1. VEHICLE DYNAMICS
CHAPTER 2 : ROAD LOADS

2. 2.1 VEHICLE AERODYNAMICS
 The study of the aerodynamics of road vehicles.
 Its main goals are reducing drag and wind noise,
minimizing noise emission, and preventing undesired lift
forces.
 Its also important to produce down-force to improve
traction and thus cornering abilities.
 An aerodynamic automobile will integrate the wheel arcs
and lights into the overall shape to reduce drag.

4. 2.2 MECHANICS OF AIR FLOW AROUND A
VEHICLE
 In fluid mechanical terms, road vehicles are bluff bodies
in very close proximity to the ground.
 Internal and recessed cavities which communicate freely
with the external flow and rotating wheels add to their
geometrical and fluid mechanical complexity.
 The objectives of improvement of flow past vehicle bodies
are to reduce of fuel consumption and improve the driving
characteristics.

5.  Vehicle aerodynamics includes three interacting flow
fields which are:
a. flow past vehicle body
b. flow past vehicle components (wheels, heat
exchanger, brakes or windshield)
c. flow in passenger compartment.

6. 2.3 PRESSURE DISTRIBUTION ON A VEHICLE
 The pressure distribution over the body surface exerts
normal forces which, summed and projected into the free-
stream direction.
 Combination of shock wave effects, vortex system
generation effects and wake viscous mechanisms
 When the viscosity effect over the pressure distribution is
considered separately, the remaining drag force is called
pressure (or form) drag.
 In the absence of viscosity, the pressure forces on the
vehicle cancel each other and, hence, the drag is zero.

8. 2.4 AERODYNAMIC FORCES
 The force exerted on a body whenever there is a relative
velocity between the body and the air.
 There are only two basic sources of aerodynamic force:
a. the pressure distribution
b. the frictional shear stress distribution exerted by the
airflow on the body surface.
 The distribution of pressure and shear stress represent a
distributed load over the surface.

9. 2.5 DRAG COMPONENTS
 Drag refers to forces acting opposite to the relative motion
of any substance moving in a fluid.
 Lift acts perpendicular to the motion.
 Drag sources - the skin friction between the molecules of
the air and the solid surface of the aircraft.
 The skin friction is an interaction between a solid and a
gas, so the magnitude of the skin friction depends on
properties of both solid and gas.

10.  The additional drag components caused by the generation
of lift is induced drag.
 Occurs because the flow near the wing tips is distorted
span wise as a result of the pressure difference from the
top to the bottom of the wing.
 Called induced drag because it has been "induced" by the
action of the tip vortices.
 It is also called "drag due to lift" because it only occurs on
finite, lifting wings.

12. 2.6 AERODYNAMICS AIDS

13. 2.6.1 BUMPER SPOILER
 A motor vehicle bumper which comprises a shield and a
spoiler hinged between two positions of stable equilibrium
 Namely a high position in which at least a part of the
spoiler projects from the shield and,
 a low position in which the spoiler extends the shield
downwards.
 The spoiler is suitable for adopting a third position of
stable equilibrium, in which it is completely retracted
behind the shield.

15. 2.6.2 AIR DAMS
 A front spoiler (air dam) is positioned under or integrated
with the front bumper.
 In racing, this spoiler is used to control the dynamics of
handling related to the air in front of the vehicle.
 To improve the drag coefficient of the body of the vehicle
at speed, or to generate down-force.
 In passenger vehicles, the focus shifts more to directing
the airflow into the engine bay for cooling purposes.
 Air dam will keep the nose steady and pointed at the
ground in high speed driving.

17. 2.6.3 DECK LID SPOILERS
 The trunk lid or boot lid is the cover allows access to the
main storage or luggage compartment.
 A spoiler’s function is to 'spoil' unfavorable air movement
across a body of a vehicle in motion.
 Spoilers are used primarily on sedan-type race cars.
 They act like barriers to air flow, in order to build up
higher air pressure in front of the spoiler.

