Design-headwind

[aerohead]

Originally Posted by NeilBlanchard

Aerodynamic drag and rolling drag are total losses, and this makes them the most important things to reduce.

Weight, on the other hand, provides higher kinetic energy and so by coasting (or using regenerative braking) some energy can be regained. So, lowering weight helps in some situations, but some of the kinetic energy can be regained and so it is not a total loss.

Yeah,it was big fun to see regen in the EV-1 and lately in friends Prius.
Until we 'cure' gridlock and un-synchronized traffic signals,it's a fabulous way to get a little back.

[aerohead]

Originally Posted by Kodak

Nice job punching all those numbers. What formula did you use to calculate the 27.4mpg headwind comparison?

The numbers make me wonder how automakers haven't employed some of these technologies. I've seen the wheel covers, but that's really it. I suppose the simple answer is that some aero changes might seem unsightly - or at least pose a major marketing risk.

Kodak,that was a goof and I've gone back and edited the mistake.
It should have read mph,instead of mpg.Sorry!
With respect to the 'unsightly' issue,it could all come down to that.
The 1929 'pregnant' Buick and 1934 Chrysler/DeSoto/Imperial Airflows are mentioned as examples of styling before their time.
If they don't sell,they'll disappear from the market.
Education could be a factor although I'm not holding my breath on that one.You'll notice Energy Secretary,Steven Chu's deafening silence on the issue.
The science is there.The votes are out on whether or not consumers are ready.
Hollywood and Madison Ave. could probably invert market tastes within 90-days with an efficiency corollary to Smoky and the Bandit,The Fall Guy,and HUMMER-toting 'Governator' Arnold Schwartzenegger .
Re-defining an Americans right of passage into adulthood and quelling fear of self-emasculation will play big roles if things are to ever change.

[aerohead]

Originally Posted by MGB=MPG

its better than it was.

is that a linear calculation/formula ?

whats the figures for a .4 and the next goal a .25

my car looks slick and is only rated at .4 ; .3 +/- seems to be current industry standard ..


approach to theoretical perfection is asymptotic . effort to achieve each incremental gain increases at an increasing rate ,,

MGB,it turns out the the difference is simply a percentage difference between wherever you 'are' and where you'd think of going,so its just an arithmetic change.
If you're at Cd 0.40 and you streamline down to Cd 0.25,then,

0.4/0.25 = 0.625,or,only 62.5% of your original drag remains,a 37.5 % drag reduction.
The horsepower necessary to overcome aero drag to maintain the same velocity would be reduced by 37.5 % also,freeing up some power for hill climbing,passing,etc..
If your engines BSFC map didn't move around do to the change ( a better bet today with EFI engines ) ,given rolling resistance as a constant,you'd pick up around 18.75 % better mpg at this speed.More at higher velocity.

[aerohead]

Originally Posted by basjoos

In addition to reducing fuel costs and tailpipe emissions, reducing drag also reduces the load on the engine and transmission, potentially extending its service life, an additional cost savings.

I've always said that driving my low Cd car is like driving in my own personal tailwind or that I am drafting myself.

Yeah,and the cool part about it all,is that nothing can 'go out of tune' over time to reduce the efficiency.It's automatically designed in as a passive high-performance feature,like' porting, polishing and c.c.-ing' the body.
Unless that deer finally strikes back,your literally on Easy Street.

[aerohead]

Originally Posted by KamperBob

Phil, nice post. I know you prize accuracy especially with data so I thought you might want to correct something. Comparing Cd 0.3 to 0.1 is a three fold change but not 300% difference. It could be either 200% increase or 67% decrease depending on baseline (direction of comparison). It doesn't change your main point. A two fold change would still back it solidly.

Thanks Bob,I'm prone to self-delusion so lets see where my synapses shorted.
I ran calcs for a Miata-sized car of 17.7 square-ft Af,90.93 ft-sec,at Cd 0.30 and 0.10.
Road load @ 0.30 was 8.637 hp
Road load @ 0.10 was 2.879 hp
Delta-HP = 5.757 hp
8.637/2.879= 3 ( this is where I'm getting the 300% ( 2.879 X 3 = 8.637 )
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On the Cd part,

0.30/0.10 = 3 ( 300 % )
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Since Cd was the only variable it was looking like the exercise could be reduced to just comparing Cds,and on a percentage basis.Is this valid?

[KamperBob]

Originally Posted by aerohead

Thanks Bob,I'm prone to self-delusion so lets see where my synapses shorted.
I ran calcs for a Miata-sized car of 17.7 square-ft Af,90.93 ft-sec,at Cd 0.30 and 0.10.
Road load @ 0.30 was 8.637 hp
Road load @ 0.10 was 2.879 hp
Delta-HP = 5.757 hp
8.637/2.879= 3 ( this is where I'm getting the 300% ( 2.879 X 3 = 8.637 )
--------------------------------------------------------------------------
On the Cd part,

0.30/0.10 = 3 ( 300 % )
--------------------------------------------------------------------------
Since Cd was the only variable it was looking like the exercise could be reduced to just comparing Cds,and on a percentage basis.Is this valid?

Percentages can be tricky. It's all about context.

Cd1 = 0.3
Cd2 = 0.1

Cd1 is 300% *of* Cd2. [multiplicative]
Cd1 is 200% *larger* than Cd2. [additive]

Same for horsepower, and it doesn't change your bottom line as I said before.

(Call me a nit picker but an error of 100% seems hard to dismiss. I thought it was kind of humorous. I hoped you did too. If not, sorry.)

I wonder what my Cd is riding motorcycle...

[aerohead]

Originally Posted by KamperBob

Percentages can be tricky. It's all about context.

Cd1 = 0.3
Cd2 = 0.1

Cd1 is 300% *of* Cd2. [multiplicative]
Cd1 is 200% *larger* than Cd2. [additive]

Same for horsepower, and it doesn't change your bottom line as I said before.

(Call me a nit picker but an error of 100% seems hard to dismiss. I thought it was kind of humorous. I hoped you did too. If not, sorry.)

I wonder what my Cd is riding motorcycle...

Bob,I see what you mean and it's not picking nits at all.
I think I've been stuck in 'multiplicative' mode.Once a value is in the calculator I just carry it through.Lazy huh?
Is '3X' a more accurate representation of the Cd comparison? Before we get too far into this it would be nice to have clarity.
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With respect to your bike,Hoerner posted Cd 0.90 for un-faired MCs.Ouch!!!!!!!!!!!!!!!!!!

[mnmarcus]

Sorry for interjecting but yes, 3X or 200 percent increase... Everyone at school looked at me like I was crazy trying to make this point... We took the smallest number/the greatest number X 100(%) and called it a percent increase. I tried to say the greatest number and smallest number was the same in one trial (hey it could have been) so we had a 100% increase with the same number! Just got glazed over stares.

[KamperBob]

Originally Posted by aerohead

Bob,I see what you mean and it's not picking nits at all.
I think I've been stuck in 'multiplicative' mode.Once a value is in the calculator I just carry it through.Lazy huh?
Is '3X' a more accurate representation of the Cd comparison? Before we get too far into this it would be nice to have clarity.
--------------------------------------------------------------------------
With respect to your bike,Hoerner posted Cd 0.90 for un-faired MCs.Ouch!!!!!!!!!!!!!!!!!!

Phil, both can be correct. I think mixing the two forms risks confusion so sticking to one is best. I think 3X is clear.

Thanks for the bike Cd data point. While 0.9 is high, frontal area is so low I still enjoy 90+ mpg without any aero mods whatsoever. And it is a bike after all. Knees in the breeze is a big part of it.

Rock on!