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Vehicle dynamics and powertrain > Vehicle Dynamics > Flashcards

Vehicle Dynamics Flashcards

1
Q

If a car is travelling at constant speed on a level surface, what are the reaction forces at the front and rear?

A
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2
Q

What is h in vehicle dynamics calculations?

A

The height of the centre of gravity

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3
Q

If a vehcile is acclerating at accleration, ax, what are the loads on the front and rear axles?

A

a

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4
Q

If a car is on an incline Θ what are the reaction forces on the front and rear axles?

A
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5
Q

What are the three stages of drive ratios?

A
  • Gear box
  • Differential
  • Tyres
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6
Q

What is an equation for clutch torque?

A

Tc=Te-IEαE

Te=engine torque

Ie is the rotational interia of the engine,

αE=engine rotational accerlation

IEαE is the torque needed to rotate the engine.

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7
Q

How do you find the driveshaft torque

A
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8
Q

How do you find the axle torque?

A
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9
Q

How can you link wheel accleration to engine accleration?

A

nt is the gear ratio

nf is the final drive ratio.

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10
Q

Taking into account the interia of the drivetrain and inefficines within the drivetrain, what is the maximum tractive force?

A

ητf=ηf*ηt

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11
Q

How do you find the equivalent mass of the rotational inertia of the drivetrain?

A
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12
Q

What is the mass factor?

A

Mass factor=(M+Mr)/M

Mr is the equivalent mass of the rotational interia of the drivetrain.

It depends on the current gear ratio.

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13
Q

How might you find the maximum accerlation of a car in a given gear?

A

ax=Fxmax / (M+MR)

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14
Q

How do you find weight shift due to the driving torque?

A

Wy=(Td-Ts) / t

Td=Fxr / Nf

Fx is the total tractive force, r is the wheel radius, Nf is the final drive ratio.

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15
Q

How can you find the roll angle of a car due to weight shift due to driveshaft torque?

A
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16
Q

What are the main parts of a brake system?

A
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17
Q

What is the braking split on a rear wheel drive vehicle?

A
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18
Q

What is the braking split on a front wheel drive vehicle?

A

The static load is higher on the front axle and much lower on the rear. With the added weight shift under braking, the front axle normal load increases and the rear further decreases

The heavier weight on the front wheels must be utilized in each hydraulic circuit if one fails • Steering offset must be minimum to reduce adverse force on steering wheel (This is why it’s a diagonal split and not left/right split)

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19
Q

Where does the vacuum of a vacuum servo come from?

A

The vacuum comes from either the intake manifold of a gasoline engine, or a vacuum pump on a diesel engine.

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20
Q

Considering air resistance, what is the equation for the braking distance?

A

Fb=braking force

C=air resistance coefficient

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21
Q

How do tyres stick to the road?

A
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22
Q

What is the braking coefficient?

A

Braking coefficient = Fx/Fz

Fx=braking force

Fz=vertical force

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23
Q

What is ABS?

A

• Senses when wheel is about to lock up and releases brake momentarily • Modulates pressure level to just hold wheel near peak slip condition on next application

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24
Q

What are the two types of ABS?

A
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25
Q

What are some of the main sources of vibration?

A

The engine Use engine mounts

Road input and tyre resonnances Use suspention

Gearbox and clutchs Transmission mounts

Exhaust system

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26
Q

What is the clamp force R within a brake system?

A

R=PAE

P=effective pressure

A=piston area

E=clamp force efficincy (less than 1 due to return springs)

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27
Q

What are c and b?

A
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28
Q

What does OEM stand for and what do they do?

A

Original equipment manufacuter

Has legal responsiblity for the product.

Buys complete modules and sub-systems from Tier 1 suppliers

Responsible for specifying sub-system performance

Also buys services such as engineering, software, advertising etc from Tier 1 suppliers

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29
Q

What is a tier 1 component supplier?

A

Supplies complete modules and sub-systems to OEM e.g. doors, axle units, seats etc.

Buys out sub-assemblies from Tier 2 suppliers

Responsible for performance of modules or sub-systems supplied to OEM

30
Q

What is a tier 1 service suppiler?

