Front Wheels Alignment
Front Wheels Alignment
27.2.1.
Need for Front Wheel Alignment
For free movement of the road-wheels with the least of effort, opposite wheels must be approximately parallel to each other when the vehicle is in motion along a straight path (Fig. 27.8). Each wheel has a tendency to negotiate a path perpendicular to its own axis of rotation. Therefore, if the front wheels are aligned for converging towards the front, then during movement in the forward direction both wheels try to roll close together. On the other hand, if the wheels are aligned for diverging towards the front, the wheels try to roll farther apart. Therefore due to free rolling tendency the average path followed by both wheels have a continuous tendency to either push together or pull apart. Consequently, while rolling forward, each wheel simultaneously tends to slip laterally, so a continuous cross tread scrub action is established, resulting in excessive tread wear, heavy steering and probably leading to poor fuel consumption
Fig. 27.8. Effects of wheel misalignment. A. Forward direction, wheels converging. B. Forward direction, wheels diverging.
A tyre tread if subjected to excessive lateral slip or scrub, a diagonal wear pattern is observed, with the tread blocks heavily worn on the leading side and an extruded feather like appearance on the trailing side. Figure 27.8A illustrates the inward feathering of the tread blocks of converging wheels. If the hand is moved inward over the tread then smoothness is felt. Figure 27.8B illustrates the usual outward feathering of the tread blocks and grooves of diverging wheels. An inward movement of the hand over the tread feels a slight roughness. For a correctly aligned and parallel-rolling wheels, an even wear pattern across the tyre tread is produced so that inward and outward movements of the hand across the thread experience smoothness.
A tyre tread if subjected to excessive lateral slip or scrub, a diagonal wear pattern is observed, with the tread blocks heavily worn on the leading side and an extruded feather like appearance on the trailing side. Figure 27.8A illustrates the inward feathering of the tread blocks of converging wheels. If the hand is moved inward over the tread then smoothness is felt. Figure 27.8B illustrates the usual outward feathering of the tread blocks and grooves of diverging wheels. An inward movement of the hand over the tread feels a slight roughness. For a correctly aligned and parallel-rolling wheels, an even wear pattern across the tyre tread is produced so that inward and outward movements of the hand across the thread experience smoothness.
Centre-point Steering
Figure 27.9 illustrates a vertical wheel and king pin arrangement, which has the following disadvantages:
(a) Large splaying-out effect of the wheel takes place.
The wheels are pushed by the force, F, which is opposed by the resistance, R. These two forces cause a couple, Fx, whose magnitude becomes very large when the front brakes are applied.
(6) Steering becomes heavy because of the distance between king pin and wheel centre. The wheel moves in an arc of radius, x, around the pin.
(c) Large bending stress occurs on the stub axle and king pin.
To overcome these problems, the wheel and king pin should have minimum possible offset distance, x. When the offset is eliminated, the centre line of the wheel meets the centre line of the king pin at the road surface. The condition is called centre point steering, which can be obtained through (a) camber (b) swivel-axis inclination or (c) dished wheels. Although the centre point steering appears to be ideal, but the spread effect of the pneumatic tyre causes the wheel to scrub and produce hard steering and tyre wear.
(a) Large splaying-out effect of the wheel takes place.
The wheels are pushed by the force, F, which is opposed by the resistance, R. These two forces cause a couple, Fx, whose magnitude becomes very large when the front brakes are applied.
(6) Steering becomes heavy because of the distance between king pin and wheel centre. The wheel moves in an arc of radius, x, around the pin.
(c) Large bending stress occurs on the stub axle and king pin.
To overcome these problems, the wheel and king pin should have minimum possible offset distance, x. When the offset is eliminated, the centre line of the wheel meets the centre line of the king pin at the road surface. The condition is called centre point steering, which can be obtained through (a) camber (b) swivel-axis inclination or (c) dished wheels. Although the centre point steering appears to be ideal, but the spread effect of the pneumatic tyre causes the wheel to scrub and produce hard steering and tyre wear.
Camber.
The down-set of the stub axle tilts the wheel outwards at the top, forming an angle between the vertical and the wheel, which is termed the camber angle. The outward inclination is called positive camber and the inward inclination is the negative camber. The camber prevents the top of the wheels from tilting inward much due to excessive load or plays in the king-pin and wheel bearings. Camber on any automobile is the result of the front end geometry. Incorrect camber usually is noticed due to abnormal tyre wear along one half of the tread. Camber is adjustable by charging the position of one end of the steering knuckle.
Although the bending stress on the stub axle and the splaying out couple are reduced, but the different rolling radius of the tyre produce a cone effect, which gives rise to tyre wear and another splaying out action. Equal camber angles are provided to balance this action; otherwise the vehicle pulls to the side of greater camber. However unequal tyre pressures can also produce this effect.
