Windshield drag?

[toaster404]

1400 touring.  3 hours without the windscreen, mixed light twisties (30 mph to 45 mph) and a good Interstate stretch at 70 mpg.  Non-scientific.  With screen, usually 36.5 mpg around here, that general type of ride.  Without screen, 44.8 mpg

Is there really a substantial mileage difference from the screen, or did I just hit an exceptional day of mileage? 

[Kev m]

My SWAG (from YEARS of running bikes with detachable windshields). NO, it's not all the shield.

There are so many variables, rate of acceleration, ambient temperatures, headwinds/tailwinds, etc.

I run a tall windshield on my Harley more often than not, and though mileage DOES fluctuate (anywhere from about 40-46 mpg) I really think I see the biggest differences with ambient temps and winterized fuels. For instance I filled up yesterday after running the windshield the entire tank, and about half the tank actually running 2-up, and I got 44 mpg. I'm sure there are times I run without the windshield that I don't get any better.

It's also possible that the windshield changes YOUR actions. More protection from the wind might mean faster average speeds or heavier acceleration since the sensations of speed are somewhat diminished by the wind protection.

It might be an interesting experiment though to get 2 identical Cali's side-by-side with similar weight riders, one with a windshield and one without, then fill them up and ride together for a day and look for differences. It wouldn't be gospel, but it could be a fun experiment.

[Moto]

An easier and more definitive experiment might use the electronic fuel mileage gauge. Since it works by counting the firings of the fuel injectors (I believe), it could be used for two or more brief runs where conditions were close to constant. You could run the bike up to a particular speed, note the "instantaneous" mileage after a mile or so, then stop, remove the windshield, and repeat. This could be done at a time of day where the wind was about constant or even calm.

If there was a concern about the slope of the highway, etc., a small round trip could be used so that both trials covered the same course took the measurement at exactly the same location, and a few repetitions could be done if the result was not obvious. The rider would have to think about the other factors Kev mentioned, and try to hold them constant. Analysis of variance could be used to look for statistical significance with a few replications.

I don't think the electronic gauge can be trusted for absolute mileage calculations, but it should be fine for comparative runs.

This seems plausible and fun if it is not too hard to take the windshield on and off (and leave it safely behind for a brief time).

Moto

[toaster404]

Makes sense, thank you.  I suspect that I simply rode a bit less vigorously, and that I likely had a little less drag on the Interstate.  I can't ride hugging the tank all day, so that doesn't count!!

The bike feels a good deal smaller without the screen, and I can tell a little bit of difference from less fork loading at highway speeds.  Not enough to make me do much. 

I also realized how hot the screen makes the bike!  So I'm looking for a smaller screen for summer.  The Guzzi one for the custom looks good, but I would have to pull the headlight braces out and drill mounting holes.  A pain.

Thanks all.

[rboe]

1981 Kawasaki 550 Ltd. Without windscreen, about 45mpg. With a slipstreamer style windscreen, high 30's. Switched to a Vetter Quicksilver and got 50-55mpg.

Big barn door windscreen are bad for mileage. Just how bad depends upon what speed you like to cruise at. When I had the 550 the speed limit was 55mph and I had to slow down to 45-50 to keep the mileage up with the large plexiglass screen on. Nice protection though. :D

I've resorted to much smaller screens now (bikini fairing? sport fairing?) and these seem to have no affect or slightly improve mileage (streamlining the wind for my chest I guess).