Quote:
Originally Posted by Daox
...What Aerohead said is true, but much more of a general rule. I assume you are talking about the rule that says something to the effect of 'A 20% reduction in Cd gives a 10% improvement in FE'. That is not going to the case at 55 mph and at 20 mph. Its just a general rule...
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Hi Daox,
Yes, you are right. That's exactly what I was trying to say.
As I do coast down tests and discover just how much the air drag and rolling drag contribute to the overall vehicle drag, maybe a glimmer of light is appearing for me, on what Aerohead is alluding to.
At lower speeds, since the air drag is almost the same proportion of overall drag as Crr is, then each contributes about 50% to the overall.
As the speed increases, let's say towards 100 mph, then 90% of the overall vehicle drag is mostly air drag, and Crr is only 10%.
So if Metro lived in Germany and could drive the Autobahn at 100 mph, his decrease in CdA because of the boat tail would give him a resounding 30% decrease in drag and:
90%/100% * 30% CdA decrease = 27% decrease in fuel consumption at that speed
However, at lower speeds, let's say 20 mph where the CdA contribution and Crr contribution are each 50%, then his fuel consumption would be:
50%/100% * 30% CdA decrease = 15% decrease in fuel consumption.
If this is not the case, then there are other factors involves that dictate overall fuel consumption on the vehicle such as tuning of the engine, gearing of the vehicle and so on.
If Aerohead has some info on this, it would be nice to elaborate.
Jim.