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Old October 5th, 2011   #1
thomasacombs
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Default Does fuel economy vary with windscreen position

Has anybody figured out how fuel mileage varies with windscreen position? It stands to reason that one would get worse fuel economy with the screen in the up position, but has anybody experimented with this?
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Old October 5th, 2011   #2
Wjbertrand
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Default Re: Does fuel economy vary with windscreen position

I tried to do this with an MCN test bike back when the 13 was introduced using the instantaneous MPG gage and I saw as much as a 3mpg difference. Having said that, it's important to point out that this was not a scientific experiment because I cannot control for imperceptible wind or slope changes during my test. I did try to maintain as constant a speed as possible though. To really test it I think one would have to burn through a full tank of fuel for each condition (shield all the way up or down) and make sure the same route was followed for each tank. A week's/tank's worth of regular commuting at highway speed for each condition might work. I doubt you would see much effect at in-town speeds.
No matter the experiment it's still difficult to control for changes in temperature and weather conditions which will also have an affect, but it would be a better test than the brief one I did.
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Old October 5th, 2011   #3
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Default Re: Does fuel economy vary with windscreen position

It sure does. On a level stretch of freeway in Colorado I noted a 4 mpg difference between the down position and up far enough to get in a silent bubble. I have attained what I feel is a good compromise between drag and silence by 1) enlarging the cutout at the base if the stock windscreen to double the area 2) Installed brackets on the lower ws mounting bolts to kick out the bottom about an inch. This moves the top of the screen about 2" closer to your face and together with a Laminar Lip makes for a turbulence-free, if still noisy, ride. Speed is critical, too. I like to cruise at 4K rpm max,where 50+ mpg is easy. Much above that the square-function drag really cuts mileage.
Big barn-door windscreens are comfy but terribly inefficient,IMHO.
Tim Shevlin #1183
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Old October 5th, 2011   #4
thomasacombs
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Default Re: Does fuel economy vary with windscreen position

Thanks. Recently I ran the tank dry because I figured I had to be getting at least 40mpg, as my commute is mostly highway. Tank ran out at 275 miles since last refill, which was way earlier than I expected. Turns out i was getting a mere 35.9 mpg (275/7.66). My computer indicated I was getting 41 mpg, which was not true.

I tend to ride with the screen almost fully up.

I'm planning a BIG trip from NYC to Tallahassee FL (including a detour to the dragons tail) and want to get the best mileage possible.
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Old October 6th, 2011   #5
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Default Re: Does fuel economy vary with windscreen position

I have had the same results as those reported above. No wind, very long straight flat stretch of road, 55 mph, the difference between full up and full down was 3-4 mpg between full up and full down on the instantaneous readout. (I let it give a reading after changing the shield position, then took the next one so the reading wasn't based on mixed positions.) Didn't test intermediate shield positions. That was when I first had the bike. Were I to do it again now, I would test full up versus 1/3 up, since it just seems as though about 1/3 up gives me the smoothest air flow, rather than shooting it upward and into my chest.
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Old October 6th, 2011   #6
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Default Re: Does fuel economy vary with windscreen position

I have found that, starting with the screen fully down, holding a fixed throttle position, then gently easing the screen up bit by bit, there comes a point when the wind noise drops and the bike noticeably speeds up by a few mph. To get back to my original fixed speed I have to ease back on the throttle. In my book that means more mpg. The screen position that achieves this is far from being fully up.
Has anybody else tried this test?
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Old October 6th, 2011   #7
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Default Re: Does fuel economy vary with windscreen position

Interesting. Intuitively, it seems that you are right.

I am planning a long trip next month. I will have to test this out. Ill do a few tanks with the shield up and down just to see how the mileage changes.
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Old October 6th, 2011   #8
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Default Re: Does fuel economy vary with windscreen position

As the screen is raised the effective vent area at the base enlarges due to tilt. (track geometry) At some point the vacuum hits a minimum before the barn door effect of the more-upright shield takes effect. Just a guess.
Tim #1183
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