Vehicle dynamics - Chapter 2 (Road Loads)

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Vehicle dynamics - Chapter 2 (Road Loads)

  1. 1. VEHICLE DYNAMICS CHAPTER 2 : ROAD LOADS
  2. 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.
  3. 3. 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.
  4. 4.  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.
  5. 5. 2.3 PRESSURE DISTRIBUTION ON A VEHICLE  The pressure distribution over the body surface exerts normal forces which, summed and projected into the freestream 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.
  6. 6. 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.
  7. 7. 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.
  8. 8.  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.
  9. 9. 2.6 AERODYNAMICS AIDS
  10. 10. 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.
  11. 11. 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.
  12. 12. 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.
  13. 13. 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.
  14. 14. 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.
  15. 15. DETERMINE AND ESTIMATE DRAG COMPONENTS 2.7
  16. 16. 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.
  17. 17. 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.
  18. 18. 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.
  19. 19. 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.
  20. 20. 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.
  21. 21. 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.
  22. 22. 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.
  23. 23. 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.
  24. 24.  However, these measures cause the flow velocity around the vehicle to increase.  This also increases the sensitivity of the vehicle’s oncentre 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.
  25. 25. 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.
  26. 26. CALCULATING THE FACTORS AFFECTING ROLLING RESISTANCE 2.14.1
  27. 27. 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
  28. 28. B. TYRE INFLATION PRESSURE/LOAD  The level of air in the tire that provides it with loadcarrying 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.
  29. 29. 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.
  30. 30. 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.
  31. 31. 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.
  32. 32. 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.
  33. 33. CALCULATING THE TOTAL ROAD LOADS 2.15
  34. 34. 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.
  35. 35. THANK YOU !

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