A

Supplies services and equipment to the OEM. This could be engineering services, software services or test systems.

31
Q

What is a tier 2 supplier?

A

Supplies sub-assemblies to Tier 1 suppliers e.g. door locks, window mechanism, struts etc

  • Buys out components from Tier 3 suppliers
  • Responsible for performance of modules or subassemblies supplied to Tier 1 suppliers e.g. disc and drum brakes, linings etc.
32
Q

What is a tier 3 component supplier?

A

Supplies individual components to Tier 2 suppliers e.g. nuts, bolts, door handles etc

33
Q

Why is there vehicle certification?

A

Standardisation

Public safety

Protection of the environment

34
Q

What is a braking force provided by an axle in terms of brake pressure? (disk brakes)

A
35
Q

What is the maximum braking force the front axle can provide when weight shift due to decleration is considered?

A
36
Q

What is the maximum braking force the rear axle can provide when weight shift due to decleration is considered?

A
37
Q

How do you find the decelleration that a wheel axel will lock up at?

A

Fb=2μNr (1) (Braking force provided by 1 axel)

Nr=Nrs ± Mah/l (2) (standard formula for weight on axel)

sub 2 into 1, F=Ma and solve for a.

38
Q

What is the difference betweent the leading and trailing shoe within a drum brake?

A

The leading shoe is pushed into the brake lining by the rotation of the wheel.

The trailing one is pushed out by the same rotation and therefore is less effective.

Having one trailing and one leading shoe means braking is equally effective in both rotational directions.

39
Q

What is the formula that produces a sound level (dB) from a pressure level? (Pa)

A

The reference pressure P0 is 20mPa

Based on the smallest sound level a person with good hearing can detect.

40
Q

What is mechanical mobility?

A

Mechanical mobility is a way of expressing the vibrational characteristic of a system and describes a systems capacity to be excited.

The higher the mobility, the higher the vibration amplitude generated by a given input force.

Using vibration velocity as the motion parameter we define mechanical mobility as the ratio of output vibrational velocity to the excitation input force i.e.

41
Q

What is mechanical impedance?

A

The inverse of mechanical mobilty.

Z=1/M

It is therefore the input force required to produce a unit output vibration velocity.

This is a measure of a system’s capacity to resist vibration input.

42
Q

What is the mobility of a mass?

A

A function of the harmonic frequency

43
Q

What is the mobilty of a damper?

A

1/c

44
Q

What is the mobility of a spring?

A

M=iω/k

45
Q

How do you add mobiltes connected in series?

A

It is like electrical resistance.

Add them up. If they are in parallel, add up the reciprocals.

Opposite for impedance.

46
Q

Explain why mechanical mobility and impedance parameters should be used in describing the vibrational behaviour of a system rather than simple motion parameters.

A

The response of a system expressed in terms of motion parameters like acceleration and displacement will depend on the force input into the system. Mechanical mobility on the other hand, is a characteristic of the system in that it is the response to a unit input force. For a linear system, the response to any input force can thus be obtained from the mechanical mobility. In like manner, mechanical impedance which is the inverse of mobility is a system characteristic. Mechanical mobility or impedance should therefore be used rather than simple motion parameters in describing vibrational behaviour.

47
Q

What must you remember about the mobilites of masses?

A

Masses can only be connected between a single moving point and “ground” (a Newtonian inertial frame of reference) so they can only be connected in parallel.

Therefore the follwing it true

48
Q

How would you re-draw this to allow you to perform mobility analysis?

A
49
Q

How would you re-arrange this diagram to find the impedance of point b if a is fixed?

A
50
Q

How could you lower the natural frequency of a suspension system?

A

Decrease the stiffness. (Increase the static deflection and softness)

51
Q

What is effective stiffness?

A
52
Q

When there is no damping force, what is the natural frequency?

A
53
Q

What is the ideal bounce frequency for a good vehicle ride?

A

1-1.5 Hz. (Clear from 4-8Hz)

54
Q

How do you find the bounce frequency?

