Steering system

FUNCTION AND PRINCIPLE

The purpose of the steering system is to allow the driver to control the direction of the vehicle by turning the front wheels. This is done by means of steering wheel, a steering column which transmits the rotation of the steering wheel to the steering gears, the steering gears which increase the rotational force of the steering wheel in order to transmit greater torque to the steering linkage, and the steering linkage which transmits the steering gear movement to the front wheels. The steering system configuration depends on vehicle design (the drive train and suspension system used, whether it is a passenger car or a commercial vehicle, etc.). At present, the rack-and-pinion type and the recirculation-ball types are in use. These are shown in Fig. 4.1.

. Requirements of steering system

The steering system plays, together with the suspension system, an important role in ensuring easy, comfortable driving all the way from the low speed range to the high speed ranges. The drive train transmits the power from the engine to the driving wheels to move the car forward; the steering system steers the car in the desired direction; and the brake system ensures positive, stable slowing and stopping.

1.         Excellent maneuverability. When the car is cornering on narrow, twisting road, the steering system must be able to turn the front wheels sharply yet easily and smoothly.

2.         Proper steering effort. If nothing is done to prevent it, steering effort will be greater when the car is stopped and will decrease as the speed of the car increases.  Therefore, in order to obtain easier steering and better feel of the road, the steering should be made lighter at low speeds and heavier at high speeds.

3.         Smooth recovery. While the car is turning, the driver must hold the steering wheel firmly. After the turn is completed, however, recovery that is the return of the wheels to the straight-ahead position should occur smoothly as the driver relaxes the force with which he is turning the steering wheel.

4.         Minimum transmission of shock from road surface. Loss of steering wheel control and transmission of kickback due to road surface roughen must not occur.

4.1.2. Principle

The relative motion between the wheels of a vehicle at the road surface should be of a pure rolling type so that wear of the tires is minimum and uniform. When the vehicle is moving on a curved path, the steering gear must be so designed that the paths of the points of contact of each wheel with the ground are concentric circular ares. For proper steering the axis of rotation of all the wheels should meet at G i.e. the instantaneous centre of rotation of the vehicle.. To satisfy this inner wells should be turned through a greater angle as shown in fig. 4.2. in which 0is greater than 0, when the vehicle is turning towards right side

STEERING GEARS

There are two types of steering gear mechanisms:

1. Davis steering gear.

2. Ackermann steering gear.

The main difference between the two steering gear mechanisms is that the Davis steering gear has sliding pairs, whereas the Ackermann steering gear has only turning pairs. The sliding pair has more friction than the turning pair, therefore, the Davis steering gear will wear out earlier and become inaccurate after certain time. Although, the Ackermann steering gear is not mathematically accurate except in the positions, contrary to the Davis steering gear which is mathematically correct in all positions. However, the Ackermann steering gear is preferred to the Davis steering gear.

1 Davis Steering Gear

The Davis gear mechanism consists of a cross link KL sliding parallel to another link AB and is connected to the stub axles of the two front wheels by means of two similar bell crank levers CAK and DBL pivoted at  A and B respectively. The cross link KL slides in the bearing and carries pins at its end K and L. The slide blocks are pivoted on these pins and move with the turning of bell crank levers as the steering wheel is operated. When the vehicle is running straight, the gear is said to be in its mid-position. The short arms AK and BL are inclined at angle 90 + a to therir stub axles AC and BC respectively. The correct steering depends upon the suitable selection of cross-arm angle a, and is given by

tan a = b/2l

where  b= AB = distance between the pivots of front axles.

                        l = wheel base

The range of b/I is 0.4 to 0.5, hence angle a lies between 11.3 and 14.1^o
 

The angles o and 0 are shown in the fig. 4.4. The value of b/i between 0.4 and 0.5, generally 0.455. The value of cot o - cot 0 corresponds to the positions when steering is correct. In fact there are three values of angle 0 which give correct steering of the vehicle first while it is turning to right, second while it is turning to left and third while it is running straight.

References

1. Automobile Engineering (Vol. 1 & 2)            -           K.M.Guptha
2. Automotive Mechanics                                   -           Joseph Heitner
3. Automobile Engineering                                 -           Harbans Singh Reyd
4. Automotive Mechanics                                   -           William H. Course