You might be surprised to learn that when you turn your car,
your front wheels are not pointing in the same direction.
For a car to
turn smoothly, each wheel must follow a different circle.
Since the inside wheel is following a circle with a smaller
radius, it is actually making a tighter turn than the
outside wheel. If you draw a line perpendicular to each
wheel, the lines will intersect at the center point of the
turn. The geometry of the steering linkage makes the inside
wheel turn more than the outside wheel.
There are a
couple different types of steering gears. The most common
are rack-and-pinion and re-circulating ball.
& Pinion Steering
Rack & Pinion Steering
is quickly becoming the most common type of steering on
cars, small trucks and SUVs. It is actually a pretty simple
mechanism. A rack-and-pinion gear set is enclosed in a metal
tube, with each end of the rack protruding from the tube. A
rod, called a tie rod, connects to each end of the rack.
The pinion gear is attached to
the steering shaft. When you turn the steering wheel, the gear
spins, moving the rack. The tie rod at each end of the rack
connects to the steering arm on the spindle
(See diagram above).
The rack-and-pinion gear set does two
It converts the rotational motion of the
steering wheel into the linear motion needed to turn the
It provides a gear reduction, making it
easier to turn the wheels.
On most cars, it takes three to four
complete revolutions of the steering wheel to make the
wheels turn from lock to lock (From
far left to far right).
The steering ratio is the ratio of how
far you turn the steering wheel to how far the wheels turn.
For instance, if one complete revolution
(360 degrees) of the steering
wheel results in the wheels of the car turning 20 degrees,
then the steering ratio is 360 divided by 20, or 18:1. A
higher ratio means that you have to turn the steering wheel
more to get the wheels to turn a given distance. However,
less effort is required because of the higher gear ratio.
Generally, lighter, sportier cars have
lower steering ratios than larger cars and trucks. The lower
ratio gives the steering a quicker response -- you don't
have to turn the steering wheel as much to get the wheels to
turn a given distance -- which is a desirable trait in
sports cars. These smaller cars are light enough that even
with the lower ratio, the effort required to turn the
steering wheel is not excessive.
Some cars have variable-ratio steering,
which uses a rack-and-pinion gear set that has a different
tooth pitch (Number of teeth per inch)
in the center than it has on the outside. This makes the car
respond quickly when starting a turn
(The rack is near the center), and also reduces
effort near the wheel's turning limits.