Spicer Steer Axles Application Guidelines - Manual (Page 28)

< 22 > back AXAG400 – February 2007
Ackermann Geometry
Ackermann Geometry is a geometric arrangement of linkages in the steering of a vehicle designed to
solve the problem of wheels on the inside and outside of a turn needing to trace out circles of different
radii. When a vehicle is steered, it follows a path which is part of the circumference of its turning circle,
which will have a centre point somewhere along a line extending from the axis of the fixed axle. The
steered wheels must be angled so that they are both at 90 degrees to a line drawn from the circle centre
through the centre of the wheel. Since the wheel on the outside of the turn will trace a larger circle than
the wheel on the inside, the wheels need to be set at different angles. The Ackermann steering geometry
arranges this concern by moving the steering pivot points inward so as to lie on a line drawn between the
steering kingpins and the centre of the rear axle. This arrangement ensures that at any angle of steering,
the centre point of all of the circles traced by all wheels will lie at a common point.
Ideally the point of intersection of the normal to the steering wheels on both sides should intersect on the
line normal to the center of the drive axle. If we did not introduce a positive tie rod angle, the tires would
try to drive at the same turn radius.
Wheelbase and track