Dual front axle alignment checks using Dunlop optical measurement equipment Fig 932a d

1 Roll or drive forward. Check the toe-in or -out of both pairs of front steering wheels and adjust track rods if necessary (Fig. 9.32(a)).

Assemble mirror gauge stand with the mirror positioned at right angles to the tubular stand. Position the mirror gauge against a rear axle wheel (preferably the nearest axle to the front) with the mirror facing towards the front of the vehicle (Fig. 9.32(b)).

Place the view box gauge stand on the floor in a transverse position at least one metre in front of the vehicle so that the view box faces the mirror (Fig. 9.32(c)). Move the view box stand across until the reflected image is centred in the view box with a zero reading on the scale. Chalk mark the position of the view box tripod legs on the ground. Bring the mirror gauge stand forward to the first steer axle wheel and place gauge prods against wheel rim (Fig. 9.34(c)).

If both pairs of steer axle wheels are set parallel (without toe-in or -out), set the pointer on the toe angle scale to zero. Conversely, if both steer axles have toe-in or -out settings, move the pointer on the toe angle scale to read half the toe-in or -out

Fig. 9.31 Dual front axle steering interconnecting relay linkage principle
Wheel Alignment Checking Procedure
Fig. 9.32 Dual front axle wheel alignment procedure

figure, i.e. with a track toe angle of 30' set the pointer to read 15'.

6 Look through the periscope tube and with an assistant move the driver's steering wheel in the cab until the hairline is central in the view box (Fig. 9.32(c)). At the same time make sure that the mirror gauge prods are still in contact with the front wheel rim. The first front steer axle is now aligned to the first rigid rear axle.

7 Move the mirror gauge from the first steer axle wheel back to the second steer axle wheel and position the prods firmly against the wheel rim (Fig. 9.32(d)).

8 Look into the periscope. The hairline in the view box should be centrally positioned with the toe angle pointer still in the same position as used when checking the first steer axle (Fig. 9.32(d)).

If the hairline is off-centre, the relay connecting rod between the two relay idler arms should be adjusted until the second steer axle alignment relative to the rear rigid axle and the first steer axle has been corrected. Whilst carrying out any adjustment to the track rods or relay connecting rod, the overall wheel alignment may have been disturbed. Therefore a final check should be made by repeating all steps from 2 to 8.

9.4.6 Steer angle dependent four wheel steering system (Honda)

This steer angle dependent four wheel steering system provides dual steering characteristics enabling same direction steer to take place for small steering wheel angles. This then changes to opposite direction steer with increased steering wheel deviation from the straight ahead position. Both of these steer characteristics are explained as follows:

Opposite direction steer (Fig. 9.33) At low speed and large steering wheel angles the rear wheels are turned by a small amount in the opposite direction to the front wheels to improve manoeuvrability when parking (Fig. 9.32). In effect opposite direction steer reduces the car's turning circle but it does have one drawback; the rear wheels tend to bear against the side of the kerb. Generally there is sufficient tyre sidewall distortion and suspension compliance to accommodate the wheel movement as it comes into contact with the kerb so that only at very large steering wheel angles can opposite direction steer becomes a serious problem.

Same direction steer (Fig. 9.33) At high speed and small steering wheel angles the rear wheels are

Fig. 9.33 Front and rear wheel steer relationship to driver's steering wheel angular movement

turned a small amount in the same direction as the front wheels to improve both steering response and stability at speed (Fig. 9.33). This feature is particularly effective when changing lanes on motorways. Incorporating a same direction steer to the rear wheels introduces an understeer characteristic to the car because it counteracts the angular steering movement of the front wheels and consequently produces a stabilizing influence in the high speed handling of the car.

Front and rear road wheel response relative to the steering wheel angular movement (Fig. 9.33) Moving the steering wheel approximately 120° from its central position twists the front wheels 8° from the straight ahead position. Correspondingly, it moves the eccentric shaft peg to its maximum horizontal annular gear offset, this being equivalent to a maximum 1.5° same direction steer for the rear road wheels (Fig. 9.33).

Increasing the steering wheel rotation to 232° turns the front wheels 15.6° from the straight ahead position which brings the planetary peg towards the top of the annular gear and in vertical alignment with the gear's centre. This then corresponds to moving the rear wheels back to the straight ahead position (Fig. 9.33).

Further rotation of the steering wheel from the straight ahead position orbits the planetary gear over the right hand side of the annular gear. Accordingly the rear wheels steadily move to the opposite direction steer condition up to a maximum of 5.3° when the driver's steering wheel has been turned roughly 450° (Fig. 9.33).

