Steering Geometry

Steering Geometry
 What is it?
 It is a geometry related to the front wheels & the steering linkages of a
vehicle.
 It is essential to know in order to solve all the problems related to
steering system.

 What are the Geometry?

• Camber
• King pin (OR) Steering axis inclination
• Included angle
• Toe In & Toe Out
• Castor
• Toe out on Turn
 Camber
 It is the tilting in (or) out of the front wheels from the true vertical position when the
wheels are viewed from the front of the vehicle

Wheel center line

True vertical line

Right front wheel of the vehicle

 Camber
 When the vehicle is not loaded, the
camber angle should be provided to
the front wheels & so that when the
vehicle is loaded, zero camber will
be set to the wheels & they will roll
straight (ie., along with the wheel
true vertical position)

 When the vehicle is not loaded:

• If the front wheels are tilted in, it
has a negative camber Right front wheel of the vehicle

• If the front wheels are tilted out, it Inside – Negative ; Out side - Positive
has a Positive camber
 Camber
 General:
 The camber angle should not exceed 2° and it will be set by the
manufacturer (it can also be adjusted).

 If there is a positive camber after the vehicle is loaded, it will cause the
out side of the tire tread to wear severely.

• If there is a Negative camber after the vehicle is loaded, it will cause the
inside of the tire tread to wear severely.
 Right front
wheel of the
vehicle

Right front wheel of the vehicle – viewed

from front of the vehicle

 When the front wheels are on ‘Bump’ (or) ‘Pot hole’:

The wheel may be tilted inward (or) outward respectively
- Negative (or) Positive camber
King Pin Inclination (or) Steering axis inclination

It is the angle between the king pin axis and the

‘True vertical line’ . It will be around 7° to 8°
Included (or) combined angle

1 – King Pin inclination angle

1
2 2 – Positive camber angle

The combination of 1 & 2, is termed as

Combined angle (or) Included angle

 The Included angle determines the point

of intersection of the tire center line
with the steering axis

 The point of intersection determines

whether the rolling wheel tends to Toe
In (or) Toe Out
 Toe In & Toe Out
 It is the amount (in degree) that the front wheels point inward (or) out ward when
they are viewed from top.
Inward – Toe In ; Outward – Toe Out
(Positive) (Negative)

Figure shows when

the vehicle is

moving in a straight

ahead position

Right front Right front

Left front
Left front
 When the vehicle takes a turn
- Outer wheel Toe In & Inner wheel Toe Out

Outer wheel
Inner wheel

Vehicle is taking a left turn

 Effect of Toe In & Toe Out

• The tire on a wheel that rolls with Toe In (or) Toe Out will wear rapidly

• The tire has to travel in the direction that the car is moving. But if the tire
has Toe In (or) Toe Out, it is dragged sideways as it rolls forward, leading
to wearing of the tires.
 Effect of combined angle variation

a b c

The point of intersection of the wheel center line & king pin axis
a) Above the ground – tendency to Toe In - Poor steering control
b) Just at the ground - no effect - Good steering control - Centre Point steering
c) Below the ground - tendency to Toe out - Hard steering
 Toe Out on turn
 The ‘Toe out angle’ Provided for the inner wheel is 1 degree lesser than the ‘Toe in
angle’ provided for the outer wheel.
The ‘Toe out angle’ is termed as ‘Toe out on Turn’

Inner wheel Outer wheel

When the vehicle takes a turn

- Outer wheel Toe In & Inner wheel Toe Out
 Toe Out on turn
• When a vehicle takes a turn, the inner wheel turns lesser radial distance than the outer
wheel with respect to the instantaneous center.

• In order to turn the lesser radius with respect to the instantaneous center, the ‘Toe out
angle’ of the inner wheel is maintained slightly lesser than the ‘Toe in angle’ of the
outer wheel at the time of turning.

• The above angle will be set for a vehicle at the time of doing the ‘wheel alignment’ in a
work shop.

• If the angle is not properly set, the turning of inner wheel can not be with respect to
the Instantaneous center.
Scrub Radius:
It is the distance between 2 imaginary points on the
road surface - the point where the wheel center line
contact the road surface & the point where the
King pin axis contacts the road surface

The effect of the ‘Scrub Radius’ is the same as the

‘Effect of combined angle variation’.
 Castor
It is the angle formed by the forward (or) rear ward tilt of the king pin axis (KPA) from vertical with
respect to the direction of motion of the wheel when the front wheels are viewed from side.
[It is normally around 2°].

 The angle is negative when

the KPA falls back of the
wheel centre line when the
wheel moves forward -
Negative Castor
 The angle is positive when
the KPA fall in front of the

 When the KPA coincides with the vertical, then wheel centre line when the

there will be ‘ Zero castor’ and it will lead the wheel moves forward -

wheel not to move freely forward Positive Castor

 Castor
Castor in By cycle

Castor in Furniture
 Reasons for having the Castor

 To maintain the directional stability & control of the vehicle

 To increase the steering returnability

 It is the tendency of the steering wheel to return to its center position after
making a turn

 Positive Castor increases steering wheel returnability

 To reduce the steering effort

Positive Castor - To maintain the directional stability & control of the vehicle

 Positive Castor aids the directional stability, because the wheel is trailing behind the
King Pin Axis (ie., the car front wheel is pulled by the KPA).

 The positive castor tends to keep the wheels pointed straight ahead and it helps to
overcome any tendency for the wheel to wander from the straight ahead position.

Negative castor does not aid the directional stability