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Coast-down

In chassis dynamometers, to allow the same balance of power to be applied to vehicle positioned on the roller as on the road, a so-called coast-down test is performed.

The vehicle is shifted to neutral and allowed to coast down from a high speed.

Vehicle behaviour on road: If the road is flat and straight and there is no strong wind, the vehicle is braked simply by internal losses (mechanical frictions) and by its air resistance (cw).

The aim on chassis dynamometers is to reproduce and analyse this period; using the measured speed progression over time, the “driving resistance” can be calculated (as a speed function), obtaining an amount of ‘power-loss’.

This driving resistance is typically described in the form of a 2nd degree equation with the constants F0, F1, F2. Here, factor F2 essentially represents the wind resistance due to the fact that this power percentage grows with the square of the speed. In order to simulate the same conditions on the roller as those found on the road, the vehicle is now attached to the roller and the parameters which lead to the same coast-down curve are determined.

Power-loss amount (measured during coast down) is then added to ‘power at the wheel’, in order to obtain the ‘power at the crankshaft’.