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The above theory is nicely thought out but from my understanding flawed. Your last diagram is not what the actual wind does. The head wind (created by forward movement of the bike) and the side wind (the 90 cross wind generated by the 'weather') is combined and cannot be seperated. This 'new' wind is called the 'apparent wind' and is talked about a lot in sailing. Effectively the wind using your example is going to be coming from about 45 degrees from the front side. This moves onto the 'wing effect'. With the wind coming from 45 degrees (or there abouts) the windward side will experience higher air pressure and the leeward side, low pressure. This measns the bike is sucked, rather than blown accross the road. The idea of sticking your leg out works in theory by disturbing the airflow on the high pressure side of the bike thus causing turbulance and reduced air pressure and effectively equalising the pressure on both sides of the bike. This I struggle to comprehend as 'the wing effect' relies on aerodynamic surfaces. Planes have very precisely shaped 'foils' (wings) to create this desired effect, and ultimately flight through lift. A motorbike is a very uneven and un aerodynamic shape with very turbulant airflow over every surface. This, it seem to me, is going to cause uneven pressure on every surface and the resultant 'wing effect' of the bike travelling with 'apparent wind' is greatly reduced, if not destroyed. Secondly, from my experience, the bike experiences buffeting worse at slower speeds. If it was the 'wing effect' and pressure then buffeting should be worse at higher speeds. With higher speeds the 'apparent wind' in moved more to the front of the bike and effectively the side wind is reduced. In conclusion, IMO the bike is pushed by the wind and not sucked and as a result sticking your knee out seems implausable. As you say, its easier with a wight board or even better, a set of beer mats and a pint in a pub! |
Sorry but the diagrams are just plain wrong. One would have spoilt air on the leeward site of the bike, and thus lower pressure. Your diagrams oestensibly show air flowing, and joining neatly at the leeward side. It just doesn't work like that.
Avo (ex Woods Airmovement BTW) |
Who's got a wind tunnel in their backyard with some smoke and HD slow speed cameras to put a end to this bs.
GB |
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It also goes some way to how I've come to think of the effect of the sidewind, so a wind coming in at right angles will, in effect hit the bike at some angle that is determined by the forward velocity of the bike and the velocity of the sidewind, presumably there is a calculation there ? It does also fit with the phenomenon of changing bike speed in a cross wind changes the affect on the bikes stability. Right, so now I'm thinking that the OP isn't quite right, and that the headwind isn't really important, but the velocity of the bike crossing the sidewind is, or the apparent wind. Ok so this makes Daves wing diagram a little more meaningfull. Right, the leg out thing (and I've never tried this, other than yesterday when to be honest on the Tenere with leather pants on it's not that easy!), so this (in theroy) still works, but not because of the reasons stated. The reason (in theory) it work is by slowing the wind on the (taking Daves diagram of the areofoil) lower part to match the speed on the upper part, and removing the 'lift', ergo more stability! Assuming you get the leg out bit matched right. I think I've got it now. Back to somthing a little less taxing, half a dozen core route vectors to work out with constantly changing traffic flows......... :happy: |
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Anyway, dug out an old book by Tom Cunliffe who knows more about this subject than I ever want to .... This is his diagram on the effects of wind to a sailing craft: http://www.smokingtailpipes.com/Gallery/ApprentWind.jpg I feel this is something of a watershed momment.. It also shows that the apparent wind is actually a combination of the side and headwinds, the apparent wind is stronger than the sidewind and the headwind! Brilliant. |
The most aerodynamic bike I owned, the hayabusa, still got affected by sidewind when I tried to wheelie it across the severn bridge. It was a little unnerving.
At it's limited top speed of 186mph, you could also feel the aerodynamics at work as you were ensconced in a bubble of quiet air while you could just feel the wind tugging at the baggy parts of the leathers. |
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There are other factors that are being left out of this you know.
If one took a section of the bike and looked at a side wind flow, the velocity would be lower at ground level, because of its friction and disturbed flow across the ground, in reality there is also likely to be uneven surface, trees, shrubs, hard shoulder and whatever, disturbing the lower air flow and the upper to a lesser extent too. Add onto that, Coander affect (stick your finger under a continuous stream of water from tap, and rather deflecting off (equal and opposite reaction) it does the exact opposite and curls round your finger)...which is to do with surface friction. GULG, rather than being diagram two, where, to be fair the air will get pushed out as it hits the bike and then try to curve back in as you have drawn it, in fact it wont curve back in, it will create a lower pressure (partial vacuum if you like). This is why the classic teardrop shape exists, and why aeroplanes (and cars) have something close to a 7 degree max angle of the panels to allow the air to flow back together without being disturbed. Anything over 7 and the air starts to spoil (hence spoilers are designed to..um..spoil, and create a lower pressure underneath to pull the car down). The real experiment to test all this is to remove gyroscopic affect, and take the rider off. Balance the bike perfectly so it stands up right on its own. Create a side wind from the left... ...Which way do you think it will fall? |
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Well...er...all the people contributing to this thread for starters...then there's the lurkers...and..um....YOU...else you wouldn't bother reading it. :sbike: |
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1. Yep, in the direction of the wind; just like a fence panel or crisp packet 2. I'm with you...who cares??? :015: But, for the record: :toothy5: ....the clue was in "removing the gyroscopic effect" ...ie wheels not turning, or in other words stationary. |
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