Quote:
Originally Posted by 3-Wheeler
It will take some dedicated equipment to record enough data, and with enough consistency, to really add value to our aero endeavors at EcoModder.
Jim
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I agree. It will also require a much higher starting speed, particularly with an aerodynamically clean car. If the car is clean (or heavy for its frontal area) then rolling resistance has larger effect, and for the differences in deceleration to show up a greater speed differential is helpful.
Repetitions on the same day going in both directions are helpful to filter out wind effects. Working on a still air day is almost essential, because cross winds increase drag when going either direction, both from tire distortion and disruption of flow due to undesirable apparent wind direction.
A fifth wheel is easy to set up, and bicycle speedometers (the magnetic counters) give consistent, repeatable speed indications.
Rolling resistance is fairly easy to measure directly, by careful towing, but requires removing axle shafts, etc. With a known rolling resistance, then aero drag can be calculated more confidentally.
Unless you have really awful tires, your Crr cannot be .0137 -- so this figure is taking into account final drive and transmission friction, which is not perfectly constant -- part of it is hydrodynamic. The usual coastdown test calculations assume one constant drag, and one exponential.
But at very least, consistent measurement speed is required.
Thanks for the impressive work and tireless efforts!