Date: Wed, 23 Jan 2008 14:21:22 -0800
Reply-To: Tobin Copley <tcopley@SFU.CA>
Sender: Vanagon Mailing List <vanagon@gerry.vanagon.com>
From: Tobin Copley <tcopley@SFU.CA>
Organization: Copley Research
Subject: Re: NOVC Comment on fuel economy, drag and velocity
Content-Type: text/plain
Interesting data, and not hugely surprising. The Bimmer *does* have pretty good aerodynamics, and adding stuff hanging off the back is going to kill those aerodynamics.
I'd be interested to see a similar comparison of bike on rear rack on a Vanagon (ideally a westfalia), with the bike attached parallel to the rear lid, wheels level and bike upright. My bikes are pretty much tucked in behind the vehicle, in what I expect is a sizeable vortex at speed. A few inches of my deep section rims (with bladed spokes!) protrude sideways into the airflow off the side of the vehicle, and my guess is those rims/spokes alone have a greater contribution to additional wind drag than the whole rest of the bike + rack (which is tucked in nicely in back).
I'm thinking on longer trips I should just knock the wheels off the bikes and either chuck the wheels inside, or toe strap the wheels to the bike frame (or rack). As it is, I carry my spare wheel sets inside half the time already (the other half I toe strap 'em to the bikes).
Never really thought that much about wind drag from the rear-mount bike carrier on my Westy. My decision criteria for whether to put bikes and/or wheel sets on the rack or in the bus are usually more along the lines of "will my bike stuff get grungy with road spray" or "do I need a less obstructed view out the rear window on this leg of the trip" or "do I want to hide the bike stuff a bit" than aerodynamic considerations... but that doesn't mean it's not worth thinking about!
T.
On Wed, 23 Jan 2008 16:36:53 EST FrankGRUN@AOL.COM wrote:
> This past weekend, SWMBO and I delivered my daughter's 21-speed full
> suspension mountain bike (XMAS present) to her at her Humboldt State
> University
> address from southern California. It was mounted to the trunk with a
> strap-based
> temporary carrier. The Vehicle in question is our 2005 BMW 330i with
> performance
> package. Average velocity along interstate 5 was 80.6 mph as reported
> by the
> computer and BMW nav system, with a high speed of 95 and a low of 44.
> Trip fuel
> economy from SoCal to Arcata was 19.8 mpg of premium at $3.76 per gallon
> average. Trip economy for the return venture was 28.4 mpg with an
> average speed of
> 82.3 mph. Same tires, same roads, no attempt to resolve net height
> variations, temperature differences, precipitation levels. Probable
> weight delta of 22
> pounds less for the return trip given the removal of bike and carrier, but
> addition of arctic temperature clothing.
>
> Take Home Lesson: 30% fuel economy loss at average speed of 80 mph with
> mountain bike perpendicular to direction of propagation. Not a lot of
> surface area.
> Large effect.
>
> For your amusement and perusal.
>
> Frank Grunthaner
>
>
> **************
> Start the year off right. Easy ways to stay in
> shape.
>
> http://body.aol.com/fitness/winter-exercise?NCID=aolcmp00300000002489
>
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