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 Suspension : Information: Alignment Basic Terms
I am planning on writing several informative articles on Suspension in the
near future, so I thought I would start of with a nice little glossary of Suspension
terms that you may find useful.
Ackerman Steering - It is simple really the greater the radius the
greater the circumference of an a circle. You may be asking yourself what does
that have to with steering and cornering. Well, when going around a corner/arc
you are simply traveling the circumference of some circle. Since the outside
wheel is further from the center it must travel a greater distance, and the
inside wheel of the steered axle must travel a tighter arc than the outside. If
both wheels turned the same amount as with a straight axel both wheels would
effectively be following different curves around the same corner which would
result in a significant reduction in grip. The concept behind Ackerman steering
allows the inside wheel to turn on a tighter arc than the outside wheel.
Toe-Out is also increased during cornering to improve
stability. Ackerman Steering is not a new concept, it has been know to
automotive engineers for decades, all the way back to the some of the very first
automobiles.
Camber - Camber is used to maximize and adjust the contact patch of the
tire. 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 camber 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) 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 viewed 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.
Included Angle - 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.
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.
Toe - Toe is the amount the tire's point outward or
inward when looking down on a car from a bird's eye view. 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 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 and a reduction in stability under braking.
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.
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 positive 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).
By Anonymous on Monday, December 15 2003
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