i) Camber: It is the angle of inclination of front wheel tyres with respect to the vertical. Camber provided may be positive or negative, Camber is also called as ‘wheel rake', (Fig 17.1).
When the wheel is tilted outward at the top comber is positive. Because of positive camber, the rolling radius at different point of the tyre tread is different result of which the tyre tends to roll like truncated cone about the centre of rotation, so tyre will wear more on outside.
If the inclination of the centre line with the vertical is inward then it is called negative camber. Negative Camber will causes more on inside . Initial positive camber is provided to the wheels so that when the vehicle is loaded they automatically come to vertical position.
Tyre life will be maximum when camber angle is zero. Camber should not be exceed 2°. Excessive camber causes improper contact of the wheels.
ii) King pin inclination:
The angle between the vertical line & centre of the king pin or steering axle when viewed from the front of the vehicle is known as King pin inclination or steering axis inclination.
The King pin inclination ranges from 3.5° t 8.5° & its average value is 5°. It reduces tyre wear & helps to provide steering stability. It also reduces steering effort particularly when the vehicle is stationary.
iii) Caster:
The angle between the King pin centre line & vertical in the plane of the wheel is called Caster angle. Caster is the slant of steering axis as viewed from the side of vehicle.
Caster is positive, when the top of the steering axis
to the vehicle & negative when it leans to the front. When the caster is positive, the projected intersection of the steering
axis ss ahead of the tyre contact point. The steering axis intersection point is called leading point & the tyre ground contact point is called trailing point. The postive caster is to provide directional stability. The greater the positive caster, greater is the stabilizing force. Amount of caster will be 2° to 2.5° with tolerance range of $\pm$ 1°.
iv) Toe-in & Toe-out :
Toe - Front wheels are angled towards one another so that they are closer together at the front than at the back when viewed from top.
Toe-in is the difference between the center to centre distance of the front & rear ends, ie. it is the amount by which the front wheels are set closer together at the front than at the rear when the vehicle is stationary. (Fig:17.5)
Toe- out is the amount by which the wheel may be set closer at the rear than the front when the vehicle is stationary. (fig:17.6)
The purpose of toe in is to neutralize the cone rolling effect of the front caused by camber angle. Therefore, the amount of toe in for any vehicle is primarily determined by the amount of camber angle. Normally in modern vehicle toe in usually 2 to 4 mm.
v) Combined angle :
Combined angle or inclined angle is the angle formed in the vertical plane between the wheel centre line & King pin centre line ( steering axis). Combined angle is equal to camber plus king pin inclination (or steering axis inclination).
Combined angle may be 9° to 10°.
The distance between the centre of tyre patch & intersection of the King pin or steering axis with the ground is called the king pin offset or scrub radius. Scrub radius should be upto abort 12 mm.
i) if the scrub radius is negative, then the wheel tends to toe-in.
ii) If the scrub radius is positive, then the wheel tends to be toe-out
iii) If the scrub radius is zero, wheel keep it straight.
vi) Under - steering & Over-steering :
While taking turn, the centrifugal force acts on the vehicle which produces side thrust. To sustain that force, the plane of the wheel must make some angle with the direction of motion of vehicle. This is achieved by the distortion of tyre tread.
The angle between the wheel inclination & the path taken by the wheel is known as slip angle. This value of slip angle depends upon amount of side force, the flexibility of tyre load carried by wheel & other factors such as camber angle & road surface.
When the slip angle is greater at the rear wheel than the front, the vehicle tends to oversteer that is to turn in the curve more than the driver intended.
When the slip angle is smaller at the rear than the front, the vehicle tends to understeer.
The understeer is opposite to oversteer & is preferable because correction by the driver involves rotating the steering wheel a little more in the direction of turn.
The oversteer reduces driver effort during cornering & has advantages whíle driving along roads having many twists & turns.
The understeer gives driving stability especially when there are strong side winds. It is better for fast driving on long straight roads.
When the slip angle at front & rear wheel is same, there is no tendency of the vehicle to move away from the path of neutral straight line. This is called neutral steer.
vii) Cornering Power:-
The Centrifugal force acts on the vehicle during turning which produce a side thrust. To sustain that force the plane, of the wheel makes some angles with the directions of the motion of the vehicle. This is achieved by the distortion of tyre which is flexible.
The angle form during taking turn to sustain the side thrust is known as slip angle & the force produce to counteracts the side thrust is called as Cornering force.
Value of slip angle depends upon the amount of side force, the flexibility of tyre, load carrying wheel & other several factors like camber angle & the condition of road surface.
At low speed & less sharp curve, the value of slip angle is small but it increases at high speed & on sharp turns, because of this excessive speed & the sharpness of the curve wheel skids sideways.
For the same slip angle, positive camber increases the cornering force & decreases negative camber.
The alternation of cornering force is due to camber angle which commonly known as camber force. The ratio of side force sustain to the slip angle is called as cornering power.
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