19. 2.6.4 WINDOW AND PILLAR TREATMENTS
 Pillars are the vertical or near vertical supports of an
automobile's window area or greenhouse-designated.
 Which is A, B, C or D-pillar moving in profile view from
the front to rear.
 Pillars are implied, whether they exist or not; where a
design's greenhouse features a break between windows or
doors without vertical support at that position
 The non-existent pillar is "skipped" when naming the
other pillars.

21. 2.6.5 OPTIMIZATION
 A mass reduction enable fuel economy improvement.
 Various global optimization methods use to solve drag
reduction problems in the automotive industry.
 The genetic algorithms (GA) has the major advantage to
seek for a global minimum.
 But this method is very time consuming because of the
large number of cost function evaluations that are needed.
 All the hybrid optimization methods have greatly reducing
the time cost by coupling a GA.

22. 2.7 DETERMINE AND ESTIMATE DRAG
COMPONENTS

23. 2.7.1 AIR DENSITY
 In simple terms, density is the mass of anything -
including air - divided by the volume it occupies.
 Lift and drag vary directly with the density of the air-as air
density increases, lift and drag increase.
 As air density decreases, lift and drag decrease.
 Air density is affected by pressure, temperature and
humidity.

24. 2.7.2 DRAG COEFFICIENT
 A common measure in automotive design as it pertains to
aerodynamics.
 The drag coefficient of an automobile impact the way the
automobile passes through the surrounding air.
 Aerodynamic drag increases with the square of speed;
therefore it becomes critically important at higher speeds.
 Reduce drag coefficient to improves the performance of
the vehicle as it pertains to speed and fuel efficiency.

25. 2.8 SIDE FORCE
 Cornering force is the lateral force produced by a vehicle
tire during cornering.
 Its generated by tire slip and is proportional to slip angle
at low slip angles.
 Slip angle describes the deformation of the tire contact
patch.
 This deflection of the contact patch deforms the tire in a
fashion akin to a spring.
 The deformation of the tire contact patch generates a
reaction force in the tire; the cornering force.

27. 2.9 LIFT FORCE
 A fluid flowing past the surface of a body exerts a force
on it. Lift is the component of this force.
 Lift is the force generated by propellers and wings to
propel aircraft and keep them in the air.
 Lift can be in any direction since it is defined to the
direction of flow rather than to the direction of gravity.
 When an aircraft is climbing, descending, or banking in a
turn the lift is tilted with respect to the vertical.

29. 2.10 PITCHING MOMENT
 The moment (or torque) produced by the aerodynamic
force on the airfoil.
 The pitching moment on the wing of an airplane is part of
the total moment that must be balanced.
 The lift on an airfoil is a distributed force that can be said
to act at a point called the center of pressure.
 If the moment is divided by the dynamic pressure, to
compute a pitching moment coefficient, this coefficient
changes only a little.

31. 2.11 YAWING MOMENT
 A yaw rotation is a movement around the yaw axis of a
vehicle that changes the direction the vehicle is facing.
 The yaw rate or yaw velocity of a car or other rigid body
is the angular velocity of this rotation.
 Yawing moment is the projection of a given torque over
the yaw axis.
 It is important in road vehicles because pitch and roll
moments are limited by the floor reaction.

33. 2.12 ROLLING MOMENT
 In a vehicle suspension, roll moment is the moment of
inertia of the vehicle's sprung mass.
 Product of the sprung mass and the square of the distance
between the vehicle's roll center and its center of mass.
 In aeronautics, the roll moment is the aerodynamic force
applied at a distance from an aircraft's center of mass.
 A roll moment can be the result of wind gusts, control
surfaces such as ailerons, or simply by flying at an angle
of sideslip.