Four wheel steer (FWS) layout (Fig. 9.34) The steering system is comprised of a rack and pinion front steering box and a rear epicyclic steering box coupled together by a central drive shaft and a pair of Hooke's universal end joints (Fig. 9.35). Both front and rear wheels swivel on ball suspension joints which are steered by split track rods actuating steering arms. The front road wheels are interlinked by a rack and transverse input movement to the track rods is provided by the input pinion shaft which is connected to the driver's steering wheel via a split steering shaft and two universal joints. Steering wheel movement is relayed to the rear steering box by way of the front steering rack which meshes with an output pinion shaft. This movement of the front rack causes the output pinion and centre drive shaft to transmit motion to the rear steering box. The rear steering box mechanism then con verts the angular input shaft motion to a transverse linear movement. This is then conveyed to the rear wheel swivels by the stroke rod and split track rods.

Rear steering box construction (Fig. 9.35) The rear steering box is basically formed from an epi-cyclic gear set consisting of a fixed internally toothed annular ring gear in which a planetary gear driven by an eccentric shaft revolves (Fig. 9.35). Motion is transferred from the input eccentric shaft to the planetary gear through an offset peg attached to a disc which is mounted centrally on the eccentric shaft. Rotation of the input eccentric shaft imparts movement to the planetary gear which is forced to orbit around the inside of the annular gear. At the same time, motion is conveyed to the guide fork via a second peg mounted eccentrically on the face of the planetary gear and a slider plate which fits over the peg (Fig. 9.35). Since the slider plate is located between the fork fingers, the rotation of the planetary gear and peg causes the slider plate to move in both a vertical and horizontal direction. Due to the construction of the guide fork, the slider plate is free to move vertically up and down but is constrained in the horizontal direction so that the stroke rod is compelled to move transversely to and fro according to the angular position of the planetary gear and peg.

Adopting this combined epicyclic gear set with a slider fork mechanism enables a small same direction steer movement of the rear wheels to take place for small deviation of the steering wheel from the straight ahead position. The rear wheels then progressively change from a same direction steer movement into an opposite steer displacement with larger steering angles.

The actual steering wheel movement at which the rear wheels change over from the same direc

Fig. 9.34 Four wheel steering (4WS) system

Fig. 9.35 Epicyclic rear steering box tion steer to the opposite direction steer and the magnitude of the rear wheel turning angles relative to both conditions are dependent upon the epicyclic gear set gear ratio chosen.

Rear steering box operation (Fig. 9.36(a-e)) The automatic correction of the rear road wheels from a same direction steer to opposite direction steer with increasing front road wheel turning angle and vice versa is explained by Fig. 9.36(a-e).

Central position With the steering wheel in the straight ahead position, the planetary gear sits at the bottom of the annular gear with both eccentric shaft and planetary pegs located at bottom dead centre in the mid-position (Fig. 9.36(a)).

90° eccentric shaft peg rotation Rotating the eccentric shaft through its first quadrant (0-90°) in a chosen direction from the bottom dead centre position compels the planetary gear to roll in an anticlockwise direction up the left hand side of the annular ring gear. This causes the planetary peg and the stroke rod to be displaced slightly to the left (Fig. 9.36(b)) and accordingly makes the rear wheels move to a same direction steer condition.

180° eccentric shaft peg rotation Rotating the eccentric shaft through its second quadrant (90-180°) causes the planetary gear to roll anticlockwise inside the annular gear so that it moves with the eccentric peg to the highest position. At the same time, the planetary peg orbits to the right hand side of the annular gear centre line (Fig. 9.36(c)) so that the rear road wheels turn to the opposite direction steer condition.

270° eccentric shaft peg rotation Rotating the eccentric shaft through a third quadrant (180-270°) moves the planetary gears and the eccentric shaft peg to the 270° position, causing the planetary peg to orbit even more to the right hand side (Fig. 9.36(d)). Consequently further opposite direction steer will be provided.

360° eccentric shaft peg rotation Rotating the eccentric shaft through a fourth quadrant (270-360°) completes one revolution of the eccentric shaft. It therefore brings the planetary gear back to the base of the annular ring gear with the eccentric shaft peg in its lowest position (Fig. 9.36(e)). The planetary peg will have moved back to the central position, but this time the peg is in its highest position. The front to rear wheel steering drive gearing is normally so arranged that

Fig. 9.36(a-d) Principle of rear steering box mechanism

the epicyclic gear set does not operate in the fourth quadrant even under full steering lock conditions.

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