Does a windshield help with a bike's aero?

[cRiPpLe_rOoStEr]

Originally Posted by j-c-c

Weight in these proportions and context, IMO is not a factor in this discussion.

Depending on where and how it's attached to the motorcycle, a much heavy windshield may affect handling.

[j-c-c]

How much weight difference are we talking (nobody is using say 3/8"? tempered glass,) we are not talking fighter jet wraparound large windscreens, and where are we mounting it that it is not mounted to same frame member as the front forks, which if heavy or of any weight, would dampen slightly crosswind effects on steering control. Size and shape IMO will impact steering far greater than any weight penalty.

[aerohead]

Originally Posted by j-c-c

Interesting, but not sure how applicable that was to the original discussion.

On the IHPVA drag table, for the image, 6th-down from the top, is a bicycle with a Grand Prix, racing-style wind screen, and it's Cd compared to the same bike without.

[cRiPpLe_rOoStEr]

Originally Posted by j-c-c

Size and shape IMO will impact steering far greater than any weight penalty.

A windshield following the movement of the handlebar, instead of being fixed to a fairing, may also affect the handling, yet I'm not totally sure about it.

[Grant-53]

I found that windshields attached to the steering forks do cause steering inputs from crosswinds. It is a function of the lateral fin area, side drag coefficient, and the distance from the center of pressure to the steering axis. I attach all my fairings to the frame near the head tube. I get the best results using both a front fairing and a tail box.

[j-c-c]

Originally Posted by j-c-c
Interesting, but not sure how applicable that was to the original discussion.
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Originally Posted by aerohead

On the IHPVA drag table, for the image, 6th-down from the top, is a bicycle with a Grand Prix, racing-style wind screen, and it's Cd compared to the same bike without.

This is from the opening post: "At highway speeds regarding mpg, in a full upright ride posture and at what point is a windscreen too big for mpg improvement?"

Your above IHPAVA reference is not likely highway speeds related, nor an upright position, nor sustainable for long in most cases being it's a crouched riding position behind a windscreen or maybe an obscure fairing.

I think this whole discussion should not lose sight of the "measurable vs significant" symmetry here.
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[JSH]

Originally Posted by j-c-c

Originally Posted by j-c-c
This is from the opening post: "At highway speeds regarding mpg, in a full upright ride posture and at what point is a windscreen too big for mpg improvement?"

Your above IHPAVA reference is not likely highway speeds related, nor an upright position, nor sustainable for long in most cases being it's a crouched riding position behind a windscreen or maybe an obscure fairing.

I think this whole discussion should not lose sight of the "measurable vs significant" symmetry here.
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I gave you real world data in post #15. A barn door windshield is good for about 5% improvement in fuel economy.

[j-c-c]

Sorry, I was referring to the "6th down from the top" as the example given, My bad.

[Nissandriver]

I am interested in knowing the answer to the question.

I think my screen might improve my aerodynamics. The bike seems to accelerate faster from 70 to 80 mph with the screen on versus with the screen removed. There's no doubt that it is much more comfortable above 50 mph with the screen than without it. It removes all the wind pressure from the upper half of my stomach, my chest, my shoulders, and most of my helmet.

I'll try a little experimenting this summer. I'll run one tank with the screen and then one without the screen and keep repeating the sequence until it gets cold. Maybe I will see some difference in mileage? I've been getting 54 mpg repeatedly since March. The three tanks in February and early March were 65 mpg but that was a lot of riding on dirt roads running between 25 and 50 mph. Since March most riding has been 50% highway at 50-60 mph and 50% in 1st and 2nd gear on forest service roads and trails.

The crosswinds are crazy on this thing! When they predict gusts above 40 mph, I don't ride anymore.

You might notice I tried to set my bags to act like a boat tail and I bought a modular helmet thinking it would be more aerodynamic than a motocross helmet. I'm tempted to raise the screen more. That might take the wind pressure off the top 2-3 inches of my helmet.

[aerohead]

Originally Posted by j-c-c

Originally Posted by j-c-c
Interesting, but not sure how applicable that was to the original discussion.
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This is from the opening post: "At highway speeds regarding mpg, in a full upright ride posture and at what point is a windscreen too big for mpg improvement?"

Your above IHPAVA reference is not likely highway speeds related, nor an upright position, nor sustainable for long in most cases being it's a crouched riding position behind a windscreen or maybe an obscure fairing.

I think this whole discussion should not lose sight of the "measurable vs significant" symmetry here.
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Whoops!
I'll look at Hoerner's stuff.
He'd reported Cd 0.90 for an upright motorcyclist without any aero aids. And I think he shows some Cds for alternate configurations. It's dated material, but may have some general utility.
Alex Tremulis used his material to design his 'Gyronaut', Bonneville LSR, Triumph motorcycle streamliner.