Does a windshield help with a bike's aero?
 

At highway speeds regarding mpg, in a full upright ride posture and at what point is a windscreen too big for mpg improvement? I am not including any weather riding/comfort improvements that a windscreen might provide.

Scanned 5? pages, didn't see this topic addressed.

Depends on the shape. Full upright would be worst, compared with semi-recumbent or cafe racer crouch.

I don't know that any windscreen is more efficient than just the rider. Unless you're ducking in the little bubble of a sport bike, I don't think they add any aero improvement and are entirely for comfort purposes.

I had a cracked windscreen completely break away at full throttle on the CB750, and I want to say I gained another 10 MPH of top speed after that. It was making that big of a difference in drag.

The folk wisdom on the Salt Flats is that it's not how you make the hole, it's how you close the hole.

http://2.bp.blogspot.com/_X_tJstZ1fV...1600/spuik.jpg
2.bp.blogspot.com/_X_tJstZ1fVE/TRILN62i6xI/AAAAAAAAAHk/-h906_09Phw/s1600/spuik.jpg

Were I to expose myself without a cage, I'd want a teadrop helmet with Bluetooth and internal rear-view cameras, an airbag vest in a backpack, and a tailbox. Maybe a bug deflector?

Sounds right to me, with the bug deflector simply being your visor. Certain days and times of the year that can be a real PITA, but usually it's a bug here or there. One time it was a yellow jacket up through the chin gap, and stuck in my face. Another time it was a yellow jacket that found the 1/4" fold in my full race leather suit at the back of the neck at 80 MPH, and his response to surviving all that was to sting me in the center of the back. Well played, but I live on, and he does not.

Kevlar neck gaiter? :)

It will depend mostly on the shape and inclination of the windscreen, but it's still most often way more efficient than having no windscreen at all. Even if the windscreen is a little more upright, like you'll be more likely to see on a Harley, if it's quite curved, may deflect better the air than the rider's torso would when directly exposed to the air flow.

I just don't see how a windscreen is likely to increase efficiency, because to provide comfort to the rider it must necessarily increase frontal area beyond the exact dimensions of the rider.

A windscreen that doesn't increase frontal area by definition would have to be smaller than the rider. What's the point of a windscreen that doesn't prevent wind from hitting the rider?

I doubt I'd notice a difference in a tuck on my bike with the windscreen there or not. Maybe my chin would be spared the feeling of a strong man shoving my head backwards.

Only advice I have is to try the Julian method of throttle blocking to test the various top speeds at a given throttle opening on a given stretch of road. My guess is the bigger the windscreen, the slower you go, which is why sportbikes have windscreens that look like this.

https://www.motorcycledaily.com/wp-c...daf4iside2.jpg

If you rest your chin on the tank, your eyes will finally peer out through the windscreen, and the top of your head will still stick above it.

https://resources.stuff.co.nz/conten...5989458428.jpg

The problem isn't the bike, it's the rider.

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.

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