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Quote:
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Originally Posted by
IAM Observer Chappy
Bike Tip of the Day:
If riding in a strong cross-wind, you would expect the bike to be blown over, off the vertical, in the same direction as the side wind blows; but it doesn�t, in fact the bike does the very opposite and leans into the wind! Why�s that?
Even on a perfectly still day, there�s an airflow created by the bike�s forward motion: this is the Head Wind.
The component of the Head Wind and Side Wind add to give a greater airflow down the windward side of the bike; this reduces the pressure to below atmospheric pressure on that side; the bike is then heeled over into the sidewind by the greater atmospheric pressure on the opposite side to the wind.
It�s the same principle that provides Lift on the underside of an aircraft wing: all due to the work of 18th Century Swiss Mathematical Physicist, Daniel Bernoulli.
To stop the bike leaning into the Side Wind, stick your knee out to act as an airbrake on the windward side.
This slows the airflow enough to get rid of the negative pressure; once the air pressure on either side of the bike is equalised, the bike sits upright.
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Now the first thing that strikes me as a bit odd here is the suggestion that you can ride a bike upright in a crosswind without drifting off the road. Or to put it another way, his implication that leaning into the wind is a bad thing which can be avoided.
If you've got a sideways force acting on the bike (from the sidewind), then there needs to be a reacting force in the opposite direction to keep you going in a straight line. I don't think that can be disputed, unless you want to completely discard Newtonian physics - and even I'm not that presumptious
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The reacting force can only come from either the contact with the road, or from some sort of aerodynamic effect which, counter to expectations, sucked you upwind.
If the reaction is coming from the tyres, then you have a wind force pushing the bike (say) to the right at the centre of pressure, which is going to be a good couple of feet above ground, and a tyre force at ground level pushing you to the left. So your forces might match horizontally, but you are still going to be creating a moment due to the offset between the forces which will try to tip the bike to the right. To counter this, you need some lean angle, which brings gravity (acting through the centre of mass of the bike/rider) into the equation to produce a balancing moment.
This is way easier to explain in person, with a whiteboard!
On to the aerodynamic effects...
The writer describes pretty accurately how aerodynamic lift is produced - air having to travel further round one side of an object than the other, resulting in higher speed and lower pressure on the 'long' side. The problem is, the 'long' side in this scenario is the downwind side of the bike, so the lift is in that direction - pushing you downwind, not sucking you upwind as he seems to conclude.
A diagram:
Imagine that the green blob is the motorcycle, and you are looking down on it from above. The bike is travelling diagonally up and to the left, and the crosswind is coming from the bottom left. The combination of the headwind and the crosswind gives an airflow shown by the black lines, which is at an angle to the bike.
(For those that commented earlier about "what if the wind comes from the other side", it doesn't matter that your bike is symmetric, rather than the asymmetric shape shown. Either shape can experience this effect, and you could in the right circumstances get the asymmetric shape to 'lift' in the opposite direction - that's why you can fly a plane upside down)
As you can see, the airflow hits the bike and splits around it, and the air going round the right hand of the bike (top in this view) has to travel further than the air going round the left, producing a pressure differential which pulls the bike...downwind.
I can't comment on whether sticking your upwind knee out helps in a crosswind, because I've never tried it. I'm also not convinced that a bike does, of its own accord, lean into the wind with no input from the rider. I shall have to wait till the next time it's windy.
I can think of a couple of hypotheses that might explain these phenomena, if they do occur, but the one given in the OP is not, in my opinion, correct. Firstly, because it assumes aerodynamic lift happens in the wrong direction. Then because it suggests that by acting to eliminate this lift, you can ride upright. But in order to ride upright, you'd need to be creating some sort of aerodynamic lift in the upwind direction. Bit of a contradiction.
Quote:
Originally Posted by
Gas_Up_Lets_Go
To add to the OP, and the 'spoon' explanation, looking at saling craft (the ones with no parts in mechanical perpetual motion). Have you ever wondered (probably not) how they can sail into the direction of the wind, yes we all know it's by tacking, but how do you convert a force pushing you (say for instance) south into a northerly travel direction?
<snip explanation>
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This is all correct, and not a bad analogy, but you've missed out one critical detail...A sailing boat will not go upwind unless it's got some sort of keel or centreboard, which provides the reaction to cancel out the 'downwind' effect of the wind, and just leave the 'forwards' effect produced by the sail shape.
Actually, the keel is also foil shaped, and the angle of the attack it presents to the water results in lift in the upwind direction, which gives your reaction.
Unless you've got a keel on your bike, then this reaction has to come from the tyres, and to keep things in equilibrium you have to lean the bike into the wind. See my point above.
Actually, the sailing boat analogy works quite well here as well - the sail is well above the keel, so although the forces are in balance horizontally, they produce a moment which causes the boat to heel over. The sailor counteracts this by leaning out to the windward side.
Of course a bike isn't actually a sailing boat - it's a sailing boat with a horrendously inefficient sail that produces drag rather than forward motion, and an outboard motor bolted to the back to push it forward despite the inefficiencies of the sail.