WLTP Chassis Dynamometer Testing: Road Load Simulation and Coast-Down Validation

WLTP Has Changed What Chassis Dynamometer Testing Means

The Worldwide Harmonised Light Vehicle Test Procedure (WLTP), mandatory for passenger car type-approval in Europe since 2017 and adopted or referenced in markets from China (CLTC borrows heavily from it) to India, imposes a far more demanding chassis dynamometer test than the older NEDC it replaced. Average test speed rose from 46 km/h to 46.5 km/h, but maximum speed increased from 120 km/h to 131.3 km/h, and critically, the cycle has four speed phases (Low / Medium / High / Extra High) that cover a much wider range of real-world conditions.

But the most important change is not the cycle profile itself — it is the road load determination methodology. WLTP requires that the chassis dynamometer be set with vehicle-specific road load coefficients derived from a coast-down test on a real road. Emissions and energy consumption results are traceable back to an actual measured vehicle, not a class average. This means the quality of your coast-down data directly determines the validity of your type-approval test.

Coast-Down Testing: The Foundation of WLTP Road Load

A coast-down test measures the deceleration of a vehicle coasting from a target speed to a lower speed on a level, wind-free road. Per UN Regulation No. 83 Annex 4a and the WLTP GTR Annex 4, the vehicle must be tested in both forward and reverse directions at each reference speed to cancel out road camber and wind effects. The result is a set of running resistance coefficients (f₀, f₁, f₂) that describe aerodynamic drag and rolling resistance as a polynomial function of speed.

• Speed accuracy: The coast-down must be triggered within ±1 km/h of the target speed; GPS-based reference speed measurement accurate to ±0.1 km/h is standard practice. OBD-sourced speed signals have too much latency for this measurement.
• Mass preparation: Test mass must be set to the Reference Mass (RM) or, for WLTP, the Test Mass (TM) defined by the regulation. Tyre pressures must be confirmed to specification immediately before each run.
• Averaging: WLTP requires a minimum of three valid runs per direction, with the average used. Outliers (runs with anomalous wind gust effects) are identified by comparing the run-to-run repeatability; any run deviating more than 2% from the mean is rejected.

Setting the Chassis Dynamometer from Road Load Coefficients

Once the f₀/f₁/f₂ coefficients are established, the chassis dynamometer must be programmed to reproduce the road load force at each roller speed. Modern chassis dynos use an AC regenerative load unit controlled by a speed-force lookup table (or a real-time polynomial calculation). The key calibration checks before each test run are:

1. Roller Surface Verification

Roller surface roughness affects tyre-roller contact friction. WLTP specifies a roller diameter of at least 318 mm (12.5 inches) for passenger cars; larger rollers (48 inches, as used in ECONOTESTS heavy-duty chassis dynos) give a more representative tyre contact patch for larger vehicles. Surface speed must agree with the reference speed measurement device to within ±0.1%.

Key metrics: Roller circumference (measured, not nameplate); roller surface temperature; parasitic losses of the inertia flywheel at each test speed.

2. Inertia Simulation Verification

WLTP uses the vehicle’s actual test mass for inertia simulation, unlike NEDC which used standardised inertia classes. The chassis dyno’s inertia simulation must match the target value within ±20 kg. This is verified by the coast-down on the roller (in-lab coast-down) at the beginning of each test campaign and compared against the expected deceleration derived from f₀/f₁/f₂.

3. Electrical Loss Correction

For BEV and PHEV testing under WLTP, the chassis dyno must measure electrical energy flow at the vehicle’s AC charge port or HV battery interface simultaneously with speed and tractive force. The applicable protocols are ISO 23274-1 (BEV) and ISO 23274-2 (PHEV); both require energy measurement uncertainty below 1%.

4. Drive Trace Validation

The WLTP drive trace is driven by a human operator or a robot driver following the speed-time profile displayed on the dyno’s operator screen. The percentage of test time spent within the speed tolerance band (±2 km/h of target) must meet the regulation’s minimum drivability criterion. Automatic chassis dyno driver robots typically achieve 99%+ time-in-band even for the Extra High speed phase.

Key metrics: % time in band per phase; maximum single overshoot; trace outlier count.

5. Emissions and Energy Measurement

For ICE and hybrid vehicles, exhaust gases are sampled via CVS tunnel (Constant Volume Sampler) for bag analysis (NMHC, NOx, CO, CO₂) and via PEMS (Portable Emissions Measurement System) for RDE correlation. For BEVs, range per cycle is the primary output, supplemented by usable battery energy measured at the socket.

Wind Tunnel — Chassis Dyno Correlation

Some manufacturers now use a combined wind tunnel and chassis dynamometer, where the vehicle sees a real headwind matched to its speed. This eliminates the aerodynamic contribution from f₂ on the roller (which is otherwise set by software) and gives the most realistic underbody airflow for thermal management development. ECONOTESTS supplies chassis dynamometers in both standard and wind-tunnel-coupled configurations for OEM R&D centres.

If your programme requires WLTP-compliant road load setup, in-lab coast-down verification, or combined chassis dyno and wind tunnel integration, our engineers can support the instrumentation and software configuration specific to your regulatory market. Talk to our team about your test plan.