Some independent suspension systems very the camber angle as the suspension spring deflects. This variation is from the positive camber shown in Fig. 27.10 to the negative camber, which is the inner tilt of the wheel. Suspension linkages alter the chamber angle when the suspension goes to jounce or rebound. Some suspensions have negative camber during both
Although the bending stress on the stub axle and the splaying out couple are reduced, but the different rolling radius of the tyre produce a cone effect, which gives rise to tyre wear and another splaying out action. Equal camber angles are provided to balance this action; otherwise the vehicle pulls to the side of greater camber. However unequal tyre pressures can also produce this effect.
Some independent suspension systems very the camber angle as the suspension spring deflects. This variation is from the positive camber shown in Fig. 27.10 to the negative camber, which is the inner tilt of the wheel. Suspension linkages alter the chamber angle when the suspension goes to jounce or rebound. Some suspensions have negative camber during both
" Fig. 27.9.
A vertical wheel and king pin arrangement having
disadvantages.
jounce and rebound so that the tyre tread centre line follows a straight path down the load. Other suspensions have positive camber during jounce and negative camber during rebound. This keeps the tyre tread surface flat on the road whejn going over bumps and dips. During a turn, high Negative camber on the outside tyre helps provide cornering power, as the tyre deflects under the high side loads due to centrifugal force on the automobile. This high negative camber also causes rapid tyre wear. The overall suspension camber is a compromise of these factors to provide the desired handling characteristics.
A vertical wheel and king pin arrangement having
disadvantages.
jounce and rebound so that the tyre tread centre line follows a straight path down the load. Other suspensions have positive camber during jounce and negative camber during rebound. This keeps the tyre tread surface flat on the road whejn going over bumps and dips. During a turn, high Negative camber on the outside tyre helps provide cornering power, as the tyre deflects under the high side loads due to centrifugal force on the automobile. This high negative camber also causes rapid tyre wear. The overall suspension camber is a compromise of these factors to provide the desired handling characteristics.
Fig. 27.10. Wheel camber (positive).
Since camber is not desirable from the consideration of a tyre wear, the angle seldom exceeds 2 degrees. This is sufficient to provide a slight outward-thrusting torque to allow road camber and slight deflections of the beam or suspension members. Excessive camber gives rise to improper contact of the wheels.
Since camber is not desirable from the consideration of a tyre wear, the angle seldom exceeds 2 degrees. This is sufficient to provide a slight outward-thrusting torque to allow road camber and slight deflections of the beam or suspension members. Excessive camber gives rise to improper contact of the wheels.
Fig. 27.11. Swivel axis inclination.
King Pin Inclination or Swivel-axis Inclination.
If the king pin is fitted outwards at the bottom, an angle between the king pin centre line and the vertical is produced (Fig. 27.11), which is known as king pin inclination (KPI) or swivel
axis inclination (SAI). Normally a king pin inclination of between 5 to 10 degrees is provided to obtain the required offset. The larger angles are necessary for a layout when the wheel is moved away from the king pin to accommodate brakes, bearings, etc.
When the wheel is turned, it moves in the plane AA such that the front of the vehicle is lifted, producing a self centring action. With vertical king pins, a simple yoke-and-pin type of steering joint can be incorporated at each end of the track rod. An inclined king pin allows the joint to move in the plane BB, upwards at one end of the track rod and downwards at the other. A ball joint is used to each end of the rod to permit this motion.
King-pin inclination is also called as steering axis inclination and king-pin or ball joint centre line is called steering axis centre line. The king-pin-centre line, if extended, contacts the road surface near the tyre tread centre line. The distance between the points of intersection of these two centre lines on the road surface is called scrub radius. A small scrub radius minimises steering wheel shock from road irregularities and reduce steering effort, and hence is desirable. The scrub radius is designed into the knuckle and spindle. If knuckle is damaged or bent, the scrub radius becomes incorrect. This damage of the knuckle can be detected by measuring both camber and steering axis inclination.
If the steering axis centreline contacts the road surface inside the tyre tread centreline, the scrub radius is positive. If it contacts the road surface outside the tyre centre line, the scrub radius is negative. The effect of unequal brake force on the front wheels increases when the scrub radius is large. This pulls the vehicles side-ways as the brakes are applied causing poor vehicle control. The wheel with high braking force pulls outward on a suspension with a positive scrub radius. Also a vehicle with positive scrub radius tends to pull outward or towards the side with an under inflated tyre.
Steering axis inclination positions the automobile body as low as possible when the wheels are in straight ahead position. As the wheels are turned in either direction, the tip of the spindle tries to move downward, but it cannot happen practically as the spindle is in the centre of the
wheel and the wheel is in contact with the ground. The spindle, therefore, pivots in the ball joint and tries to lift the vehicle. The weight of the automobile produces a strong tendency to turn the wheels to a straight ahead position after the turn has been completed. This force is sufficient to provide excellent directional stability so that the vehicle has a natural tendency to run straight ahead. The steering axis inclination angle is selected as a compromise between steering effort, neutral returning force and wheel pull sensitivity.
The inclination of the swivel axis or the tilting of the king pin can be obtained with the help of one of the following methods.
