WHEEL ALIGNMENT, Toe-in and Toe-out

Toe-in and Toe-out

The right and left front wheel are not parallel but the front portions are some what closer together than the rear portions. This condition is called toe-in. Conversely, if the front portions are wider apart the rear portions, it is known as toe-out. Toe-in refers to the difference in the distance between the front and rear centres of the front tyres.  The difference is generally 2-5 mm.

Referring to the figure below, the front wheels are provided with camber so they tend to roll outward, along dotted line b. However, since the wheels are installed to the automobile by the upper arms, without toe-in, they would roll in the direction of the vehicle movement, along line a, causing road surface slip, tyre damage and unstable steering. Toe-in is provided to prevent these flaws so that the wheels then tend to roll inward, along dotted line c, thereby counteracting the outward roll and instead, roll straight without slipping.

Role of Toe angle : The main function of toe angle is to cancel out the camber thrust generated when camber is applied. When the front wheels are given positive camber, they tilt outward at the top. This causes them to attempt to roll outward as the car moves forward, and, therefore, to side-slip. This subjects the tyres to wear.

 When an automobile turns a corner, the camber thrust on the outside tyres 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. Some vehicle models take advantage of this effect and add a slightly negative camber for straight-ahead driving so that positive camber will be reduced during turning, reducing the camber thrust and yielding sufficient cornering force for the turn

Therefore, toe-in is provided for the front wheels to prevent this by cancelling outward rolling due to camber. Since camber approaches Zero in most recent vehicles, the toe angle value is also becoming smaller (some vehicles have zero toe angle).

Reference:

Type of Tyres and Toe angle. The two angle given to bias-ply tyres differs from that given to radial-ply tyres, even when the camber is the same. The reason is that, since the tread and shoulders of a bias-ply tyre are subject to greater deformation than those of a radial-ply tyre, the former type generates greater camber thrust. Therefore, bias-ply tyres are given more toe angle than raidal-ply tyres.

Suspension rigidity and Toe angle. During driving, forces from various directions are brought to bear on the suspension, with the result that the wheels tend to toe out-slight toe-in even when the camber is zero.

References

1. Automobile Engineering (Vol. 1 & 2)            -           K.M.Guptha
2. Automotive Mechanics                                   -           Joseph Heitner
3. Automobile Engineering                                 -           Harbans Singh Reyd
4. Automotive Mechanics                                   -           William H. Course