The following forces act on vehicle in motion:

(1) Aerodynamic drag:.

• Aerodynamic means the behavior of the air motion relative to the car body. Aerodynamic drag is also called as air resistance.
• Air drag force acts in the direction of vehicle motion.
• The total aerodynamic drag of a vehicle include many factors such as profile drag (57%), induced drag (8%), skin friction (10%), interference drag (15%) and cooling and ventilation drag (10%).
• Stream line of air flow around the vehicle should be continuous and separation of the boundary layer with its attendant vertices should be avoided. Skin drag coefficient should be decrease by smooth and well-polished of body surface.
• The accessories such as mirror, door handle aerials and badges which project outward from normal surface of body produce interference drag and projection below the vehicle such as axle, propeller shaft, tow bar also contribute interference drag hence such projection should be avoided.

(2) Aerodynamic lift:

• It is the vertical component of the resultant force caused by the pressure distribution on the vehicle body.
• The aerodynamic lift and pitching moment are undesirable effects. The aerodynamic lift tends to reduce the pressure between the tyre and the ground. This causes the loss of steering on the front axle and loss of traction on the rear axle.
• Pitching causes rear wheel lift off the ground and reduces available traction. It is the rocking chair or rotating action about the transverse axis through the vehicle parallel to ground. Due to pitching, the front suspension moves out of phase with the rear resulting in rocking effect in a vehicle.

(3) Side force:

• The imbalance of the wheel due to centrifugal force acts on the vehicle during turning which produces a side thrust.
• To sustain that force, the plane of the wheel makes some angle with the direction of motion of the vehicle. This is achieved by the direction of tyre which is flexible.
• The angle form during taking turn to sustain the side thrust is known as slip angle and the force produce to counter acts the side thrust is called as cornering force.

(4) Yawing movement (Bouncing):

• It is vertical movement of the complete body .When complete body of vehicle goes up and down which is known as bounce or bouncing. Depending upon the movement of front end or rear end the bounce is known as front end bounce or rear end bounce.

(5) Rolling movement:

• It is the movement of a vehicle about its longitudinal axis produced due to centrifugal force act during cornering.
• The retarding and cornering forces are applied at road levels but the centre of gravity of a vehicle is at a certain height. During cornering, a turning couple is produced about the longitudinal axis of the vehicle owing to centrifugal force acting at centre of gravity and forces acting at the point of contact of road and tyre patch. This results in a motion known as rolling.
• A combination of rolling and pitching is called diagonal pitch.

(6) Remedies:

• In order to control all the above suspension movements; anti roller bar, stabilizer, pitch and roll control bars, hydraulic systems, mechanical levelling devices etc. are provided to vehicles.

The body shape & size of the vehicle must have acceptable aerodynamic characteristics.There are three forces acting on a vehicle.

i) Air drag force acting in a direction of vehicle motion. ii) Aerodynamic lift acting vertically upward. iii) Cross wired force acting in lateral direction. The aerodynamic aspects will be more expensive but lowering aerodynamic drag the fuel economy of the fastest vehicle can be improved.

a) Perodynamic drag Perodynamic means the behaviour of the air motion relating to car body. Perodynamic drag is also called air resistance. Total aerodynamic drag of a vehicle include many factors such as profile drag(57%), skin friction(10%), interference drag(15%) & cooling and ventilation drag (10%).

Total aerodynamic drag, $P_x=\frac{1}{2}\, C_x pV^2A $

Where
Cx= dimensional drag coefficient
P= air density
V= Vebsity of car
A = Projected area of vehicle from the front
So, from the above equation it is necessary to select a careful profile of size of car to achieve low profile drag coefficient.

b) Aerodynamic lift : It is the vertical component of the resultant force caused by the pressure distribution on the vehicle body

c) Side force: The imbalance of the wheel due to centrifugal force acts on the vehicle during turning which produce a side thrust. To sustain that force the plane of wheel makes some angle with the direction of motion of vehicle. This is achieved by distortion of tyre which is flexible

d) Yowing movement: It is vertical moment of complete body. When complete body of vehicle goes up & down which is known as bounce & bouncing. Depending upon the movement of front end or rear end the bounce is known as front end bounce or rear end bounce.

e) Rolling movement It is the movement of a vehicle about its longitudinal axis produced, due to centrifugal fee acting dusting cornering.