A
55
Q

How should rear and front suspensions compare?

A

Rear suspension bounce frequency should be 12% to 15% higher than front suspension bounce frequency, produces flatter ride

56
Q

What is the dynamic pitch index and what should it ideally be?

A

Ideally be 1

57
Q

When elements (impedances or mobilites) are connected in series, what can you say about their velocity and force?

A

When elements are connected in series, the same force (as current in the electrical analogue) is transmitted through all elements. Velocities are, however, different.

58
Q

When elements (impedances or mobilites) are connected in parralel, what can you say about their velocity and force?

A

When elements are connected in parallel, the velocity (voltage) is common and the force (current) is shared between them

59
Q

When does resonance occur?

A

Vibration resonance is said to occur when the impedance of a system is minimum

60
Q

Describe the three different types of automotive certification and where they are used.

A

o Self-certification: Manufacturer is responsible for ensuring that all legal requirements are met and certifies this by affixing a label of specific format to driver’s door. Used in USA and Canada.

o Type Approval: Manufacturer submits vehicle to govt appointed test authority who carries out all required tests and grants type approval. Used in the EU.

o Type Definition: Japanese variation of type approval – manufacturer submits detailed documents on design and manufacture to approval body and must maintain detailed manufacturing and inspection records.

61
Q

State the relationships between the full-car, half-car and quarter-car models which may be used for investigating the ride performance of a motor vehicle

A

Full car model considers a vehicle chassis with inputs at all four wheels of the vehicle i.e. rigid body suspended on front and rear axles with a full set of suspensions. The half-car model is derived from the full car model assuming no roll and symmetry about the fore-aft axis thus considering one front wheel and one rear wheel. The quarter-car model is a further simplification, derived from the full car model assuming no roll and no pitch thus considering just one corner of the car i.e. single wheel, suspension and sprung mass.

62
Q

What is transmissibility?

A

This simply represents the non-dimensional ratio of the response amplitude to the input amplitude.

63
Q

What is brake proportioning?

A

The purpose is to balance the force between the front and rear axels. It tries to bring both axels up to lock at the same time, meaning less pressure is sent to the rear axel.

64
Q

What is EBD?

A

Electronic brake distribution

65
Q

What is the equation for vehicle slip?

A
66
Q

What is bracking efficiency?

A
67
Q

What are the phase characteristics of the three basic elements?

A
  • for a mass, the velocity lags the input force by 90°
  • for a viscous damper, the output velocity and input force are in phase
  • for a spring, the output velocity leads the input force by 90°

One significance of the phase difference between input force and output velocity is that the part of velocity in phase with the force produces energy that is absorbed by the element whereas that part which is 90° out of phase produces conservative energy which is subsequently released e.g. potential energy. Thus while the energy in a mass and spring can be released again, the damper dissipates its energy as heat.

68
Q

What is the difference between traction and power limited accleration?

A

Traction limited

  • The amount of tractive force that a vehicle is able to generate is limited by the contact friction between the tyres and the road regardless of the tractive force available from the engine/transmission. The maximum acceleration is therefore limited by this tractive force and is known as traction limited acceleration.
  • Power limited
  • Power limited acceleration is determined solely by the maximum torque that the engine can deliver to the road wheels irrespective of what the wheel can transmit to the road: it depends on engine torque, transmission losses and drivetrain inertia
69
Q

Explain the term mass factor?

A

When calculating the acceleration performance of a vehicle, its mass, 𝑀, must be augmented by the apparent mass, 𝑀r , of the rotating inertias (crankshaft, flywheel, gearbox, propeller shaft, axle(s) and wheels. This can be accounted for by multiplying the vehicle mass by the dimensionless mass factor

70
Q

How are the second third and fouth gear teeth numbers determined?

A

For the second, third, fourth and fifth gears, a similar iteration process will be carried out to check shaft distance A, the axial load balance, using addendum modification if needed.

71
Q

What is the equation for the wheel hop frequency?

A
72
Q

What is the equation for the body bounce frequency?

A

Vehicle dynamics and powertrain (2 decks)

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