Overview
ISO 9816:2018 - "Passenger cars - Power-off reaction of a vehicle in a turn - Open-loop test method" defines repeatable open-loop test procedures to quantify how a passenger car reacts in a turn when motive power is suddenly reduced by releasing the accelerator. Intended for passenger cars (as defined in ISO 3833), the standard provides a controlled laboratory-style method to capture vehicle responses (steering, yaw, lateral motion) following a sudden power-off event. The 2018 (third) edition includes updates addressing alternative powertrain systems.
Key technical topics and requirements
- Test principle: Start from a steady-state circular run; introduce a sudden power-off by rapid release of the accelerator while maintaining the initial steering-wheel angle; record driver input and vehicle response.
- Two alternative methods:
- Constant-radius method - fixed path radius; increase lateral acceleration by raising speed. Requires less space but more driver skill.
- Constant-speed method - fixed speed; increase lateral acceleration by increasing steer angle (smaller radii). Keeps engine speed constant; requires more test surface.
- Measured variables (required): instant of power-off initiation (t0), steering-wheel angle, yaw angle or yaw rate, longitudinal velocity, lateral acceleration, sideslip angle or lateral velocity. Optional: longitudinal acceleration.
- Instrumentation & accuracy: Multi-channel recording with time base; transducers as per ISO 15037-1. Typical recommended maximum errors include instant of power-off initiation ±0.05 s and yaw angle ±2°.
- Test conditions: Warm-up and environmental limits per ISO 15037-1. Initial lateral acceleration around 4 m/s², increased in successive runs in increments ≤ 1 m/s² (0.5 m/s² recommended where response changes rapidly).
- Powertrain and gearing: Manual transmissions tested in the lowest possible gear except first; engine speed ≤ 80% of engine speed at maximum power point. Automatic/CVT/ adaptive gear behaviors must be recorded.
Practical applications and users
ISO 9816:2018 is useful for:
- Vehicle dynamics engineers validating handling and stability under power-off disturbances.
- OEMs comparing design variants (suspension, steering, powertrain calibration).
- Test laboratories creating repeatable, comparable datasets for development and benchmarking.
- Regulators and safety researchers studying correlations between handling metrics and accident avoidance (with caution - the open-loop maneuver is not representative of all real-world driving behaviour).
Practical outcomes include objective measures of directional stability, transient yaw/side-slip behavior, and influence of drivetrain characteristics on cornering safety.
Related standards
- ISO 15037-1 - Vehicle dynamics test methods: general conditions for passenger cars (instrumentation, data processing).
- ISO 8855 - Vehicle dynamics vocabulary (terms and definitions).
- ISO 3833 - Vehicle classification (definition of passenger cars).
Keywords: ISO 9816:2018, power-off reaction, vehicle in a turn, open-loop test method, passenger cars, vehicle dynamics test, constant-radius, constant-speed, lateral acceleration, yaw angle.