ISO 4138:2021

Overview

ISO 4138:2021 - "Passenger cars - Steady-state circular driving behaviour - Open-loop test methods" specifies repeatable open‑loop test procedures to quantify the steady‑state circular handling behaviour of passenger cars (per ISO 3833) and light trucks. The standard defines controlled test manoeuvres that are not representative of everyday driving but are designed to produce comparable, discriminatory measures of vehicle steady‑state behaviour for vehicle dynamics and road‑holding evaluation. This is the fifth edition (2021) and updates the 2012 edition, including an additional variation of the constant‑speed method.

Key topics and requirements

• Scope and purpose: Provides open‑loop methods to determine steady‑state circular behaviour as a component of overall vehicle dynamics and road‑holding.
• Three principal test methods:
  • Method 1 - Constant radius (speed varied; steering measured)
  • Method 2 - Constant steering‑wheel angle (speed varied; radius calculated from vehicle motion)
  • Method 3 - Constant speed (radius varied; steering measured or varied). Method 3 includes three variations, one with slowly increasing steering‑wheel angle.
• Equivalence principle: Any steady‑state equilibrium (speed, steering angle, radius) can be achieved by holding one parameter constant, varying a second, and measuring the third; methods are equivalent if they span the same steady‑state conditions.
• Measured variables: Longitudinal velocity, lateral acceleration, steering‑wheel angle; lateral acceleration may be derived from yaw rate, roll or sideslip when required.
• Instrumentation & data processing: Requirements and guidance for transducer installation, inertial measurements, path radius calculation and data processing to ensure repeatable results.
• Test conditions and procedures: Warm‑up, initial driving condition, and detailed stepwise procedures for each method including driver requirements and test area considerations.
• Data analysis & presentation: Methods for plotting, differentiating gradients, normalizing results for comparison and extracting characteristic values. Normative annexes provide presentation formats and steering‑ratio determination; an informative annex covers theoretical background.

Applications

• Vehicle dynamics development: Comparative handling assessment during chassis or suspension tuning.
• Performance benchmarking: OEMs and tier‑1 suppliers evaluating design iterations or competitor vehicles.
• Tyre and chassis testing: Assessing steady‑state response to steering inputs and lateral loads.
• Research and validation: Academic and R&D groups studying handling behaviour, model validation and simulation correlation.
• Test laboratories: Establishing repeatable protocols for certification or product comparison.

Who should use this standard

• Automotive engineers, test drivers and vehicle dynamics specialists
• OEM test departments and independent proving grounds
• Tyre manufacturers and chassis suppliers
• Research institutions and vehicle safety analysts

Related standards

• ISO 3833 - Vehicle categories (reference for passenger car definition)
• ISO 15037‑1:2019 - General conditions for passenger car vehicle dynamics tests
• ISO 8855 - Vehicle dynamics vocabulary

Keywords: ISO 4138:2021, steady‑state circular driving behaviour, open‑loop test methods, passenger cars, vehicle dynamics, road‑holding, constant‑radius, constant steering‑wheel angle, constant‑speed, lateral acceleration.