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Geektastic - Drafting

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Sun Jun 28, 2015 6:45 am PostPost subject: Geektastic - Drafting
AK
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This is something that I had never really considered, but makes sense and seems widely known when you think about it Laughing

Dafting/following a car closely reduces Drag on following car - yes, well and good.

In the order of 50% reduction in Cd when 0.5m behind to ~10% 3m back.

But.... the lead car also benefits from a reduction in drag - at around 1-1.5m between cars.

Reading 'Competition Car Aerodynamics' at 8am on Sunday morning.... living the life: lol:
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Thu Jul 02, 2015 6:34 pm PostPost subject:
Ibiza125
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Hmm interesting theory!

Surely it's more of an advantage for the following car though?
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Thu Jul 02, 2015 9:54 pm PostPost subject:
AK
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100%.... dont have the book to to hand to give figures but the front car is a tiny gain conpared to rear at about 1m
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Fri Jul 03, 2015 8:15 am PostPost subject:
nefarious
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AK wrote:
the front car is a tiny gain conpared to rear at about 1m


Not so. In a simple model (i.e. basic car shapes and ignoring rolling resistance), the lead car's gains are very similar to the following car's when the distances are short (i.e. virtually touching). As the distance increases, the effect on the lead car drops off much more rapidly than that on the following car, which is still seeing a near 30% drag reduction at just over a car length (while the benefit to the lead car has fallen to almost zero).

The situation also moves to the advantage of the lead car when you introduce some real world factors. Firstly - lift. In the scenario where two cars are very close (i.e. less that 1/4 car length), the lead car sees almost zero effect on lift, where the following car will have lift more than halved. Depending on its aero package, this could have a significant impact on rolling resistance (to the advantage of the lead car).
Secondly, if you look at more realistic car shapes, you introduce more turbulence, which will affect the following car more severely than the lead car (up to a certain point, where the flow over the lead car is so turbulent that the benefits of drafting are marginal anyway).

You only have to look as NASCAR to see this in action - when a car drops out of the "train" it quickly loses out to the entire convoy. Also, you see them maintain the same train order for lap after lap - if the advantage to the following car was so much greater, they'd constantly be swapping places.
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Fri Jul 03, 2015 8:22 am PostPost subject:
AK
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I'll consult my geek book.... as its actually NASCAR shape tested in MIRA.
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Fri Jul 03, 2015 8:31 am PostPost subject:
AK
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MIRA tested, SCSA Stock cars - look like NASCAR type saloons

Car line astern, no offset


Change in Cd

At 3m

Following- -18%
Lead - -8%

At 2m

Following- -22%
Lead - -16%

At 1m

Following- -35%
Lead - -19%

At 0.5m

Following- -45%
Lead - -17%

Change in Lift/Downforce -ve being lift

For following car...

At 3m

Following- -17%

At 2m

Following- -22%

At 1m

Following- -28%

So, yup... crap for grip but good on straights Smile
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Fri Jul 03, 2015 8:55 am PostPost subject:
nefarious
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Interesting numbers - presumably some of the quirks in there are due to the specifics of the NASCAR aero package.

The big surprise is that the benefits to the lead car actually appear to peak at a distance of about 1m, whereas in the "simple" model, they continue increasing all the way to the touch point.

Also, I presume that the "change in lift/downforce" figures are percentage change in lift (either positive or negative). Does that mean that for a car that suffers lift at speed, this problem is actually reduced by following?
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Fri Jul 03, 2015 11:09 am PostPost subject:
foz01
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Taking of Nascar I was always of the opinion that they were a bit shit, they only went to fuel injection last year or something Laughing however seeing them up close and hearing them at the festival of speed Shocked Cool
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Fri Jul 03, 2015 11:21 am PostPost subject:
Oggy
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It certainly helped 'bump drafting' when we raced in Legends
I'm not an expert in this by any means but the more cars you had bump drafting the quicker it made the train go up the main straight as I suppose you were punching a bigger hole/gap

I remember when I first started racing and I would get passed on the straight by 2 or 3 cars in line I never understood why until I started doing it myself........if you got it wrong it ended badly Laughing
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Fri Jul 03, 2015 11:21 am PostPost subject:
AK
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lift/downforce, % change.... is more lift. i perhaps should have noted that as % lift as it may get confuzled with -ve lift = downforce

By that I would take it that a car with lift at speed would suffer yet more lift
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