On road determination of vehicle drag coefficient using the new constant power plate method

This paper presents a novel on-road method for determining a vehicle’s aerodynamic drag coefficient using a constant power approach with a towbar-mounted drag plate. The technique involves fixing the throttle pedal and measuring the vehicle’s equilibrium speeds in two configurations: baseline and with the added drag plate. From these speeds, the vehicle’s baseline drag coefficient can be calculated. Two formulations are introduced - one for an idealised plate with negligible self and interference drag, and another for practical setups where the support structure introduces additional self-drag and interference. The method was applied to a Citroen Berlingo van using an aluminium plate and stand, yielding a measured drag coefficient of 0.416. Validation against traditional coastdown testing showed a close agreement, with only a 6.1% difference. A sensitivity analysis demonstrated that the new method is less dependent on variables such as vehicle mass, air density and rolling resistance compared to coastdown testing. The potential to extend the method to estimate a vehicle’s rolling resistance is discussed, though limited by current GPS accuracy. Overall, the new constant power plate method offers a simple, robust alternative to coastdown testing and demonstrates strong potential for its usage in future aerodynamic assessment and vehicle development.