The tilt steering mechanism allows selection of the steering wheel position (in the vertical direction) to match the driver’s driving posture. The tilt steering mechanisms are classified into the upper fulcrum type and the lower fulcrum type. Here, the lower fulcrum type is explained.

2. Construction

The tilt steering mechanism consists of a pair of tilt steering stoppers, tilt lock bolt, breakaway bracket, tilt lever, etc.

3. Operation

The tilt steering stoppers turn together with the operation of the tilt lever. When the tilt lever is in the lock position, the peaks of the tilt steering stoppers are lifted up and the stoppers push against the breakaway bracket and tilt attachment, locking the break away bracket and tilt attachment. On the other hand, when the tilt lever is moved to the free position, the height difference on the tilt steering stoppers is eliminated, and the steering column can be adjusted in the vertical direction.

SERVICE HINT:

If the wheels are given excessive positive or negative camber, this causes uneven tire wear. If the wheels are given excessive negative camber, the tires wear quicker on the inside; if the wheels are given excessive positive camber, the tires wear quicker on the outside.

Negative Camber

When a vertical load is applied to a cambered tire, force is generated in the horizontal direction. This force is called “camber thrust” and operates to the inside of the vehicle for negative camber and the outside of the vehicle for positive camber. During cornering, because the vehicle leans to the outside, the tire camber becomes more positive, the camber thrust to the inside of the vehicle is reduced, and cornering force is reduced. The negative camber suppresses the positive camber of the tires during cornering and helps maintain proper cornering force.

Camber During Cornering

When a vehicle turns a corner, the camber thrust on the outside tires acts to reduce the cornering force due to the increase in positive camber. Centrifugal force tilts the turning vehicle due to the action of the suspension springs, changing the camber.

HINT:

Cornering is always accompanied by centrifugal force, which tires to force the vehicle to turn in a larger arc than intended by the driver unless the vehicle can generate a sufficient counterforce – that is, centripetal force – to balance this. This centripetal force is generated by the deformation and side-slipping of the tread that occurs due to friction between the tire and the road surface. This called cornering force.

Camber During Cornering

When a vehicle turns a corner, the camber thrust on the outside tires acts to reduce the cornering force due to the increase in positive camber. Centrifugal force tilts the turning vehicle due to the action of the suspension springs, changing the camber.

HINT:

Cornering is always accompanied by centrifugal force, which tires to force the vehicle to turn in a larger arc than intended by the driver unless the vehicle can generate a sufficient counterforce – that is, centripetal force – to balance this. This centripetal force is generated by the deformation and side-slipping of the tread that occurs due to friction between the tire and the road surface. This called cornering force.

The telescopic steering mechanism allows forward or backward adjustment of the steering wheel position to suit the driver’s posture.

2. Construction

The telescopic mechanism consists of the sliding shaft tube, two wedge locks, stopper bolt, telescopic lever, etc.

3. Operation

The wedge locks move together with operation of the telescopic lever. When the telescopic lever is in the lock position, the telescopic lever presses the wedge locks against the sliding shaft tube, locking the sliding shaft tube. On the other hand, when the telescopic lever is moved to the free position, a gap is created between the wedge locks and the sliding shaft tube and the steering column can be adjusted in the forward or backward direction.

SERVICE HINT:

If the wheels are given excessive positive caster, the straight-line stability is improved, but cornering becomes difficult.

Straight-line Stability and Wheel Recovery

Straight-line stability due to caster angle When the steering axis rotates, during cornering, if the wheels have caster angle, the tires are inclined relative to the ground and jack-up torque is generated that attempts to lift the vehicle body as in the figure. This jack-up torque functions as a recovery force that attempts to return the vehicle body to the horizontal and maintains the straight-line stability of the vehicle.

Wheel recovery due to caster trail If the wheels are given caster angle, the contact point of the line extending from the steering axis is forward of the center of tire-road contact. Therefore, since the tires are pulled from the forward direction, the force pulling the tires holds down the force attempting to destabilize the tires and maintains straight-line performance. Also, when the tires face sideways due to steering or disturbance during straight-line travel, side forces (F2 and F’ 2) are generated. These side forces act as rotation forces around the steering axis due to the caster trail and are forces attempting to return the tires to their original positions (recovery force). At this time, if the caster trail is long, for the same magnitude side force, a larger force works to return the steering wheel. Therefore, the longer the caster trail, the greater the straightline performance and recovery force.

