The steering gear mechanism turns the front wheels of a vehicle to change its direction while keeping the back wheels straight. An Ackerman steering gear is an exact mechanism that fulfills the steering condition to avoid skidding by ensuring the front wheels turn about the same instantaneous center, which lies on the axis of the fixed back wheels. Compared to the Davis mechanism, the Ackerman mechanism has the whole steering assembly behind the front wheels rather than in front, and uses turning pairs rather than sliding pairs.

1. By PDPU Students

2.  The steering gear mechanism is used for
changing the direction of two or more of the wheel
axles with reference to the chassis. Usually, Back
wheel axle is fixed in direction with reference to
the chassis and the steering is done by means of
the front wheels.

3.  When the vehicle takes turn, the front wheels
along with the respective axles turn about
corresponding povited points.
 Also, the back wheels remain straight and do not
turn. Therefore, steering is done by means of the
front wheels.

4.  In order to avoid skidding(slipping of the wheels of
sideways),the two front wheels must turn about
the same instantaneous centre which lies on the
axis of the back wheels. if this does not happen,
the skidding on the front or back wheels will
definitely take place, which will in turn produce
more wear and tear on the tyres.

5. Steering principle
A C
E F
B
a
b
Iφ θ
cot
cot
cot cot
EI EF FI a FI
AE AE b
FI FI
CF b
a FI FI a
b b b
θ
φ
θ φ
+ +
= = =
= =
+
∴ − = − =

6.  This is an exact steering gear mechansm. This
mechanism fulfils the above steering condition. But due
to presence of more sliding members, the wear will be
increased and this eliminates the accuracy.

10.  It is much simpler than Davis mechanism.
 Difference between Davis and ackerman
mechanism:
◦ The whole mechanism is on back of the front wheels ,
whereas in Davis ,it is in front of the wheels.
◦ It consists of turning pairs, whereas in Davis, it consists
of sliding pairs.

12.  When the vehicle moves along straight path, the
longer links AB and CD are parallel and the
shorter links BC and AD are equally inclined to the
longitudinal axis of the vehicle.
 When the vehicle moves left, the position of the
gear is shown by dotted lines in fig.

13. THANK
YOU