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Old 05-19-2010, 11:07 AM   #1 (permalink)
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Lightbulb actual cd numbers

i have seen people do all these areodynamic mods like front air dam, under body smoothing, kammback, mirror deletes, but has anybody re evaluated there new CD to see what each mod or collection of mods actually decreases the CD by. i was curious if we could attach a reduction number to say" adding a front air dam reduces CD by .2 on average"
this way people can quantify and more accurately put in order the mods they want to
if not the reduced CD then maybe the decreased rolling resistance would be a better easier number to get
i am a numbers guy as you can tell i like hard evidence, thanks

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Old 05-19-2010, 11:56 AM   #2 (permalink)
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Measuring Cd is virtually impossible to do without the use of a windtunnel. It is made very difficult partly because we also cannot accurately measure tire rolling resistance without the use of a ramp/hill and a flat coasting run; like this:

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Old 05-19-2010, 01:35 PM   #3 (permalink)
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Cd can be gotten easy enough if you have a level "runway" and a means of measuring time and speed. See the Instructables "Measure the drag coefficient of your car" page. I have done the coast down testing like they do a couple times, and have modified the simple spreadsheet to meet my needs.
My findings can be seen here.
50% grill block + windshield wiper block + folded passengers mirror + smooth underpanneling under the engine compartment + smooth underpanneling behind the rear axel = took my Cd from 0.32 to 0.28 (a 12.5% reduction)
All of the above (except the passengers mirror was folded back out) + 4' kammback (on a sedan) = took my Cd from 0.28 to 0.245 (another 12.5% reduction)
Things to be aware of when doing coast down testing, you need to do several bi-directional runs and average the results. I have been doing 5 runs in each direction, ASME recommends 10. Also, to get accurate results, you need both high speeds (to get the Aero) and low speeds (to get the rolling resistence), so if you can't do a 60mph to 15mph run in one pass, you may have to do 60-30 and 35-10 (overlap is ok), stitch them together and run the spreadsheet. If you need help figuring out how to make it work, I would be happy to give pointers.
I believe coast down testing is the method used by basjoos to determine his Cd, and I think aerohead has used it to gauge impacts also. Not everyone has run it through the spreadsheet as I have, but it is easy to overlay the graphs on top of each other and see that you are coasting longer/faster.
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Old 05-19-2010, 01:44 PM   #4 (permalink)
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Dude, but many of us here try to quantify our mods, though outside of a windtunnel it's only an approximation, as Neil noted. Look and ye shall find. There are a few how-to threads, calculators and spreadsheets on coastdown and rolldown testing. Also, many members have included results in their modding threads (MetroMPG's cardboard boattail is good for a 15% reduction). In some cases it's easier to test the reduction in fuel consumption, which is usually about twice the Cd reduction (very ballpark here).

Quote:
Originally Posted by talldudenumber5 View Post
to see what each mod or collection of mods actually decreases the CD by. i was curious if we could attach a reduction number to say" adding a front air dam reduces CD by .2 on average"
It's impossible to say that "such-and-such mod reduces Cd by 0.02". At best, you can put it in %, but beware that this may not always be true. Every car is different, so the same mod (airdam, bellypan, wheelskirts, etc.) on different cars may have a totally different effect. It also depends on other mods, for example MetroMPG noticed while testing the cardboard boattail that it behaves differently depending on whether he had a bellypan or not, similarly whether the side mirror was folded or unfolded, etc.

Another thing to keep in mind is that a good coastdown session takes lots of time, so noone wants to test each individual mod and configuration separately.

EDIT: I see Wyatt posted first
I'll add that my rear wheel skirts + 45cm (18") Kammback reduced my Cd from 0.334 to 0.324 (only about 3%), with fuel consuption reduction around 7% @ 70km/h and 8% @ 100km/h. The testing was in less than ideal conditions, I'm still looking for a better place (straight and flat for 4-5km with no traffic).
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Old 05-19-2010, 01:53 PM   #5 (permalink)
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Quote:
Originally Posted by Piwoslaw View Post
Dude, but many of us here try to quantify our mods, though outside of a windtunnel it's only an approximation, as Neil noted. Look and ye shall find. There are a few how-to threads, calculators and spreadsheets on coastdown and rolldown testing. Also, many members have included results in their modding threads (MetroMPG's cardboard boattail is good for a 15% reduction). In some cases it's easier to test the reduction in fuel consumption, which is usually about twice the Cd reduction (very ballpark here).
MetroMPG's cardboard boattail was good for a 15% increase in FE (I think this is what you are saying). Metro tends toward doing FE comparissons at a set speed, using cruise control and a Scanguage to measure results. I don't have cruise control, so my FE numbers are kind of sketchy. There are lots of ways to do comparisons, the big thing is documenting what you do and making sure YOUR comparison is apples to apples.
I should have said, my first set of aero-mods was good for about a 7% increase in FE (from a 12.5% reduction in Cd) and my second set was good for about a 6% increase in FE (from another 12.5% reduction in Cd), so the "about 2% Cd = 1% FE" rule is pretty good.
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Old 05-19-2010, 02:09 PM   #6 (permalink)
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Well, in my case it was the other way around, but I'm willing to blame traffic for that.
Like I said, very ballpark.
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Old 05-19-2010, 03:00 PM   #7 (permalink)
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I have done coastdown tests on my Scion xA, using that Instructables page as a guide. The spreadsheet shows the *critical* nature of the RR coefficient. A small change in the RR number yields a big change in the Cd. Since you are only estimating the RR number, the Cd is relative. You would have to have done ABA testing on the same day, in order to "know" what the Cd changes are.

That's a lot of testing, and it is still an estimate/approximation and it is relative to your vehicle only.
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Old 05-19-2010, 05:54 PM   #8 (permalink)
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Delta-mpg=Delta-Cd

A back door for us is the relationship between mpg and Cd and it is based in scientific investigation and was reported in 1963 in a SAE paper.
For any vehicle,with an established mpg baseline,determined during constant speed evaluation,a 10% change to Cd will yield
5 % change to mpg at 55-mph
6 % change to mpg at 70-mph
8.5 % change to mpg at 80-mpg ( my value,reverse-engineered from data presented by Chrysler,1941 )
The first two values were verified as still valid as of 1991,in a telephone conversation with General Motors Aerodynamics Laboratory.
There is no reason why we cannot use these relationships.

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