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Quote:

Originally Posted by Gas_Up_Lets_Go (Post 122700)

One thing I absolutaly agree with you, it's easier with a whiteborad, and being in the same room!

------

I have infront of me a number of crude sketches depicting various windflows over a symetrical (assuming the bike is symetrical - which it isn't, well not perfectly anyway), and I'm still happy that the OP is correct, regarding the flow of air over the bike.

What has perplexed and vexed me over the weekend is the relationship of the headwind to the sidewind, given that the headwind isn't actually a moving force, it's just the realionship to the bike that has the effect of an oncoming wind. I can safely say it gave me a headache just thinking about it!

At some point, where a sidewind exceeds the 'power' of the headwind then things are going to go wrong, so this might as well be ignored for now. Lets concentrat on a relatively small sidewind force of 9 meters per second (about 20mph) , against say a 27 meters per second (about 60mph). To further simplify this, I'm going to take a sidewind at 90 degrees to the bike, the results change as the asngle of incidence becomes smaller than 90 degress and greater than 90 degrees.

You'll have to forgive the rough drawings.

Under normal (no sidewind) riding the bike would part the oncoming air (headwind) in a nice equal pattern

http://www.smokingtailpipes.com/Gallery/Headwind.jpg

Assuming the bike is stationary, the sidewind would part something like this.

http://www.smokingtailpipes.com/Gallery/Sidewind.jpg

But when both are present, the sidewind, having less velocity than the headwind would not be able to pass around the motorcycle in the same way due to the velocity of the headwind being greater than the sidewind, as airflow is easily defelcted, the result would be that the sidewinfd flow would pass to the rear of the bike, this is what would cause the lower pressure on the widward side of the bike

http://www.smokingtailpipes.com/Gall...26sidewind.jpg

Therefore you get higher windflow down the side of the bike on the windward side, thus lower atmospheric pressure.

I've looked at the possibility that the power of the sidewind is somehow expelled as energy, but there is no other energy that is being shown, the scrub off the tyres isn't really evident (you don't get black line down the road) and the bikes dont move in the direction of the wind anyway, there is no noise (but that is difficult to demonstrate or prove due the the windnoise being created around your helmet), no heat is being created, so I can't see any evidence of the winds power being converted to energy. Therefore I can only conclude that the wind is still there and being deflected. Of course a bike isn't a 2 dimensional object, so wind can deflect over and under the machine, I haven't even thought about the complications a 3d view would give.

Another thing that strikes me, this topic is 4 or 5 pages long and other than the OP there has been no other attempt to describe why a bike moves in the way it does when in a sidewind....... lots of disagreements, but no alternative theory.

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

Quote:

Originally Posted by uberthumper (Post 122704)

. You mentioned having some trouble figuring out the relationship of headwind to sidewind...
.

Not quite as you understand it, or maybe as I've said it..... My issue is this, and I agree now that the rather crude diagrams are a little off-kilter!, the side wind cannot be affected by the headwind, mainly because the headwind doesn't exisit, it is just a relative wind in relation to the forward movement of the bike (Newton rules! equal and opposite actions), the sidewind has no 'forward' (in relation to the bike) movement, just the direction it is heading in, so in conclusion my third diagram is way off, and I'm now inclined to think that the actual flow of the wind is more likely to flow as in the second diagram

Quote:

Originally Posted by DickyC (Post 122763)

The ....<snipped, as it's quite long>.....a pint in a pub!

Dicky, that's well put and explain's the gaps in uberthumpers explanation of his theory to me, Thanks.

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:

Quote:

Originally Posted by DickyC (Post 122763)

This 'new' wind is called the 'apparent wind' and is talked about a lot in sailing.

I knew the old Day Skipper Course would come into play at some point (althoug I confess I was never a 'true' sailor, only ever under power, and I only took the course so I could use certain launchways...)

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.

Quote:

Originally Posted by Gas_Up_Lets_Go (Post 122803)

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.

Naturally. Clever chap that Mr Pythagoras. ;)

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?

Quote:

Originally Posted by Avo (Post 122895)

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?

Over?

Quote:

Originally Posted by Keithblade (Post 122914)

Who cares!:guitar player:

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: