Date: Mon, 24 Oct 2011 18:27:24 -0700
Reply-To: Alistair Bell <albell@SHAW.CA>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Alistair Bell <albell@SHAW.CA>
Subject: Re: Aerodynamics related to fuel consumption (for an 84 tin top)
In-Reply-To: <1319504948.26431.YahooMailClassic@web83602.mail.sp1.yahoo.com>
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It is not the frontal area that kills the aerodynamics of the van, it is the big low pressure area at the rear. Don 't get me wrong, reducing frontal area would improve things, but fairing the rear would work wonders :)
I think my improvement in gas milage with canoe on top (which increases frontal area) is that the canoe modifies the airflow at the rear.
alistair
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
>
> --- On Mon, 10/24/11, Don Hanson <dhanson928@GMAIL.COM> wrote:
>
> From: Don Hanson <dhanson928@GMAIL.COM>
> Subject: Aerodynamics related to fuel consumption (for an 84 tin top)
> To: vanagon@GERRY.VANAGON.COM
> Date: Monday, October 24, 2011, 5:29 PM
>
> For appx. 3 weeks I ran with my Quick-N-Easy roof rack on my vanagon,
> having to carry some long stuff. It's a full width two-bar rack, with the
> cross bars made from Alaskan yellow cedar, appx. 2x4" and just a little
> narrower, side to side, than my stock mirrors. The 'bars' have large radius
> corners. I was working about 60mi round trip from home, but otherwise
> driving in the same area and at the same speeds I do in 'real life' (when I
> am doing what I want, not working for pay)
>
> My vanagon is a 5sp with a 2.0 liter Jetta inline gas motor and I've
> always kept track of my fuel mileage, every tank for about 4 years now.
> I've averaged ~23mpg really consistently. If I use premium fuel without
> corn, I get almost 2 mpg more. If I drive at 75 all day, I get a few mpg
> less. On my 4 or 5 fill-ups while sporting that roof rack, my gas mileage
> declined by almost 2 miles per gallon.
>
> Then, two tanks ago I took the rack off...it is Quick and Easy, so why
> not, plus I'm out of work again so no need...The gas consumption again is
> 22.++ or 23 and 'small change'.... So there it is, a statistical sample of
> one, but it is accurate. I fill at the same pump, letting the same auto
> shut off stop the flow (and spew some backwash out on the station's
> driveway) each time. I fill at about 275 miles run. I had the same weight
> load aboard. I only carried a long ladder one time (30 miles) on the roof.
>
> In case anyone is interested in how aerodynamic drag sucks down the gas...
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