ACTAR Vehicle Dynamics 3 — Questions and Answers
Question 1: What is the 'center of gravity height' effect on rollover threshold?
- Higher CG lowers the rollover threshold, making rollover more likely at lower lateral accelerations (Correct answer)
- Higher CG raises the rollover threshold, making rollover less likely
- CG height has no effect on rollover threshold
- Higher CG only affects pitch, not roll dynamics
Correct answer: Higher CG lowers the rollover threshold, making rollover more likely at lower lateral accelerations
A higher center of gravity increases the overturning moment relative to the stabilizing moment, reducing the lateral acceleration needed to initiate rollover.
Question 2: What is the significance of the 'static stability factor' (SSF) in rollover analysis?
- SSF = track width / (2 × CG height); higher SSF means greater rollover resistance (Correct answer)
- SSF = CG height / track width; higher SSF means greater rollover resistance
- SSF = wheelbase / CG height; used to predict braking stability
- SSF = weight / tire contact area; used to predict hydroplaning risk
Correct answer: SSF = track width / (2 × CG height); higher SSF means greater rollover resistance
The Static Stability Factor equals half the track width divided by the CG height; a higher ratio indicates the vehicle can sustain greater lateral acceleration before rolling over.
Question 3: How does 'load transfer ratio' (LTR) differ from static stability factor in rollover analysis?
- LTR is a dynamic measure using actual wheel loads during a maneuver, while SSF is a static geometric ratio (Correct answer)
- LTR and SSF are identical measurements expressed in different units
- LTR measures longitudinal weight transfer; SSF measures lateral weight transfer
- LTR is used only for trucks; SSF applies only to passenger cars
Correct answer: LTR is a dynamic measure using actual wheel loads during a maneuver, while SSF is a static geometric ratio
LTR uses the actual difference in wheel normal forces during a dynamic event (LTR = (F_right - F_left)/F_total), making it a real-time dynamic measure versus SSF's static geometric estimate.
Question 4: What is 'dwell time' in steering input analysis and why is it relevant to crash reconstruction?
- The time between a driver's reaction and first steering input
- The duration a steering wheel is held at a particular angle before being returned or reversed (Correct answer)
- The time required for electronic stability control to activate
- The delay between brake application and wheel lockup
Correct answer: The duration a steering wheel is held at a particular angle before being returned or reversed
Dwell time is how long a driver maintains a steering angle, which affects the magnitude of lateral displacement achieved and can indicate intentional versus panic steering.
Question 5: A vehicle with a wheelbase of 110 inches has its CG located 48 inches behind the front axle. What percentage of vehicle weight is on the front axle (static)?
- 43.6%
- 56.4% (Correct answer)
- 48.0%
- 52.0%
Correct answer: 56.4%
Front axle load % = (distance from CG to rear axle / wheelbase) × 100 = (110−48)/110 × 100 = 62/110 × 100 ≈ 56.4%.
Question 6: What phenomenon causes a vehicle's natural tendency to return to straight-line travel after a steering disturbance?
- Positive camber thrust
- Caster angle and pneumatic trail providing self-aligning torque (Correct answer)
- Negative toe-in creating drag forces
- Differential torque biasing toward the outside wheel
Correct answer: Caster angle and pneumatic trail providing self-aligning torque
Positive caster angle places the steering axis ahead of the tire contact patch, and pneumatic trail creates a self-aligning torque that naturally returns the wheels to straight-ahead.
Question 7: In the context of vehicle dynamics, what is 'jounce' and 'rebound'?
- Jounce is suspension compression (wheel moving up); rebound is suspension extension (wheel moving down) (Correct answer)
- Jounce is lateral tire deflection; rebound is the tire returning to its original shape
- Jounce is vehicle pitch forward under braking; rebound is the nose rising after braking
- Jounce is the initial impact force; rebound is the secondary force after collision
Correct answer: Jounce is suspension compression (wheel moving up); rebound is suspension extension (wheel moving down)
Jounce (bump) describes the suspension compressing as the wheel moves upward toward the body, while rebound describes the suspension extending as the wheel moves downward.
ACTAR Vehicle Dynamics 3
What is the 'center of gravity height' effect on rollover threshold?