Overview
ISO 20918:2007 - Road vehicles: Braking threshold pressures for heavy commercial vehicle combinations with fully pneumatic braking systems - provides a standardized method for measuring the system threshold pressure using a roller brake tester. Intended for use in workshops and garages, the standard defines how to identify the control-line (trailer coupling-head) pressure at which brakes begin to generate measurable braking force above rolling resistance. ISO 20918:2007 is not for Type Approval and does not address high‑pressure compatibility, friction control, or dynamic influences.
Key topics and requirements
- Purpose: Determine the pressure at which braking force starts to develop (system threshold pressure) for heavy vehicles and trailers with fully pneumatic braking systems.
- Recommended threshold: A nominal value of around 70 kPa measured at the trailer coupling head is given as the target system threshold pressure, with specified allowances for small actuators and certain vehicle classes (see standard for exact conditions).
- Measurement method: Use a roller brake tester to detect when braking force exceeds rolling resistance while increasing control-line pressure.
- Apparatus requirements:
- Minimum sampling rate recommended: 5 Hz
- Pressure gauges: ±10 kPa accuracy below 500 kPa, ±2% above 500 kPa
- Roller brake tester force resolution: 100 N (≤5 000 N) and 500 N above, with accuracy ±100 N (≤5 000 N) or ±2% thereafter
- Compressed air source able to reach 1 000 kPa and control within 10 kPa up to 200 kPa
- Adapter to measure control line pressure (or digital diagnostic tools meeting accuracy requirements)
- Test procedure (practical steps):
- Charge air system, apply/release brakes several times
- Fit pressure adapter or connect diagnostic tool
- Place axle on roller brake tester, gradually increase service/control pressure
- Note minimal coupling-head pressure when braking force rises above rolling resistance
- Repeat three times per wheel/axle and record average
- Limitations: Excludes high‑pressure compatibility (coefficient of friction effects) and dynamic influences; not for regulatory Type Approval.
Applications and who uses it
- Vehicle maintenance workshops and heavy-vehicle garages for brake diagnostics and set-up
- Fleet maintenance and inspection teams for ensuring braking balance and wear optimization
- Brake equipment manufacturers and roller brake tester providers for device specification and calibration
- EBS system developers and vehicle OEMs as a minimum reference for default threshold values (EBS defaults should, at minimum, comply with this standard)
Related standards
- ISO 21069-1 - Roller brake tester testing for pneumatic systems (normative reference)
- ISO 611 - Braking vocabulary and terms used in ISO 20918
Keywords: ISO 20918:2007, braking threshold pressure, roller brake tester, heavy commercial vehicle, pneumatic braking systems, system threshold pressure, trailer control line, roller brake test, workshops and garages, EBS compliance.