Cord measurement equipment Fig 928a and b

1 Check rear wheel toe angle by using the procedure adopted for front wheel toe angle measurement (Fig. 9.26(a)). Use the convention that toe-in is positive and toe-out is negative.

2 Keep the wheel clamp and measuring gauge assembly on both rear wheels.

3 Attach a second pair of wheel clamps to both front wheels.

4 Remove the rear wheel toe elastic cord from the two measuring gauges.

5 Hook a front to rear alignment elastic cord between the stub shaft deep outer groove of the front wheel clamps and the single hole in the measuring gauge rotors set at 90° from the middle hole of the three closely spaced holes (Fig. 9.28(a and b)).

Late ral offset

(a! Front to rear alignment with rigid rear axle suspension

40'toa +15'loe -K'oot +25' in

Latere I offset

Fj-oftttorear wheel alignmenl with independent rear suspension

Fig. 9.27(a and b) Front to rear wheel alignment procedure

6 Apply a slight tension to the front to rear alignment cord using the metal plate adjusters.

7 With all four wheels pointing in the straight ahead direction, read and record the left and right hand measuring gauge scales (Fig. 9.27 (a and b)). To determine the thrust axis deviation (TAD) angle subtract the left reading from the right reading and divide the difference of the reading by two.

Thrust axis deviation (TAD) angle

where R = Right hand measuring gauge reading L = Left hand measuring gauge reading

8 The lateral offset can be approximately determined from the formula

Lateral offset = Wheel base x tarn-

however, the makers of the equipment supply Table 9.2 to simplify the conversion from thrust axis deviation angle to lateral offset.

Example 1 (Fig. 9.27(b)) Determine the rear wheel toe-in or -out and the front to rear lateral offset for a 3000 mm wheelbase vehicle having a rigid rear axle and 13 inch diameter wheels from the following information:

Left rear wheel toe angle reading = 0' Right rear wheel toe angle reading = 0' Left side front to rear measurement reading — out (out = negative) = —30'

Right side front to rear measurement reading — in (in = positive) = +30'

Rear wheel combined toe angle = 0' + 0' = 0' Thus wheels are parallel.

b) Lateral offset:

Thrust axis deviation

From lateral offset Table 9.2, a thrust axis deviation of 30' for a wheel base of 3000 mm is equivalent to a lateral offset to the right of 22 mm when the vehicle moves in a forward direction.

Example 2 (Fig. 9.27(b)) Determine the rear wheel toe-in or -out and the front to rear lateral offset of a vehicle having independent rear suspension from the following data:

Wheelbase = 3400 mm

Wheel diameter = 13 inches

Left rear wheel toe angle reading — out (out = negative) = —40'

Right rear wheel toe angle reading — in (in = positive) = +15'

Left side front to rear measuring gauge reading — out = —55'

Right side front to rear measuring gauge reading — in = +25'

Rear wheel combined toe angle = (—40') + (+15') = 25'

From toe conversion table a toe angle of —25' for a 13 inch diameter wheel is equivalent to a toe-out of 2.65 mm.

b) Lateral offset:

Thrust axis deviation (TAD) angle

From lateral offset Table 9.2, a thrust axis deviation of 40' for a wheelbase of 3400 mm is equivalent to a lateral offset to the right of 33.5 mm when the vehicle is moving in the forward direction.

9.4.4 Six wheel vehicle with tandem rear axle steering geometry (Fig. 9.28) For any number of road wheels on a vehicle to achieve true rolling when cornering, all projected lines drawn through each wheel axis must intersect at one common point on the inside track, this being the instantaneous centre about which the vehicle travels. In the case of a tandem rear axle arrangement in which the axles are situated parallel to each

Table 9.2 Lateral offset tables

Lateral offset of front wheels In relation to rear wheels (Measurements In millimeters)

Table 9.2 Lateral offset tables

Lateral offset of front wheels In relation to rear wheels (Measurements In millimeters)

Wheelbase

10°

20°

30°

40°

50°

60°

mm

1800

4.0

7.5

11.5

15.0

19.0

23.0

2000

4.5

8.5

13.0

17.5

22.0

26.0

2200

5.0

10.0

15.0

20.0

24.5

30.0

2400

5.5

11.0

16.5

22.0

27.5

33.5

2600

6.0

12.0

18.5

24.5

30.5

37.0

2800

6.5

13.5

20.0

26.5

33.5

40.5

3000

7.0

14.5

22.0

29.0

36.5

44.0

3200

8.0

15.5

23.5

31.5

39.0

47.5

3400

8.5

17.0

25.0

33.5

42.0

51.0

3600

9.0

18.0

27.0

36.0

45.0

54.5

3800

9.5

19.0

28.5

38.5

48.0

58.0

(Measurements in inches)

Wheelbase

10°

20°

30°

40°

50°

60°

ft In

60

0.2

0.3

0.5

0.6

0.8

0.9

66

0.2

0.3

0.5

0.7

0.8

1.0

7 0

0.2

0.4

0.6

0.7

0.9

1.1

7 6

0.2

0.4

0.6

0.8

1.0

1.2

80

0.2

0.4

0.7

0.9

1.1

1.3

86

0.2

0.5

0.7

1.0

1.2

1.4

9 0

0.3

0.5

0.8

1.0

1.3

1.5

9 6

0.3

0.5

0.8

1.1

1.4

1.6

10 0

0.3

0.6

0.9

1.2

1.5

1.7

10 6

0.3

0.6

0.9

1.2

1.5

1.9

11 0

0.3

0.7

1.0

1.3

1.6

2.0

11 6

0.3

0.7

1.0

1.4

1.7

2.1

A negative (-) value Indicates front wheels offset to left A positive (+) value indicates front wheels offset to right

A negative (-) value Indicates front wheels offset to left A positive (+) value indicates front wheels offset to right other, lines projected through the axles would never meet and in theory true rolling cannot exist. However, an approximate instantaneous centre for the steered vehicle can be found by projecting a line mid-way and parallel between both rear axles, this being assumed to be the common axis of rotation (Fig. 9.30). Extended lines passing through both front wheel stub axles, if made to intersect at one point somewhere along the common projected single rear axle line, will then produce very near true rolling condition as predicted by the Ackermann principle.

Improvements in rear axle suspension design have introduced some degree of roll steer which minimizes tyre scrub on the tandem axle wheels.

This is achieved by the camber of the leaf springs supporting the rear axles changing as the body rolls so that both rear axles tend to skew in the plan view

Fig. 9.28 Six wheel tandem rear axle vehicle steering geometry

so that the imaginary extended lines drawn through both rear axles would eventually meet. Unfortunately lines drawn through the front steered stub axles and the rear skewed axles may not all meet at one point. Nevertheless, they may almost merge so that very near true rolling can occur for a large proportion of the steering angle when the vehicle is in motion. The remainder of the rear axle wheel misalignment is absorbed by suspension spring distortion, shackle joints or torque arm rubber joints, and tyre compliance or as undesirable tyre scrub.

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