Ackermann Steering Mechanism
The Ackermann steering gear has only turning pairs. Fig. (a) shows the steering gear for straight drive. The turning pairs are : AK, KL, LB and AB. The two short arms AK and BL are of equal length and are connected by pin joints with front wheel axle AB at A and B respectively. AC and BD are the stub axles so that CAK and DBL form bell crank levers. ABLK form a four-bar linkage. KL is the track rod.
Determination of angle α
Consider the Ackermann steering gear, as shown in figure (b) taking a right turn. The instantaneous centre I lies on a line parallel to the rear axis at a distance of approximately 0.3 l above the rear axis. It may be seen that the whole mechanism of the Ackermann steering gear is on the back of the front wheels. From figure (c), we have,
Projection of arc K′K on AB = Projection of arc L′L on AB
or K1K1′ =L1L1′
or AK[sin α – sin (α – φ)] =BL[sin(α + θ) – sin α]
Now AK=BL
so, sin α – sin(α – φ) =sin (α + θ) – sin α
or, sin α – sin α cos φ + cos α sin φ=sin α cos θ + cos α sin θ – sin α
or, sin α(2 – cos φ – cos θ) =cos α(sin θ – sin φ)
or tan α= sin sin φ /2 cos cosθ …(3.15)
The value of θ and φ are known for correct steering. Hence the value of α can be determined from Equation 3.15.
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