Coastdown

Coastdown is a method used in vehicle dynamics to characterize the forces that oppose motion as a vehicle slows down after the driver releases the throttle. By analyzing how quickly the speed decreases, engineers infer the magnitudes of aerodynamic drag and rolling resistance, and, in some setups, drivetrain losses.

Typically conducted on a level test surface or a closed track, the vehicle is allowed to coast from a set speed without propulsion. Speed is recorded over time using GPS, radar-based sensors, or wheel-speed sensors, and the deceleration a = dv/dt is related to velocity by the equation m dv/dt = -1/2 ρ CdA v^2 - Cr m g (ignoring small losses). By performing runs at different initial speeds and fitting the data, CdA (drag area) and Cr (rolling resistance coefficient) can be extracted.

Coastdown results are used to calibrate energy consumption models, validate aerodynamic and rolling resistance estimates, and support vehicle development and regulatory testing. In electric vehicles, coastdown is particularly useful for predicting range under real-world conditions. There are also coastdown tests on chassis dynamometers to control wind, road grade, and surface conditions more tightly.

Limitations include sensitivity to road grade, wind, temperature, surface roughness, and braking system behavior. The method assumes steady environmental conditions during each run and may require corrections for incline or crosswinds. Differences between on-road and on-dyno environments must be accounted for when comparing results.