Bandit Alley
MODEL SPECIFIC => BIG BANDIT BANTER => Topic started by: banditone1250s on July 02, 2007, 07:13:17 PM
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I was thinking that a lot of air got around, through, or over my windscreen and now I think something else. I think that a lot of it comes from the "hole" in the very front bottom of the screen. I wonder why that "hole" is there??? I have looked at some after market windscreens and they do not seem to have that same "hole" in the lower front. Any body have any ideas about the subject. I would appreciate any thoughts that you may have...Banditone1250s... :burnout:
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I was wondering that myself.
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My new MRA has the same hole:
(http://i132.photobucket.com/albums/q13/ZenMan33/WS9.jpg)
So does the new Givi screen:
(http://i132.photobucket.com/albums/q13/ZenMan33/WSGivi.jpg)
So does the Zero Gravity screen for older Bandits (only pic I could find):
(http://i132.photobucket.com/albums/q13/ZenMan33/WSZero.gif)
It's there to reduce turbulence and buffeting. The air flowing through the hole up on the inside of the screen mixes with the air flowing up over the top of the screen to smooth it out. The airspeed is reduced by the time the air mixes back together. Without the hole, the air coming off the top would "tumble", and you'd get a lot of buffeting in your face.
What you have to realize is, these small fairings and sporty windscreens are not designed to give a big bubble of dead air devoid of any wind at all. They are designed to reduce air pressure to a more comfortable level.
If you want total dead air space, you'd have to have a windshield the size of a barn door.... even a convertible car has turbulence with the top down.
Here's the pamphlet I received with my Vario screen:
FUNCTIONAL DESCRIPTION
Please carefully read these instructions. You will get the most out of the adjustable spoiler if you thoroughly understand it's features.
The spoiler serves to reduce air pressure on the rider's helmet and upper body without causing disturbing air turbulence.
The spoiler has six different positions, enabling the rider to make adjustments to suit his needs.
The higher the spoiler is set, the more air will flow between the spoiler and the windscreen.
The airflow speed is reduced by means of the angle that opens in the direction of the rider, thereby providing at higher speeds the same relaxed atmosphere that a rider experiences at slow speeds.
It then goes on the explain how to adjust the spoiler.
What does the hole have to do with the spoiler? It works on the same principle, of slowing and mixing air to reduce airspeed and eliminate turbulence. I'm no expert on aerodynamics, but I understand the engineering behind these windscreens a lot better now than I did before.
I know from my V-Strom that a tall windscreen without a spoiler creates a lot of buffeting. Ask any V-Strom owner and they'll tell you it's a big problem. Go to the V-Strom forums and there are threads upon threads on the subject.
The hole helps, but the spoiler helps a lot more. That's why I bought a screen with a spoiler. :wink:
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I made a crude visualization on how I think it works:
(http://i132.photobucket.com/albums/q13/ZenMan33/WSdiag-1.jpg)
When the air curves around the screen, it slows it down. The air coming up from the vent straightens it out.
A spoiler adds more curves to slow the airspeed down even more.
(http://i132.photobucket.com/albums/q13/ZenMan33/WSdiag2.jpg)
That's how I understand it, anyway... :roll:
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I tried to make sense of it from the perspective of flaps on an airplane. Your drawings are correct in that with the slot, the turbulent air is lessened (is that a word?) or "placed" higher in the slipstream. With the curve, the air actually accelerates and the pressure drops (Bernoulli's principle) relative to that air going through the slot. I think the slot helps to place the pressure differential of the two airflows closer together and therefore reduce a ton of turbulence.
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ZenMan is right - the hole is there to reduce turbulence and negative pressure. The screen on my FJR was great but in the fully up position there wasn't enough air going through the front and while it deflected an awful lot of air away from me, the pressure effect felt as if there was a hand on my back pushing me forward. The air vent reduces the pressure so it's there for a reason and not a whim on the part of the designer.
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Like I said, I'm no expert on aerodynamics. :roll:
I figured with the drawings at least we'd have a reference to figure it out from. Your airplane wing analogy is a good way of looking at it.
So if the curve speeds it up creating negative pressure, like lift on a wing, then the hole must be there to equalize it, thus reducing turbulence. The spoiler controls the amount of air passing over the top, thus reducing airspeed.
However it works, the result is a comfortable airspace to ride in without turbulence. Works for me. :bigok:
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Test! We need a test!
Somebody block the hole and see how it changes the buffeting!
(Not me, I'm still trying to find the time to adjust the handlebars and post pictures ;) )
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Test! We need a test!
Somebody block the hole and see how it changes the buffeting!
Thinking the same thing. I may try to stuff a towel in there some time to test the effects. If I'm real motivated, I'll tape some strings of yarn on the outside and inside of the screen. It would be cool to see some wind tunnel/smoke visuals. Just thinking too, I wonder if a long piece of yarn from the lip of the screen would show how far the smooth slip stream extended, how high it went, and at what point it starts to burble?!
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I'm wondering if you stuffed a towel in there if it would stay? If the towel doesn't stay you could tape it off from the front for this experiment. Just use a tape that will peel off easily without leaving behind a bunch of residue to clean up.
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Try cutting a piece of cardboard just a little bit bigger than the hole, and taping it over the hole using a clear packing tape.
The yarn I would go about 3-4", if they were too long the slipstream would grab them and pull them out too far, or they'd just flop around too much.
