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Wheel Alignment and Balancing

What is wheel alignment and balancing of an automobile? Why alignment and balancing is required and what is its importance?

Wheel Alignment

The wheel alignment relates to the relative position of the wheels with respect to the wheel attaching parts, and the ground. The proper wheel alignment reduces steering effort, provides directional stability and control, reduces tyre side slip and wear. The important alignment factors are as follows:

1. Camber

It is the inward or outward tilt of a wheel from the vertical when viewed from the front of the vehicle, as shown in figure (a) below. A wheel that tilts outward from the top has a positive camber and if it tilts inwards, then it has a negative camber. The camber is measured in degrees and can be adjusted on many vehicles. Generally, positive camber is provided on vehicles to compensate for the wheels tilting inwards due to the weight of the vehicle. The positive camber also places the centerline of the wheel closer to the steering axis. The distance between the wheel centerline and the steering axis centerline at the point where they intersect the road surface is called camber offset. The smaller the offset, the smaller is the effort required to steer the vehicle. But, excessive camber will, however cause uneven tyre wear and loss of traction.

2. Kingpin inclination (Steering axis inclination)

The kingpin (ball joint centerline) inclination is the inward tilt of the kingpin from the vertical as viewed from the front of the vehicle, as shown in Figure (b). Sometimes, an extended line drawn through the king pin inclination intersects the ground at a point 0 different from the center of the tyre's ground contact M. The difference in these two points is called the scrub radius.

This helps in return of the wheels to the straight-ahead position after a turn has been made. This also helps to reduce camber offset in a way that it brings the wheel centerline and the steering axis centerline closer together where they intersect the road surface, which has the corresponding effect of reducing the effort required to steer the vehicle.

Wheel Alignment and Balancing

3. Caster

It is the tilt of the kingpin towards the front or the rear of the vehicle, as shown in Figure (c). If the tilt is towards the rear, the wheel has a positive caster and if it tilts towards front, then it has a negative caster.

This positive caster generates a steering-aligning torque. It tends to force the wheels to travel in a straight-ahead position, and it also assists in return of the wheel to the straight-ahead position after a turn has been made, thus maintaining directional stability.

4. Toe-in

The front of the wheels is drawn inwards such as that the distance between the front ends is slightly less than the distance between the back ends. In Figure (d), the distance 'A' is less than the distance ‘B’. The difference in these distances is called toe-in and is measured in millimeters. In the reverse case, when the front of the wheels are angled outwards, the condition is called toe-out,

The purpose of toe-in is to ensure parallel rolling of wheels, to stabilize steering, and to prevent both side-slipping and excessive wear of the tyres.

Wheel Balance

The weight of the wheel (wheel rim & tyre assembly) should be distributed evenly to create the balanced condition. If the weight is distributed unevenly, centrifugal forces cause the wheel to vibrate as it turns. A slight imbalance causes vibrations on the steering wheel. More serious imbalance causes complete body vibrations. There are two types of wheel balance:

1. Static wheel balance

The static balance is the equal distribution of weight around the wheel. A statically balanced wheel does not rotate by itself regardless of the position in which it is placed on its axis. On the other hand, a statically imbalanced (off axis) wheel tends to rotate by itself until the heaviest portion reaches the bottom. During vehicle operation, static imbalance causes a bouncing action that leads to uneven tyre wear.

2. Dynamic wheel balance

The dynamic balance is the equal distribution of weight on each side of the wheel's centerline (as seen from the front or rear) During vehicle operation, a dynamically imbalanced (off-center) wheel experience side-to-side deflection, leading to vibration in the steering wheel or body and to bald spots on the tyre.

Notes:

  1. In order to correct static imbalance, a weight is attached to the wheel directly opposite to the heaviest spot.
  2. In order to correct dynamic imbalance, equal weights are placed 180 degrees apart from each other, one on the inside of the wheel and one on the outside, at the point of imbalance.

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