>What are: (a) Caster, (b) SAI, (c) Included Angle (appears to be the sum
>of Camber and SAI) and (d) Thrust Angle, and what do they tell us?
>
>Any information on this most confusing subject would be greatly
>appreciated!
I'm pretty certain that SAI stands for 'Steering Axis Inclination',
which I'll describe shortly
Caster and SAI are very, very similar. To illustrate caster, think of
a bicycle's front wheel, fork and steering head. When viewed from the
side, it will be immediately apparent that the axis around which the
forks and wheel rotate when steered left or right runs at an angle
that is nowhere near vertical. That axis is tipped back in such a way
that if you draw an imaginary line on the axis, you'll find that the
line intersects the ground IN FRONT of the wheel/tire's point of
contact with the ground by a considerable distance. This is referred
to as a POSITIVE caster angle. NEGATIVE caster refers to a steering
axis which is angled such that it intersects the ground BEHIND the
wheel/tire. If you have a bicycle handy, get on it and steer the
front wheel right or left and watch what happens to the CAMBER of the
wheel. That is, notice that the farther you steer the wheel in one
direction or another, the wheel itself leans considerably and is no
longer in a vertical plane. You'll notice the same thing on a road
car when the wheels are steered sharply
the camber of the wheels
change with steering input. How does this happen? Back to the bicycle
analogy: Imagine that you designed a bicycle so that the steering
axis was angled back so far that it was PARALLEL to the ground. Of
course, you wouldn't be able to steer the bike at all. Turning the
handlebars would only lean the wheel over.
Why have caster at all? Why not just run the steering axis vertical?
Good question. Ever notice when you're steering through a corner that
IF you were to let go of the steering wheel, it would snap back to
center? That tendency is due to the positive caster angle. The
greater the angle, the greater the self-centering force. Caster has a
huge impact on the directional stability and steering effort. That
is, the greater the positive caster, the lighter the steering effort
(to a point, I believe) AND the greater the tendency of the car to
wander. Lesser caster angles give great directional stability, but
the steering effort becomes greater.
Steering axis inclination is very similar. The caster angles
described above are observable when looking at the car from the side.
SAI is observable when looking at the car from the front (or rear).
Both angles tell us something about the steering axis. If you view a
front wheel from the front of the car, the axis around which that
wheel rotates when steered left or right is also slanted, almost
always so that an imaginary line drawn on the axis intersects the
ground near the center of the tire's contact patch. The other end of
this imaginary line would (theoretically) intersect the axis of the
opposite wheel's steering axis at some point in space above the car.
In other words, the steering axis is inclined toward the center of
the car. This inclination also has an impact on steering effort.
Usually the axis is designed to intersect the ground NEAR the
geometric center of the tire's contact patch. But usually it's offset
so that it intersects the ground INBOARD of the geometric center of
the contact patch. The distance of this offset is called 'steering
offset'. The greater the offset, the more steering effort is
required. SAI and steering offset are manipulated especially in front
wheel drive cars to tune out 'torque steer' to some extent.
I'm not sure I'm familiar with 'included angle'.
Good luck!
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