Lesson 1/Learning Event 2
That is, the left wheel tries to pivot to the left and the right wheel to
the right. The wheels are able to pivot out to some extent because the
steering knuckles and tie rod ends must have a slight amount of clearance to
permit easy steering. To offset this, the wheels are aligned with a slight
amount of toein.
Toedin wheels are closer together at the front than at the rear. Toein is
measured in inches. The amount is found by measuring the distance between
the front wheels, first at the front outer edges of the tires and then at
their rear outer edges. The amount of toein is the difference in the two
measurements, which is usually about 1/32 inch to 1/8 inch. If the wheels
are closer together in the rear than in the front, they are said to be toed
out. The amount of toein can be adjusted by shortening or lengthening the
adjustable tie rod.
Ideally, the toein, measured with the vehicle standing still, should
exactly equal the amount the wheels pivot outward when the vehicle runs at
cruising speed. Then, the wheels will roll perfectly straight ahead at
cruising speed with no side slippage.
Side slippage of the tires must also be considered when turning a corner, to
ensure positive steering control and to prevent excessive tire wear. Each
wheel must be at a 90 angle to the center of rotation if it is to roll
easily and not scuff the tread of the tire.
This is no problem forfifth wheel steering. Pivoting the axle assembly
moves the front wheel at the outer edge of the turning radius ahead of the
inner wheel, and the 90 angle is obtained. With Ackerman steering,
however, the wheels must toeout on a turn.
With Ackerman steering, when turning a corner, one straight line cannot be
extended from the center of both front wheels. Instead, two lines must be
extended at different angles in order to pass through the center of the
wheels. For both wheels to be at a 90 angle to the center of rotation, the
inner wheel must pivot more than the outer wheel. (The exact difference in
the amount that the wheels should pivot is the value of the angle formed by
the lines extended from the center of rotation through the center of the
front wheels. For instance, if the lines form a 3 angle, the inner wheel
must pivot 3 more than the outer wheel.)