The Parallelogram linkage steering system was a common
system used on older vehicles. Today, this is only used on
large cars and trucks because of the weight and number of
parts. Each joint in this system must allow for rotation and
changes in angles, and therefore has a significant amount of
"free play" or movement.
The Pitman Arm is supported by the "Steering Box" sector
(or pitman) shaft. The Steering Box is attached to the frame
on the driver's side of the vehicle. The "Idler Arm" is
attached to the frame on the opposite side and supports the
The "Steering Box" reduces turning effort by increasing
the force applied through a screw thread (worm gear)
principle, and turns the force at 90 degrees to rotate
the pitman arm back and forth to move the linkage.
Rack and Pinion
The Rack and Pinion System is used with most
today's small and intermediate size cars and minivans.
The reduced weight and fewer parts helps to allow vehicles
to meet fuel economy and emission standards. Because
there are fewer moving parts, the steering system has less
"free play" and results in a more positive steering system.
The Rack and Pinion system is not suitable for large cars or
trucks because of the tendency of the rack to flex under hard
cornering with heavy vehicles.
The Rack and Pinion
The Rack is a flat gear machined at one end of the center
portion of the center rod. The pinion is a gear machined
at the end of the steering shaft that, when rotated, causes
the rack to move back and forth in the Rack Housing. Note
the ball and socket design of the Inner Tie-Rod end.
Regardless of the system used, the outer Tie-Rod ends
must be attached to the Steering Arm - which is often an
integral part of the steering knuckle assembly that is
used to support the wheel. Note that the steering arm is
not straight - but is angled in to the center of the vehicle.
This becomes important when discussing the
"Toe Out on Turns" angle in the Alignment section
of this lesson.
Power Steering is actually Power
hydraulic pump is driven by the engine to produce pressure
to reduce the amount of effort required to turn the wheels
away from the straight-ahead position. On a Parallelogram
system, the power assist can be used in conjunction with a
piston on the Ballnut (see the above steering box diagram)
or may be used with a hydraulic piston attached to the steering
linkage. The Rack and Pinion system uses a hydraulic piston
attached to the Rack to reduce the amount of effort required.
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