Does a windshield help with a bike's aero?

[redpoint5]

Originally Posted by j-c-c

My thinking was a rider has to be completely a high drag obstruction, and a small to medium windscreen might deflect enough air smoothly to lower the total overall drag, even by adding some drag of its own, ideally there would be a sweetspot, but I'm not reading that so far with these replies.

As freebeard explained, it matters less how you punch the hole, and more how you close it.

The boxfish has an abrupt square face, but because it tapers at the rear, it's pretty efficient.

[cRiPpLe_rOoStEr]

Originally Posted by j-c-c

My thinking was a rider has to be completely a high drag obstruction, and a small to medium windscreen might deflect enough air smoothly to lower the total overall drag, even by adding some drag of its own

Sure.




I guess you can (well, barely at least) notice the inclination I was talking about.

ideally there would be a sweetspot, but I'm not reading that so far with these replies.

There is no rule-of-thumb without taking the rider's height and position into the equation. I guess the stock windscreen of a Suzuki DR 800S for instance, even though it's not curved enough to decrease the amount of drag from its mere presence, would benefit me more than a really short friend who maybe can't even have a comfortable and safe riding position in one.

[j-c-c]

I would think without any proof, that every example you posted would be a better solution drag wise than nothing at all. As none are IMO full size, and are slightly larger than a what I would call a small/token windscreen. or not too hot and too cold.

[redpoint5]

There's no chance that the sheet of lexan on that scooter improves aero, unless the rider sitting behind it has a two foot wide square head.

Take something of that size, drive 55 mph, and then hold it out of your car. It will tear your arm off.

Windscreens aren't for aerodynamics, they are for comfort.

[JSH]

Yes, a large barn-door windscreen will reduce drag and increase fuel economy. When I first got my 2002 BMW R1150R I put a 22" Parabellum Sport / Touring screen on the bike. When I took it off and replaced it with a small national cycle screen the average fuel economy dropped a couple MPG. (~ 42 to 40 mpg)

I got rid of the Parabellum because if I got a gust of crosswind just right it would shake the bars and I don't like the feeling of a windshield steering the bike at 70 - 80 mph. It was also hot in the South in the summer (but nice when riding to work in the winter with temps in the 30's)

The clear National Cycle shield was a good compromise (This set-up was great for commuting and two-up touring. I would dare say better than the R1200RT I have today although I really like the electronic adjusting windscreen on the RT)

Parabellum


National Cycle


National Cycle (Clear)

[JSH]

Originally Posted by Piotrsko

Im old enough to remember Craig Vetters early efficiency wrap around fairings for Honda 750's They were pretty good at increasing bike speed and bug deflection

Vetter is still at it. This is the fairing he built for a personal trip to Alaska. The reduced drag allowed him to increase the final drive gearing and reduce cruising RPM 15% while still maintaining highway speed. He managed 59 mpg at 65-70 mpg with that ancient carburated single even into a strong headwind.

[cRiPpLe_rOoStEr]

Originally Posted by j-c-c

I would think without any proof, that every example you posted would be a better solution drag wise than nothing at all.

Seems like you got the point.

As none are IMO full size, and are slightly larger than a what I would call a small/token windscreen. or not too hot and too cold.

Yet some full-size windscreens, as long as they're curved and inclined enough to act as a wind deflector, could improve aerodynamics.

[j-c-c]

Originally Posted by cRiPpLe_rOoStEr

Seems like you got the point.




Yet some full-size windscreens, as long as they're curved and inclined enough to act as a wind deflector, could improve aerodynamics.

However, I don't yet see consensus here on these two points.

[cRiPpLe_rOoStEr]

Originally Posted by j-c-c

However, I don't yet see consensus here on these two points.

You won't see consensus here on that matter. There will always be someone claiming every windshield other than the ones fitted to crotch-rockets is never an aerodynamic improvement

[redpoint5]

There is consensus; try it and measure the results.