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Why do I have a new episode of mythbusters going on in my head? :grin:
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Why do I have a new episode of mythbusters going on in my head? :grin:
That, or someone is going to end up with a Darwin award. "Motorcyclist dies with yarn wrapped around neck, news at 11."
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:toofunny: :toofunny: :toofunny:
"Motorcyclist dies with yarn wrapped around neck, news at 11."
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Oh, if people can do the Mad Bastard Scooter Rally wearing capes, a bit of yarn should be fine ;)
linky (http://www.cmgonline.com/magazine/gallery2/v/MBSR2007/Sat_Mark_cape.jpg.html)
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Towel test already performed by your humble tester last summer, with the stock screen and Laminar Lip Touring model.
(http://i73.photobucket.com/albums/i234/H2RICK/BC%20Trip/th_DSCF0536.jpg) (http://i73.photobucket.com/albums/i234/H2RICK/BC%20Trip/DSCF0536.jpg)
I did NOT cover the vent in the stock screen since the ugly setup I had with the Laminar Lip and towel/duct tape seemed to work satisfactorily. No buffeting and the APPARENT wind blast was reduced (subjectively) by about 30% or so. Very unscientific and subjective and I'm sure the towel thing did a lot to disturb the airflow....but it worked for me when compared to just the stock screen alone or the stock screen with the "untowelled" Lip on it.
Now if I was 5' 9" or so (instead of 6') it would have worked even better. :grin:
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Wrong hole, Rick. :lol:
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Yep, Zenman, I've heard that about twice in my lifetime. I blamed it on the booze..... :duh:
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All joking aside, the hole we're talking about is the one down at the bottom of the screen, above the headlight. Not the space between the windscreen and the spoiler, where you have your towel taped up. :roll:
I agree, probably the booze... :lol:
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Well, it's been awhile since this thread has been revisited. It's Labor Day weekend and the girls' are sick so we're not taking the dog to the river for swim lessons. So, what to do to fill my boredom?! Hey, is that a ball of yarn? Why yes it is!
Taped 2" long pieces: a). front of screen, centered, right at hole. b). front of screen, centered, half way up screen. c). two approx. 3" either side of center right at the top edge of screen. d). inside of screen, centered, half way up. Also, taped about a 3' piece coming off the top edge of screen, centered.
By the time I reached 20 mph, all of the strings were flowing smoothly and did not change...only went up to 60. The long string "rode" nearly horizontal (canted up slightly) at a position that was about 1" higher than the top edge of the screen, therefore hitting me low chest/upper abs. I then placed a wash cloth into the hole. From zero to 60 all of the strings were flopping around aimlessly except for the one in item "a" above. At some point I actually had to grab the long string to pull it up as it was laying dead down towards the forks. It then stayed up, but swayed from left to right with a lot of burbling in the length of the string.
So there you have it, my Labor Day weekend.
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I'm assuming that this test was done on the stock windscreen?? The test you have done is also done on aircraft wings and sails on sailboats. Sailors refer to them as tell tails, and use them to trim their sails. Yarn works quite well but old recording tape from a cassette player is even better. Any curved shape with air driven over it fast enough will produce lift and when the tell tails are all streaming back thats whats happening however when the tell tails no longer stream back then the wing or whatever curved shape you are testing is stalled. When a wing stalls the air is turbulent, detaching itself from the surface, hence the buffeting. The 3 foot yarn that you used in the centre is a bit long, you can get the same or better result with a yarn about 6" long as it will give you a truer angle. Your a curious person Skyrider, I'll bet you took clocks apart when you were a kid..
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I'm assuming that this test was done on the stock windscreen??
Your a curious person Skyrider, I'll bet you took clocks apart when you were a kid..
Yes, the stock screen. And, yes, I took all kinds of stuff apart as a kid...usually never worked again either. Thankfully I have two girls and they're not interested in mechanical stuff.
Good idea with recording tape. I only thought of yarn because one of the types of helicopter I used to fly used yarn as a trim device...quite effective.
Sky
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Interesting, I'm a pilot myself. The only time I used tell tails was sailing but I'll bet you used the yarn on the helicopter for yaw? I once test flew a friends ultralight which I referred to as a lawn chair with wings and the plane came with a yarn on the front to check the yaw, no more needle ball, just yarn! Of course he also had a little tube with a wire down the middle and a shirt button on the wire, a hole in the bottom blew the button up and that was the airspeed which worked fine in level flight but when you turned steep the centrifugal force pushed the button down, cool. Anyways interesting to read your report...
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Pilots - that makes three of us, at least. I see yarn on experimental helicopters all the time still - go figure. Isn't that what the skid ball is for?
But on to more important things Roberto.....did you ever equip your bike with the Givi gear?
Phil :motorsmile:
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But on to more important things
Phil :motorsmile:
More important than flying?! Get out!
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I have purchased the Givi windscreen but haven't done anything on the bags yet, still soaking up the info. Put some grip puppies on yesterday and sometime in the next day or two I'm going to install the new frame sliders. I'm going to put a new can, probably a Yoshi on but will wait for winter. By the way does anyone know the proper torque for the engine covers as I have to replace a few bolts when I install the sliders??
Yup I could talk flying forever, over 9000 hours and loving every moment....
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But on to more important things
Phil :motorsmile:
More important than flying?! Get out!
OK - you're right. And I get to do it for a living - WOOHOO!!
What I should have said was "Now on to less expensive things" :duh: