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ACHEV
| Question | Answer |
|---|---|
| # ACHEV Chapter 1 – EXAM-ORIENTED Flashcards (Savaresi) | |
| ## Format: Question | Answer |
| Define the main goal of a suspension system | Filter the road-to-vehicle interaction while balancing comfort and road holding |
| List and explain the three main vertical dynamics motions | Heave (vertical), roll (rotation around longitudinal axis), pitch (rotation around lateral axis) |
| Why is heave the most important motion? | It directly affects vertical comfort and is dominant in suspension analysis |
| Describe the quarter-car model | A 1D model with two masses (sprung M and unsprung m) connected by spring-damper and tire stiffness |
| Define sprung and unsprung mass | Sprung: vehicle body above suspension; Unsprung: wheel assembly below suspension |
| Why is the quarter-car model used? | It simplifies analysis while capturing essential vertical dynamics |
| Write and explain the damper force law | F = -c·Δż, force opposes relative velocity and dissipates energy |
| Write and explain the spring force law | F = -k·Δz, force restores original position (elastic) |
| Explain why damper is dissipative and spring is conservative | Damper converts energy to heat, spring stores and releases energy |
| Draw and explain damper characteristic curve regions | 4 regions: rebound/compression at low/high speed, different damping behavior |
| Why is the real damper nonlinear? | Different behavior in operating regions and adjustable characteristics |
| Explain the role of gas spring inside a damper | Filtering, volume compensation, cavitation prevention |
| Explain cavitation and its consequences | Low pressure creates bubbles → noise, vibration, damage |
| Why is a preload gas chamber used? | To keep pressure above zero and avoid cavitation |
| Explain semi-active damping concept | Damping c(t) is controlled in real time |
| Compare EH and MR dampers | EH: mechanical valve control; MR: field-controlled fluid viscosity (wider range, more expensive) |
| Define controllability range of a damper | Range between minimum and maximum achievable damping |
| Explain comfort objective in suspensions | Minimize body acceleration (z̈) |
| Why does suspension behave as a low-pass filter? | It attenuates high-frequency road disturbances |
| Explain bandwidth limitation | High bandwidth requires high actuator power |
| Explain stroke limitation | Physical limit of suspension travel restricts compensation ability |
| Explain road-contact objective | Keep Fz as constant as possible to maintain tire grip |
| What happens when Fz = 0? | Loss of contact → no acceleration, braking, or steering |
| Write expressions for longitudinal and lateral forces | Fx = μxFz, Fy = μyFz |
| List components of vertical load Fz | Weight + aerodynamic load + dynamic load |
| Explain trade-off between comfort and road holding | Improving one typically worsens the other |
| Explain why down-peaks and up-peaks cannot be independently controlled | Reducing one also reduces the other due to system dynamics |
| Explain stroke as a design constraint | Limited resource that must be optimally used |
| Describe progressive vs regressive spring behavior | Progressive: stiffness increases; Regressive: stiffness decreases |
| Why are progressive springs preferred? | Prevent hitting end-stop by increasing force with displacement |
| Write coil spring stiffness formula and explain parameters | K = (G·d^4)/(8·n·D^3), depends on material and geometry |
| Compare pneumatic and hydro-pneumatic suspensions | Pneumatic: air-based, simple; Hydro: oil + accumulator, flexible layout |
| Advantages of pneumatic suspensions | Lower friction, easier fluid management, better damping control |
| Advantages of hydro-pneumatic suspensions | Remotized spring, flexible packaging |
| Explain stick-slip phenomenon | Sudden drop in friction when motion starts |
| Why is stick-slip undesirable? | Causes vibrations and poor comfort |
| Which suspension has lower stick-slip? | Pneumatic |
| Write the full quarter-car dynamic equations | Two coupled second-order ODEs for M and m |
| Why is the system 4th order? | Two masses → each has position and velocity |
| Define system state variables | x = [z, ż, zt, żt] |
| What is the system input? | Road profile zr(t) |
| Why is zr considered a disturbance? | It is external and not controlled |
| What are typical system outputs? | z (body displacement), zt (wheel displacement) |
| Explain why system is SIMO | One input, multiple outputs |
| Explain model validity limit | Invalid when tire loses contact (force cannot be negative) |
| Explain controllable damping model | c becomes input → nonlinear system |
| Why does controllable damping introduce nonlinearity? | Multiplication between c(t) and velocity states |
| Explain actuator dynamics for damping control | First-order system modeling valve response |
| Write actuator equation | ċ = -βc + βcin |
| Interpret actuator as filter | Low-pass filter with bandwidth β |
| What is physical meaning of β? | Speed of actuator response |
| How does actuator affect system order? | Adds one state → 5th order system |
| Define equilibrium condition | All derivatives equal zero |
| Does equilibrium depend on damping? | No, because damping acts only with motion |
| Explain equilibrium physical meaning | Static deflection due to weight |
| Explain linearization around equilibrium | Small perturbations → linear model |
| Define perturbation variables | δz, δzt, δzr |
| Why fix damping for linear model? | To remove nonlinearity |
| What type of system is obtained after linearization? | Linear Time-Invariant (LTI) |
| Explain physical meaning of tire stiffness kt >> k | Tire is much stiffer than suspension |
| Provide typical parameter values | M=400kg, m=50kg, k=20kN/m, kt=250kN/m, c≈1.3kNs/m |
| ------------------------------------------------------------------------------------ | |
| K = (G·d^4)/(8·n·D^3), depends on material and geometry | |
| EXAM TIP | Be ready to draw schemes: quarter-car model, damper curve, Fz behavior, suspension layout |
| EXAM TIP | Always explain physical meaning, not only equations |
| EXAM TIP | Clearly state trade-offs (comfort vs road holding) |
| EXAM TIP | Mention assumptions (linearity, no contact loss, 1D model) |
| Advantages of hydro-pneumatic suspensions | Remotized spring, flexible packaging |
| Explain stick-slip phenomenon | Sudden drop in friction when motion starts |
| Why is stick-slip undesirable? | Causes vibrations and poor comfort |
| Which suspension has lower stick-slip? | Pneumatic |
| Write the full quarter-car dynamic equations | Two coupled second-order ODEs for M and m |
| Why is the system 4th order? | Two masses → each has position and velocity |
| Define system state variables | x = [z, ż, zt, żt] |
| What is the system input? | Road profile zr(t) |
| Why is zr considered a disturbance? | It is external and not controlled |
| What are typical system outputs? | z (body displacement), zt (wheel displacement) |
| Explain why system is SIMO | One input, multiple outputs |
| Explain model validity limit | Invalid when tire loses contact (force cannot be negative) |
| Explain controllable damping model | c becomes input → nonlinear system |
| Why does controllable damping introduce nonlinearity? | Multiplication between c(t) and velocity states |
| Explain actuator dynamics for damping control | First-order system modeling valve response |
| Write actuator equation | ċ = -βc + βcin |
| Interpret actuator as filter | Low-pass filter with bandwidth β |
| What is physical meaning of β? | Speed of actuator response |
| How does actuator affect system order? | Adds one state → 5th order system |
| Define equilibrium condition | All derivatives equal zero |
| Does equilibrium depend on damping? | No, because damping acts only with motion |
| Explain equilibrium physical meaning | Static deflection due to weight |
| Explain linearization around equilibrium | Small perturbations → linear model |
| Define perturbation variables | δz, δzt, δzr |
| Why fix damping for linear model? | To remove nonlinearity |
| What type of system is obtained after linearization? | Linear Time-Invariant (LTI) |
| Explain physical meaning of tire stiffness kt >> k | Tire is much stiffer than suspension |
| Provide typical parameter values | M=400kg, m=50kg, k=20kN/m, kt=250kN/m, c≈1.3kNs/m |
| --- | |
| EXAM TIP | Be ready to draw schemes: quarter-car model, damper curve, Fz behavior, suspension layout |
| EXAM TIP | Always explain physical meaning, not only equations |
| EXAM TIP | Clearly state trade-offs (comfort vs road holding) |
| EXAM TIP | Mention assumptions (linearity, no contact loss, 1D model) |
Created by:
Filotì