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ISO 28981:2009 — Mopeds — Running resistance setting on chassis dynamometer
Coastdown method for determining and reproducing moped running resistance on chassis dynamometer | Vehicle testing guide
Introduction to ISO 28981
ISO 28981:2009 specifies methods for setting the running resistance of a moped on a chassis dynamometer. This standard provides a standardized procedure for simulating on-road driving conditions in a laboratory environment, enabling reproducible measurements of fuel consumption, emissions, and performance without the variability of real-world road testing.
The standard covers the coastdown method: measuring deceleration on a real road to determine running resistance, then reproducing that resistance on a chassis dynamometer for controlled laboratory testing.
Key Technical Requirements
Road Test Procedure
The standard specifies precise requirements for the test road: gradient less than 0.5%, wind speed below 3 m/s, and dry surface conditions. The moped is accelerated to a specified starting speed, then coasted in neutral with the engine off.
| Parameter | Requirement |
|---|---|
| Road gradient | < 0.5% |
| Wind speed | < 3 m/s |
| Reference speed | As specified for moped category |
| Coastdown starting speed | Above reference speed |
| Measurement accuracy | Speed ±1%, Time ±0.1 s |
| Rider position | Standard riding posture |
| Minimum runs | At least 5 in each direction |
Ambient conditions significantly affect coastdown measurements. Air temperature, barometric pressure, and wind must be within specified ranges, and measurements corrected to standard reference conditions.
Engineering Design Insights
The coastdown method is elegant in its simplicity: by measuring how quickly a moped decelerates when coasting in neutral, the total running resistance (aerodynamic drag + rolling resistance + driveline friction) can be determined without complex instrumentation. The running resistance force F is calculated from the coastdown time and vehicle mass.
The standard specifies a running resistance curve fitting procedure, typically using a quadratic function of speed: F = a + bv + cv². The coefficient a represents rolling resistance, b represents speed-dependent friction, and c represents aerodynamic drag.
Using the coastdown method eliminates the need for expensive wind tunnel testing or complex analytical models, making emissions and performance testing accessible to a wider range of testing facilities.
Dynamometer Setting
The chassis dynamometer is set to reproduce the target running resistance force at the reference speed. The standard specifies requirements for dynamometer accuracy, including inertia simulation and power absorption characteristics.
Frequently Asked Questions
Q: Why is the coastdown method preferred over wind tunnel testing?
A: Coastdown testing measures total running resistance under real road conditions without the complexity and expense of wind tunnel facilities.
A: Coastdown testing measures total running resistance under real road conditions without the complexity and expense of wind tunnel facilities.
Q: How does rider position affect results?
A: Rider posture significantly affects aerodynamic drag. The standard specifies a standard riding position to ensure reproducibility.
A: Rider posture significantly affects aerodynamic drag. The standard specifies a standard riding position to ensure reproducibility.
Q: Can this method be used for electric mopeds?
A: Yes, the coastdown method applies regardless of powertrain type.
A: Yes, the coastdown method applies regardless of powertrain type.
Q: What is the reference speed used for?
A: The reference speed is the speed at which the dynamometer setting is verified, typically corresponding to the moped most common operating speed.
A: The reference speed is the speed at which the dynamometer setting is verified, typically corresponding to the moped most common operating speed.
