iloveriding
Trackday Trickster
Joined: 24 May 2007
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 Posted: 20:13 - 24 Sep 2007  Post subject: Dead Air question |
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As the topic says,, I seem to be having a reoccurrence of this situation. I regularly ride my faired Bandit to work on the M5. I quickly get up to about 85mph and try to stay there. I carry a tank bag so cant get down under the screen.
I have noticed that if I keep to the 85/90mph for a while all the buffeting goes and its like I am in the centre or the storm, really calm and it feels like the bike is just being eased along. My revs dont change they stay at a constant 7000.
Has any one else experienced this and why does this happen??
Drop below 85 and the wind comes back
Tony 
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TheShaggyDA
Repost Police
Joined: 14 Jun 2004
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| Posted: 20:36 - 24 Sep 2007 Post subject: |
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From another forum. You've just found the sweet spot, but without the need for a lower fairing, possibly due to your tankbag 
| Quote: | Basically lowers are smaller “windshields” attached to the fork-legs... they need not affect the height of your windshield at all... what they do is interrupt the airflow that normally would go under the windshield, usually diverting it laterally so the relatively still air being the windshield is more stable...
Basically, windshield are just air deflectors... the air stream hits the Plexiglas causing a high-pressure area, or as the aerodynamicists call it a stagnation area. The basics of Bernoulli’s principle are that the faster the moving air stream the lower the pressure ( the same principle that makes the venturi in our carburetors work). So you start with a relatively high pressure air moving up and around the windshield... High pressure air will try to seek out low pressure areas... now because this is unattached flow, the area behind the windshield is already low pressure relative to the air in front, so the air moving over and around the shield tries to fill in behind – in its rush to do it, it burbles and swirls as it trips over the glass edge... cause the familiar buffetting.
Now, enter air from below... if air enters from behind the windshield moving from bottom to top, it energizes this air right behind the windshield – speeding it up and, you guessed it, lowering the relative pressure even further, causing even a more acute “sucking” action that pulls in the higher pressure air stream moving up and around the windshield... hence, instead of being thrown higher, the swirls and burbles are thrown back (as the air tries to fill in the low pressure area) and of course smacks the rider – perhaps not like it would without the windshield, but noticeable none the less...
So what lowers do, is sort of put an air dam farther below the windshield interrupting this energizing flow and somewhat reducing the attraction of the airflow for the riders face... This wasn’t widely noticed until about thirty years ago until riders began to wonder why fairings (like Vetter, for instance) seemed to deflect more air even though the frontal/Plexiglas area was no larger than many of the conventional windshields of the day... the shape of the lower portions of the frame mounted fairing, was reducing (indeed in those cases, almost eliminating) air moving up under the windshield.
Your windshield may indeed be too low for your preferred riding style, but you can do a simple test to see if there is significant air moving behind the shield by simply riding along in clear air, and well clear of traffic, and at highways speeds; let go of the left grip and position your forearm in front of you crossways, horizontal and a little above the tank, and see if you can feel any noticeable reduction in buffeting... what you’re trying to do is slightly block any air moving up from underneath... you may have to play with it for a moment, but likely you’ll find a sweet-spot where you notice a reduction... I was just reading a note by a VTX1800 owner, who has the Memphis Fats windshield – his contention is that if one wants truly turbulence free air, then lowers are necessary (at least on the VTX1800 when using that particular combination). |
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Last edited by TheShaggyDA on 20:39 - 24 Sep 2007; edited 1 time in total |
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Noxious89123
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Joined: 10 Jun 2007
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