Nachlauf and Vorlauf Geometry

In general, the caster angle must be increased in order to increase caster trail. However, even if the caster angle remains the same, the caster trail can be set as desired by offsetting the steering axis to the front or rear from the wheel center. Nachlauf geometry enables caster trail to be increased by offsetting the steering axis to the front from the wheel center. Vorlauf geometry enables caster trail to be decreased by offsetting the steering axis to the rear from the wheel center. In actual vehicles, various settings are made by Nachlauf and Vorlauf geometry in order to match vehicle characteristics.

The new PPS(Progressive Power Steering) varies, the steering operation effort in accordance with the vehicle speed. The steering effort is lighter at low speed and heavier at high speed.

2. Operation

The hydraulic reaction type progressive power steering uses a thinner torsion bar than in ordinary power steering to reduce the steering effort required during stationary steering or when traveling at a low speed. However, this causes the steering required effort to become too small (that is, the steering wheel feels too “light”) when the vehicle speed increases. To prevent this, the required steering effort is increased as when there is a thicker torsion bar, by the provision of a hydraulic reaction chamber for suppressing the rotation of the control valve shaft (in the control valve housing) by means of four hydraulically operated plungers. The hydraulic pressure applied to the hydraulic reaction chamber is low at low vehicle speeds and high at high vehicle speeds. It also increases as the hydraulic pressure in the power cylinder rises as a result of steering operation. In the hydraulic reaction type progressive power steering, the required steering effort varies according to the vehicle speed and steering operation.

A steering linkage is a combination of the rods and arms that transmit the movement of the steering gear to the left and right front wheels. The steering linkage must accurately transmit the movement of the steering wheel to the front wheels as they move up and down while the vehicle is running. There are various linkage arrangements and joint constructions designed to do this.

2. Construction

The steering linkage consists of the following parts.

(1) Tie rod

(2) Tie rod end

(3) Knuckle arm

(4) Pitman arm (R/B type)

(5) Relay rod (R/B type)

(6) Steering knuckle (R/B type)

(7) Idler arm (R/B type)

(8) Drag link (R/B type)

This type of steering column adjusts the tilt mechanism and telescopic mechanism electrically. A motor is used for each mechanism, and operations with a switch.

2. Construction

The power tilt mechanism section consists of the tilt motor, tilt worm shaft, tilt worm gear, and slider. The power telescopic mechanism section consists of the telescopic motor, sliding tube, and telescopic screw.

The switches for operating these motors are on the steering column cover.

HINT: If the tilt away auto switch is ON, when the ignition key is removed, the tilt position of the steering column automatically moves to the highest position and the telescopic position moves to the shortest position for the driver to get on and off comfortably. Also, since the ECU stores the column position read by sensor into memory, when the ignition key is inserted again, the steering column returns to its original position.

3. Operation

(1) Power tilt operation

Operating the switch up or down operates the tilt motor. The tilt worm gear and tilt worm shaft start to rotate and the slider slides. The steering column linked with the linkage tilts up or down.

(2) Power telescopic operation

Operating the switch left or right operates the telescopic motor. The telescopic worm gear starts to rotate and the sliding tube slides forward or backward.

(1) Component parts

<1> Steering wheel Handles the steering operation.

<2> Steering column Joins the steering wheel and the steering gears.

<3> Steering gears Convert the steering torque and rotational deflection from the steering wheel, transmit them to the wheel through the steering linkage, and make the vehicle turn.

<4> Steering linkage A steering linkage is a combination of the rods and arms that transmit the movement of the steering gear to the left and right front wheels. Also, there are two types of steering.

• Rack-and-pinion type

• Recirculating-ball type

2. Requirements of steering system

The steering system has the following requirements.

(1) Requirements

<1> Excellent maneuverability When the vehicle is cornering on a 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 vehicle is stopped and will decrease as the speed of the vehicle increase. 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 vehicle 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 roughness must not occur.