Steering Geometry

3.11 Steering Geometry:

Steering geometry is the geometric arrangement of the parts of a steering system, and the value of the lengths and angles within it. It refers to the angular relationship between the front wheels and parts attached to it and vehicles frame. And this geometric arrangement of linkages in the steering of a car designed to solve the problem of wheels on the inside and outside of a turn needing to trace out circles of different radii. This geometry can be easily defined if the wheelbase, kingpin locations, and moment arm lengths are known.

The steering geometry includes:

1. Caster angle

2. Camber angle

3. King-pin inclination

4. Toe-in & Toe-out.

5. Steering gear ratio & backlash etc.

1) Caster Angle:

The angle between the vertical line and the king pin center line in the plane of the wheel (When viewed from the side) is called the Caster angle. [An angle b/w vertical line and kingpin centreline (from s.v.).]

Fig 1: Caster angle

This is the angle between backward or forward tilting of the king pin from the vertical axis at the top. This is about 2º to 8º. The backward tilt is called as positive caster. The forward tilt is called negative caster.

The main purpose of caster angle is to create self-centering effect in the steering. It provides the directional stability. It positive caster increase the efforts required to steer and tries to keep the wheels straight ahead. In heavy duty trucks negative caster is preferred. This makes the steering easier. Excessive caster makes steering heavier, less responsive and increased effort.

Caster is the angle of the steering axis of a wheel from true vertical, and has little effect on tyre wear. Positive caster tilts the tops of the steering knuckles toward the rear of the vehicle. Negative caster tilts the tops of the steering knuckles toward the front of the vehicle. Unequal caster will cause the vehicle to steer toward the side with less caster angle. Most common problem affecting caster is worn strut rod and control arm bushings

2) Camber angle:

The angle between wheel axis to the vertical line at the top is called camber angle. It is approximately ½º to 2º. [Inward or outward tilt of wheel (from f.v.)]

Fig 2: Camber angle

It is the angle between the centre line of the tyre and the vertical plane. When viewed from the front of the vehicle when the angle is outward, so that the wheels are farther apart at the top the camber is “Positive” when the angle is inward, so that the wheels are closer together at the top, the camber is “Negative”. The usual value of camber angle should not exceed 2º. When the camber angle is positive, it causes slip out prevention lightens the perpendicular load and lessen the required steering effort. If it is a Zero camber, it prevents uneven wear of tyres. When the camber angle is negative, the camber thrust increase with increase in tyre inclination relative to the road surface.

3) King-pin inclination:

Fig 3: King-pin inclination

The kingpin is set at an angle to the vertical plane when viewed from the front or rear of the vehicle. This angle is known as the king pin inclination. The purpose of the KPI is to produce vertical displacement of the vehicle in during steering in an upward direction. It is the angle between vertical line to the king pin axis. The inclination tends to keep wheels straight ahead and make the wheels to get return to the straight position after completion of a turn. The inclination is normally kept 7º to 8º.

Most of the steering systems have a kingpin which is attached to steering knuckle to a support. In some later design kingpin is replaced by ball and socket joint. In this design, the Steering knuckle and knuckle-support are combined into a single part, This part is called steering knuckle. No kingpin is used in this case. The steering knuckle is supported at the top and bottom by control arm.

4) Toe-In & Toe-Out.

Fig 4: Toe-in & toe-out

Toe is critical to tire wear. Toe-in: front of the wheels are set closer than the rear. Toe-out: front of the wheels farther apart than the rear. Rear-wheel-drive vehicles are often adjusted to have toe-in, front-wheel-drive vehicles are adjusted to have a slight toe-out.

Toe-in: It is the amount in minimum at the front part of the wheel point’s inwards approximately 3 to 5 mm. It prevents side slipping excessive tyre wear, proper rolling of front wheels and steering stability.

Fig 5: Toe-in

Purpose of Toe-in

To ensure that wheels are rolling parallel.

To stabilize steering and prevent slipping towards sides.

To prevent excessive tyre wear.

To offset the effect of small deflections in the wheel support system.

Toe-out: It is the difference in angles between two front wheels and vehicle frame during turning. It is used to prevent dragging of tyre during turn. While taking the turn, the inside wheel makes larger angle than outer wheel to satisfy the condition of correct steering. The toe-out is shown in Figure

Fig 6: Toe-out

5) Steering Ratio:

The amount of input from the driver to which the output of wheel turned is called as “Steering Ratio”. The Steering ratio is dependent on many factors like diameter of the Pinion, Rack travel, Steering wheel diameter, Moment arm length.