Quote:
Originally Posted by JulianEdgar
Well, of course. But 'contextual' is the whole issue here: the context is that in some cases, full wheel covers increase drag. Isn't that the point?
I am sorry, but I have seen lots of advice here - including from you - that is stated without any caveats whatsoever. Here's an example that was being used in a signature:
"At 55 mph, a 10% drag reduction translates to a 5% increase in fuel economy. At 70mph,a 10% drag reduction translates into a 6% increase -Phil Knox (Aerohead), Aerodynamics Seminar #2"
No caveats there.
(Nothing on what proportion of total drag is made up by CD vs rolling resistance - which is car-dependent? Nothing on the change in engine efficiency - BSFC if you like - as the rpm changes? For a discussion of this sort of thing, see Barnard, Road Vehicle Aerodynamics Pages 54-55, and Stone, Motor Vehicle Fuel Economy, Page 136.)
|
*The 5%,10% etc. rule of thumb is from PhD, Gino Sovran,General Motors Research Laboratory, contributor to SAE publications,and personal friend and mentor of Wolf Heinrich Hucho.These relationships have been shared by GM,Ford,Chrysler,and Hucho,extending into recent times.
*The relationship is predicated solely upon drag reduction,and I've addressed the implications of BSFC degradation for not gear-matching in the seminars,as per Gino Sovrans admonition in the SAE literature.Hucho repeats this cautionary tale in his textbooks. I'm quite familiar with the relationship. Once upon a time, I had a personal relationship with Glen Scharf,of GMs Aerodynamic Laboratory.I could simply call on the phone,share data,and he was gracious enough to help me navigate the drag implications of delta-drag,based upon delta-mpg.Glen is the individual who stated that my modified CRX had Cd 0.235,based upon my Bonneville speed record, coastdown data from the Chrysler Proving Grounds,and tank mileage.