911 Steering gearbox angular ratios

The steering gearbox has two main functions: it produces a gear reduction between the input steering wheel and the output drop arm (Pitman arm) and it redirects the input to output axis of rotation through a right angle.

Generally, the steering road wheel stub axles must be capable of twisting through a maximum steering angle of 40° either side of the straight ahead position. The overall angular gear ratio of a steering gearbox may be as direct as 12:1 for light small vehicles or as indirect as 28:1 for heavy vehicles. Therefore, lock to lock drop arm angular displacement amounts to 80° and with a 12:1 and 28:1 gear reduction the number of turns of the steering wheel would be derived as follows:

Lock to lock steering wheel 80 x 12 turns for 12:1 = 360

= 2.66 revolutions

Lock to lock steering wheel 80 x 28 turns for 28:1 reduction 360

6.22 revolutions

Lock to lock steering wheel turns

Fig. 9.1 Relationship of overall angular gear ratio and steering wheel lock to lock revolutions

Lock to lock steering wheel turns

Fig. 9.1 Relationship of overall angular gear ratio and steering wheel lock to lock revolutions

From these results plotted in Fig. 9.1 it can be seen that the 12:1 reduction needs the steering wheel to be rotated 1.33 turns from the straight ahead position. The 28:1 reduction will require more than twice this angular displacement, namely 3.11 turns. Thus with the 12:1 gear reduction, the steering may be heavy but can be turned from the straight ahead position to full lock and back again relatively quickly. However the 28:1 reduction will provide a light steering wheel but the vehicle will be compelled to corner much slower if the driver is to be able to complete the manoeuvre safely.

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