steering, and also plays a roll in anti-dive characteristics under braking.
TOE: Toe is the amount the tire's point inward or outward from dead ahead when the steering is perfectly centered (viewed from directly above the tire). Toe is measured in inches (usually very small increments of inches to be exact); toe-out indicates the wheels point slightly away from vehicle center in a straight on path, while toe-in indicates a slight bias towards vehicle center. Zero toe would be a case where the wheels point dead ahead when the steering is centered. Toe plays an important part in straight line stability and vehicle turn-in characteristics. Toe-in gives makes the car easier to keep pointed straight during normal driving and under heavy braking, while toe-out makes the vehicle feel more eager to enter corners but will cause directional stability to suffer (and stability under braking to suffer greatly).
TOE CURVE: Just like camber, toe amount changes as the suspension undergoes movement. Suspension designers generally take full advantage of this and design the suspension to take on reduce toe amounts as the suspension compresses, thus allowing the vehicle to remain directionally stable during normal driving yet more eager to change direction under braking.
SET BACK: Measures the difference between wheel location from one side to the other relative to each other (when viewed from above). Let's say we were to draw a line through the center of the car (from fore to aft), then draw a line perpendicular to this beginning at the leading edge of, say, the left front tire. If the right front tire didn't precisely touch that line (let's say for simplicity it was 1/4" behind that line), you would have a front axle set back of 1/4". Basically this just tells you how dead on the front wheels are located relative to each other when viewed from the side of a car on an alignment machine. If the set back were a fairly large value (say nearly 1/2-1"), it's probable that something in the suspension or frame has been bent.
SAI (Steering Axis Inclination): This is a measure of the steering's pivot axis vs. the tire's true pivoting axis (as viewed from the front of the car). Virtually every suspension design doesn't actually have the steering system pivot the wheel in a perfectly vertical axis, because the mounting point of the steering's tie rod to the spindle is usually further out from the center of the vehicle than the upper mounting location of the spindle to the upper A arm (in your suspension). In other words, the steering system pivots about an axis that is tilted inwards towards the center of the car at it's upper mounting location. However, the wheel pivots about an axis that is perpendicular to the ground (imagine a second line drawn vertically though the center of the hub). The difference in angle of these two lines, one being the steering axis and the other being the wheel axis, is called the SAI. Whenever the SAI is out of spec, it's usually due to a bent suspension component, as this concept is centered around suspension hard parts and their mounting points to the chassis of the car.
INCLUDED ANGLE: Ok, so I lied! When I said the wheel pivots about an axis perpendicular to the ground, I wasn't being perfectly accurate for most any independent suspension design. Our wheels almost always have a camber angle (hopefully a small negative angle at normal ride height), and this throws off our nice little concept of SAI. To get a really accurate idea of the difference in pivoting axes between the steering system and wheel, you need to take into account the camber of said wheel. So, say if you had an SAI of 15 degrees and a negative camber on the wheel of 1 degree, you would get an included angle of 14 degrees. The wheel is canted inwards 1 degree from our previous true vertical measuring point, so this concept will give us a truly accurate idea of the angles everything will be pivoting on at normal ride height.
SCRUB RADIUS: The difference between where the SAI line and the vertical wheel centerline intersect the ground (as viewed from the front of the car). A vehicle is said to have a postive scrub radius if the SAI line falls closer to vehicle center than the tire, and a negative scrub radius if it falls outside the tire centerline. Scrub radius is important to both vehicle stability under braking and acceleration, plus steering feedback during at the limit adhesion. A negative scrub radius hurts steering feel (most fwd cars have either zero to negative scrub), but keeps the steering wheel from yanking around when one of the steered wheels loses traction (a positive scrub radius can yank the wheel out of your hands when only one steering wheel loses traction during a turn, which is of course a bad thing).
ACKERMAN STEERING: A design concept which allows the inside wheel of the steered axle to travel a tighter arc than the outside. When one thinks about a vehicle turning, it becomes obvious that for the front end to maintain optimal traction, the inside wheel in the turn always has to be making a slightly sharper turn than the inside wheel. If both turned an equal path, the two tires would effectively be following different curves around the same point and wasting a whole lot of grip in the process. So ackerman steering geometry is created to allow the inside wheel to turn a somewhat tighter arc than the outside, maximizing traction during a turn. Also, increased ackerman will enhance toe-out during cornering, allowing the vehicle to become even more nimble in changing direction. Ackerman is however, one of those things the average Honda enthusiast never need worry about, I just figured I would mention it since we are on the topic of aligment.
Of final note, it should be remembered that almost all alignment prinicples are in constant flux when the suspension is moving. Camber, caster, toe, SAI, scrub; they can all change with suspension movment and suspension part changes. I wrote most of these down knowing that 99% of you will never need to know them or even be interested in knowing them, but on that off-chance you are wondering what exactly is going on in your suspension, well, now you know . Hope this helps someone, peace.
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