Date: Mon, 24 Oct 2011 21:18:37 -0700
Reply-To: "Danny C." <bruiserbabie@YAHOO.COM>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: "Danny C." <bruiserbabie@YAHOO.COM>
Subject: Re: Aerodynamics related to fuel consumption (for an 84 tin top)
In-Reply-To: <CAHTkEuKAFceBEZydbGvTAKTd_nwdwz8CG5a=HOnsEVAmOR3gXw@mail.gmail.com>
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Seriously!!! your worried about what drag there is on a roof rack. WOW!!!
Danny C.
Portland, Oregon
'84 Westy (Vanny)
________________________________
From: Don Hanson <dhanson928@GMAIL.COM>
To: vanagon@GERRY.VANAGON.COM
Sent: Monday, October 24, 2011 9:02 PM
Subject: Re: Aerodynamics related to fuel consumption (for an 84 tin top)
>
>
> On 2011-10-24, at 6:09 PM, Richard Koerner wrote:
>
> > Thanks Don, very good info. I've been thinking about building my own
> Quick-N-Easy roof rack for my 85 tin top and wanted to factor in
> aerodynamics into the decision. Of course, it might "blow" off all the
> empirical data by making a (sortof) aerodynamic box (maybe 10" tall or
> something) to hold lightweight floppy things like inflateable kayaks and
> paddles and even camp chairs and whatever. Maybe the large Vanagon frontal
> area pushes the slipstream high enough above the roofline and potential
> homemade "Rocket Box" or whatever they call them....hence no apparent
> decline in mileage. Anyway, it's not a huge outlay of cash and effort to
> give it a try for extended cross-country expeditions. Would be bummed by a
> -5 MPG result, but a couple MPG reduction would be acceptable. Still
> thinking about my design....rounded frontal end on my "box" seems
> appropriate. Also a factor is garageability with the higher roof.
> >
> > Any perceptable increase in wind noise? Whistling and that sort of thing?
> >
> > Rich
> > San Diego
> >
>
Actually, the Quick N Easy rack system is probably one of the more aero
ones out there, but anything that causes air turbulence is very harmful to
overall aerodynamic efficiency. A simple round tube metal crossbar rack on
the roof of a vehicle....those will create a very significant airflow
disruption all the way back along the vehicle.
Yes, a boat on the roof, that can really help. The Vanagon's shape most
likely causes turbulence to begin at the top of the windshield...and all
back along the roof, there is no laminar flow (better for moving through the
air)..Stick a boat up there, and the air remains attached to the boat...till
it gets to the back...then the big suction..... I recall 12degrees is the
Magic number...If you can slope something down to a point with the slope
never exceeding 12 degrees, the air will remain attached...like a drop of
water's shape...
It is counter-intuitive (Unbelievable?) how significant even a small detail
can influence the aerodynamic efficiency of a vehicle. One time in the
Pony Express open highway high speed event, my navigator and I were doing
our 'practice run' where we had a short two-way 'pre-run' to test things
before doing our 100 mile high speed run...We were limited to remain within
10mph of our stated 'target speed' which in that race was the Super Sport
speed of 165mph....but we knew the radar gun guy was going to not be there
to check...so I wanted to really try to break 200mph on one of our passes
through a 15 mile section they'd given us for practice. On our first pass,
my navigator forgot to raise his window all the way and we could only run at
165, flat out. I was appalled! How could we average 165 if we could only
max out at 165?...then the light went on..."Aha!...Put up your window on the
return leg..." Going back with the window up we easily hit 190 and still
climbing.......At that speed, yes, a down window robbed us of 25+mph in
speed...Those flag mirrors you see on street cars?....probably about 20mph
at speed, easily.
The term "Streamline"...that came from the old days before windtunnels
when guys would put dot's of oil or something else, all along the surface of
the bodies of their vehicles or airplanes or trains....then go run them at
speed and stop to check out how the liquid drops had been 'streamed' along
the surface of the body...'streamlines' I saw guys doing that
still at the track, testing different wings and tail sections on
racecars....kinda crude, but hey, we aren't F-1 when we race in the Sports
Car Club of America events...And it really worked...You could see where
there was turbulence because the oil was blown around in crooked lines or
backwards or in an unpredicted way, while where everything was 'laminar'
(smooth) the streamlines flowed smoothly along...
Don Hanson
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