(i) Upper independent suspension members are mounted slightly to the rear of the lower members.
(ii) The axle beam of a light truck is tilted by
(a) fitting wedges between axle and spring,
(b) mounting the axle towards the front of a laminated spring, and
(c) inclining the laminated spring.
axis inclination (SAI). Normally a king pin inclination of between 5 to 10 degrees is provided to obtain the required offset. The larger angles are necessary for a layout when the wheel is moved away from the king pin to accommodate brakes, bearings, etc.
When the wheel is turned, it moves in the plane AA such that the front of the vehicle is lifted, producing a self centring action. With vertical king pins, a simple yoke-and-pin type of steering joint can be incorporated at each end of the track rod. An inclined king pin allows the joint to move in the plane BB, upwards at one end of the track rod and downwards at the other. A ball joint is used to each end of the rod to permit this motion.
King-pin inclination is also called as steering axis inclination and king-pin or ball joint centre line is called steering axis centre line. The king-pin-centre line, if extended, contacts the road surface near the tyre tread centre line. The distance between the points of intersection of these two centre lines on the road surface is called scrub radius. A small scrub radius minimises steering wheel shock from road irregularities and reduce steering effort, and hence is desirable. The scrub radius is designed into the knuckle and spindle. If knuckle is damaged or bent, the scrub radius becomes incorrect. This damage of the knuckle can be detected by measuring both camber and steering axis inclination.
If the steering axis centreline contacts the road surface inside the tyre tread centreline, the scrub radius is positive. If it contacts the road surface outside the tyre centre line, the scrub radius is negative. The effect of unequal brake force on the front wheels increases when the scrub radius is large. This pulls the vehicles side-ways as the brakes are applied causing poor vehicle control. The wheel with high braking force pulls outward on a suspension with a positive scrub radius. Also a vehicle with positive scrub radius tends to pull outward or towards the side with an under inflated tyre.
Steering axis inclination positions the automobile body as low as possible when the wheels are in straight ahead position. As the wheels are turned in either direction, the tip of the spindle tries to move downward, but it cannot happen practically as the spindle is in the centre of the
wheel and the wheel is in contact with the ground. The spindle, therefore, pivots in the ball joint and tries to lift the vehicle. The weight of the automobile produces a strong tendency to turn the wheels to a straight ahead position after the turn has been completed. This force is sufficient to provide excellent directional stability so that the vehicle has a natural tendency to run straight ahead. The steering axis inclination angle is selected as a compromise between steering effort, neutral returning force and wheel pull sensitivity.
The inclination of the swivel axis or the tilting of the king pin can be obtained with the help of one of the following methods.
(i) Upper independent suspension members are mounted slightly to the rear of the lower members.
(ii) The axle beam of a light truck is tilted by
(a) fitting wedges between axle and spring,
(b) mounting the axle towards the front of a laminated spring, and
(c) inclining the laminated spring.
Fig. 27.12. Dished wheel.
By slightly dishing the wheel (Fig. 27.12), the amount of camber and king pin inclination can be reduced. The light pressed-steel wheel must not be excessively dished, as it decreases the strength.
By slightly dishing the wheel (Fig. 27.12), the amount of camber and king pin inclination can be reduced. The light pressed-steel wheel must not be excessively dished, as it decreases the strength.
Negative Offset (Negative Scrub Radius).
In the earlier days positive offset was used which is obtained when the centre line of the wheel meets the swivel axis at a point just below the road. The offset distance, measured at the road surface between the two centre lines, should be equal to balance the inward or outward pull at both the wheels. If one front tyre deflates, the positive offset on that side increases. This causes the vehicle to pull severely to that side so that is becomes difficult for the driver to maintain control especially during application of the brake (Fig. 27.13). Consequently, to improve road safety negative offset is provided by the manufacturer. In this case the swivel axis is inclined more than the earlier in order to obtain an interaction point well above the road surface. With this layout the effect of tyre deflection shortens the offset. The rolling resistance, however, increases, but the shorter offset robs the deflated tyre of leverage to change the direction of the vehicle.
Fig. 27.13. Positive offset.
The negative offset also provides an additional safety when the front brakes are unbalanced due to poor adhesion of one wheel or failure of one front brake. Under these adverse conditions the vehicle can still be brought to rest in a straight line. Figure 27.14 illustrates when the right hand front brake fails, the geometry causes the braking action of the left hand wheel to steer the wheels to the right to compensate the loss of brake drag on the right hand side of the vehicle. Also the negative offset layout is specifically suitable for a diagonally connected split line braking system
The negative offset also provides an additional safety when the front brakes are unbalanced due to poor adhesion of one wheel or failure of one front brake. Under these adverse conditions the vehicle can still be brought to rest in a straight line. Figure 27.14 illustrates when the right hand front brake fails, the geometry causes the braking action of the left hand wheel to steer the wheels to the right to compensate the loss of brake drag on the right hand side of the vehicle. Also the negative offset layout is specifically suitable for a diagonally connected split line braking system
Fig. 27.14. Negative offset
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