The Carlisle & West Cumbria IAM have a brilliant guy who produces the newletter. He was a RN Diver, and has a Physics Degree, tought High School Science and now teaches people how to drive, ride and is a IAM Senior Observer.

His party trick is explaing the physics behind why cars & bikes corner, slip angles, reaction forces, corner forces, slip force, thrust demand, camber thrust etc etc.

He put this snippet in the monthy news letter and I though you'd be interested as people often ask how to ride in the wind. Well here's the explanaition of what happens and why...


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.


Simples!

__________________
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Never underestimate the power of stupid people in large groups.

What happens when the wind is so strong it's trying to blow you off the road?

Speed up, then the 'apparent wind' (combination of head and cross wind) is less sideways and more dominated by the headwind.

Go on, I dare you.

At 80mph the 'apparent wind' was very much a sidewind. The sort of sidewind that blows trucks off course.

So my bike is like an aircraft wing!
So what happens when the wind blows from the other side?
Unconvinced
K

Where does it say that ?

__________________
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Never underestimate the power of stupid people in large groups.

speed up always helps and lower gear so your driving all the time and no looseness in bike (does that make sense)

Well its BASIC physics 101! Don't shoot the messenger. The OP is perfectly correct. I may not be able to ride my bike well but I do know the physics involved and the OP is correct.

Try this little home experiment. You will need running water through a tap and a tea spoon. Run the water, in parallel to the water, but about 1" appart hold the tea spoon from the tip of the handle with the BACK against the water. Now when you close the 1" gap to nothing you would expect the flow and pressure of the water to push the back of the tea spoon away, yes? See what actually happens, this is called 'lift' and is the principle of flight and part of the aerodynamics that the clever OP is refering to.

Learn from this man he knows what he is talking about. I don't so ignore me, lol

Keep safe and remember; 'the mind works best when used like a parachute. Open'.

Kidest,

Ryland

Back to the original topic...

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 .

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.

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.

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

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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





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






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



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.

__________________
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Never underestimate the power of stupid people in large groups.