INTRODUCTION - RACK AND PINION STEERING BOX

INTRODUCTION

The vehicle has two steering ratios.

Technical specifications for petrol engines:

- reduction ratio: 51 mm/rev

- steering wheel angle/wheel angle ratio: 15.7 °steering wheel/ °wheel

- steering wheel turns: 2.8 for a turning circle (between pavements) of 10.1 m

Technical specifications for diesel engines:

- reduction ratio: 60-44 mm/rev

- steering wheel angle/wheel angle ratio: 13.4 °steering wheel/°wheel to the centre and 18.2° steering wheel/°wheel to the end of travel

- steering wheel turns: 2.8 for a turning circle (between pavements): 10.76 m

VARIABLE RATIO RACK AND PINION STEERING BOX (diesel versions)

Specifications

The innovative feature lies in the possibility of reducing the difference between the maximum force applied to the steering wheel, when the vehicle is cornering or committed to manoeuvres with high steering angles, and the minimum force applied when the steering wheel is virtually or actually in a straight line position.

Diagram showing different forces applied to a constant ratio steering box and a variable ratio steering box

1. Constant ratio steering box

2. Variable ratio steering box

This feature is produced thanks to the particular shape of the rack teeth capable of determining variable ratios between the force applied to the steering wheel and the force transmitted to the wheels depending on the position of the rack in relation to the pinion.

The rack teeth are cut with a variable pressure angle and module from the centre towards the ends so that the pinion meshes with the different step teeth depending on the steering angle.

A more direct ratio is produced in this way when the pinion is working in the centre part of the rack and a more reduced ratio gradually as the pinion meshes close to the two ends of the rack during steering.

This increases driving sensitivity in straight lines and improves driving comfort round bends and during manoeuvres.

SECTION OF VARIABLE RATIO STEERING BOX WITH DIAGRAM SHOWING RACK AND PINION TEETH

SECTION OF CONSTANT RATIO STEERING BOX WITH DIAGRAM SHOWING RACK AND PINION TEETH (Petrol Version)

The diagram below shows the section of the constant ratio steering box and the diagrammatic representation of the rack and pinion teeth.