The steering is designed to give the necessary radial movement to the drop arm, the outer end of which is connected to the drag link or pull-and-push rod. The steering gear ratio varies, but on modern vehicles considerable leverage is always provided so that the effort exerted by the driver is comparatively less. Steering gear is a device for converting the rotary motion of the steering wheel into straight line motion of the linkages or angular turning of the front wheels.
COMPONENTS OF STEERING SYSTEM
(a) Steering Wheel. This consists of a wheel connected by three or four spokes to a central hub. The rim is often notched on underside to provide a good grip for the driver’s fingers. The hub is bored and either splined or fitted with a keyway so that it may be secured to the top end of the steering shaft, more often called the inner column. The size of the wheel depends on the weight of the vehicle and the type of gearing used.
(b) Outer Steering Column. This is a tube in which the inner column rotates. It has an internal bearing or bush at or near the top end, its lower end fits on to the steering box or it is made integral with the steering box.
(c) Inner Steering Column. The inner steering column is housed in bushes or bearings which are carried in the outer column. The top end; which protrudes from the outer column, is either serrated, splined or fitted with key or key-way. The steering wheel lower end is fitted with a worm thread and the worm is often brazed to the column.
(d) The Steering Gear Box. It houses the steering gears, steering cross-shaft and forms the reservoir for the lubricant.
(e) Cross or Rocker Shaft. The rotary movement of the inner column and worm is converted to give a turning movement to the steering rocker or cross shaft. The shaft extends beyond the steering box and carries the steering drop arm.
(f) Drop Arm. This is tough steel forging, which is bored at either end. The bore of the larger end is internally serrated, splined or keyed, to fit on the cross-shaft. The smaller end is fitted with a ball joint which connects the drop arm to the drag link or pull and push rod.
OPERATION OF STEERING SYSTEM
When the steering wheel is turned, the movement is conveyed through the steering gear to the drag link, which is either pulled backward or pushed forward by the steering gear drop arm. The drag link moves the right side steering arm causing the stub axle to move about its kingpin. Movement is also imparted to the track arm that is fixed to the stub axle. The right side track arm pulls the track rod, and with it the left side track arm which is fixed to the stub axle. The left side stub axle is thus moves about its king pin at the same time as the right side stub axle causing the front wheels to turn simultaneously in the same direction.
LAYOUT OF STEERING SYSTEM
The accompanying illustrations show two examples of steering and front axle layout. The image below “Complete Steering Parallel Drag Link” is more common with a parallel drag link or pull and push rod. The transverse drag link type is shown in image “Complete Steering – Traverse Drag Link”. It is generally used on vehicles fitted with transverse front suspension.
CASTOR ACTION AND CAMBER
(a) Castor Action. The front wheels of a vehicle in motion always tend to return to a straight-ahead position and this is noticed especially after taking a turn. This is brought about by providing the front wheels with a castoring or trailing action, by tilting the king pin from the vertical towards the rear at its top, so that when a line is extended through the centre of the king pin it meets the ground slightly ahead of the point where the tyre touches the ground.
(b) Castor Angle. It is the amount in degree by which the top of the axle beam or kingpin is tilted towards the back of the vehicle. It is measured by the line drawn through the centre of the king pin to the vertical
(c) Camber. Camber is the amount in degrees that the front wheels are titled outwards at the top and inwards at the bottom from a vertical position. When a vehicle is in motion the tendency of the front wheels is to turn about their pivot (king pin). The kingpins inclined at the top towards the centre to a greater degree and wheels are inclined outwards at the top to lesser angle so that an approximate centre point steering is obtained.
KING PIN INCLINATION AND TOE IN
(a) King Pin Angle . The amount in degrees by which the king pin is inclined at the top towards the centre of the vehicle is known as king pin angle. Kingpin angle is measured by the line drawn through the centre of the king pin to the vertical.
(b) Toe-in of Front Wheels . The inclination of the front wheels towards each other at the front is known as Toe-in. Toe-in is given to counteract the tendency of the front wheels to fly outwards when travelling straight ahead due to wheel camber and the play in the track rod and stub axle linkage.