35. 2.13 CROSSWIND SENSITIVITY
 The disturbances such as crosswinds should be minimized.
 The increasing of this disturbances will make the driver
have difficulties in compensating it.
 The sensitivity of a vehicle to cross-wind depends on
many factors involving the design of the suspensions and
the aerodynamics of the body.
 Streamlined bodies with smooth transitions and paralleled
underbodies lead to low drag coefficients.

36.  However, these measures cause the flow velocity around
the vehicle to increase.
 This also increases the sensitivity of the vehicle’s on-
centre handling under non-idealized flow conditions.
 A method for estimating the aerodynamic loads on vehicle
due to crosswind on a road section is also presented.
 The aim to find a relationship between steering feel and
crosswind sensitivity.
 Aerodynamic loads under real conditions were estimated
and the data were thereafter used in a study.

37. 2.14 ROLLING RESISTANCE
 A term used to describe the energy generated by the
friction of a tyre rolling over a road surface
 Tyre inflation pressure, tread compound, tread design and
temperature can affect the rolling resistance
 The tyres are usually manufactured with a higher degree
of silica built into the compound
 Silica allows the tyres grip performance to remain high,
especially in wet conditions, whilst rolling resistance is
improved.

39. 2.14.1 CALCULATING THE FACTORS AFFECTING
ROLLING RESISTANCE

40. A. TYRE TEMPERATURE
 The temperature grades representing the tire's resistance to
heat generation and its ability to dissipate heat.
 Tested under controlled conditions on a specified indoor
laboratory test wheel.
 Sustained high temperature can cause the material of the
tire to degenerate and reduce tire life
 While excessive temperature can lead to sudden tire
failure

41. B. TYRE INFLATION PRESSURE/LOAD
 The level of air in the tire that provides it with load-
carrying capacity.
 Affects the overall performance of the vehicle.
 A number that indicates the amount of air pressure -
measured in pounds per square inch (psi).
 Manufacturers of passenger vehicles and light trucks
determine this number based on the vehicle's design load
limit.

43. C. VELOCITY
 Rolling without slipping is a combination of translation
and rotation where the point of contact is instantaneously
at rest
 When an object experiences pure translational motion, all
of its points move with the same velocity as the center of
mass
 The object will also move in a straight line in the absence
of a net external force.

44. D. TYRE MATERIAL AND DESIGN
 Tires provide a gripping surface for traction and serve as a
cushion for the wheels of a moving vehicle
 Rubber is the main raw material used in manufacturing
tires, and both natural and synthetic rubber is used.
 The other primary ingredient in tire rubber is carbon
black.
 In the tire design, the main features of a passenger car tire
are the tread, the body with sidewalls, and the beads.

46. E. TYRE SLIP
 Slip is the relative motion between a tire and the road
surface it is moving on.
 Can be generated either by the tire's rotational speed or by
the tire's plane of rotation being at an angle to its direction
of motion.
 In rail vehicle dynamics, this overall slip of the wheel
relative to the rail is called creepage.
 Its distinguished from the local sliding velocity of surface
particles of wheel and rail, which is called micro-slip.

47. 2.14.2 TYPICAL COEFFICIENTS
 Tire Rolling Resistance Coefficient is calculated by
dividing the measured rolling resistance force
 Comparing Rolling Resistance Coefficients only allows
comparing tires within a single size.
 Larger tires generate higher Rolling Resistance Forces
than smaller tires.
 Larger tires will often have a lower Rolling Resistance
Coefficient than smaller tires.

48. 2.15 CALCULATING THE TOTAL ROAD
LOADS

49. 2.15.1 FUEL ECONOMY EFFECTS
 Vehicle weight, aerodynamic drag, driving style and
rolling resistance can effect the amount of fuel.
 Tyres can affect up to 1/3 of the vehicle's total fuel
consumption. Each tyre creates drag.
 A vehicle's aerodynamics and its travelling speed have an
extremely large effect on how much fuel is consumed.
 An environmental factors are impossible to control but
have a direct effect on fuel consumption.

50. THANK YOU !