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:

http://upload.wikimedia.org/wikipedi...attack.svg.png

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.

http://www.lyc.ns.ca/lycimages/Hiking_out.jpg

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.

It wasn't. I agree entirely with what you said - if you're interested enough to read my posts you'll see why, if you're not (and I don't blame you) then you'll just have to take my word that I'm not arguing with you. :D

I was disputing Markymark's post, and GULG's response to it. Because I'm boring and have a background in physics and engineering. Sorry ;)

It was this or giving Mr Binks something to moan about!


What I am going to do when he goes........


Dave - The keel, yes, I just knew you were going to mention that. Trouble with this type of subject is there is always someone who understands it, and can provide a valid argument, and I would never presume to deny Newtons 'opposite and equal' observations.

I shall put your theory to George, and await his reply....... although it will be long, and thorough, and confusing, and will probably drive Keith to the edge of reason and understanding of this 'boring' subject (form a man who understands Pensions and the FSA, I think is a little rich!).

Anyway, the cats are out and need bringing in........:brave:

My dad and my sister are both chartered accountants. I, however, approach the field of finance with a deep distrust, combined with the same disinterest you have for physics. This is probably why I can explain why bikes go round corners, but have no money :icon_mrgreen:.


For the sake of full disclosure, I am a mechanical engineer and spend my days designing gearboxes - I am not in any way an aerodynamicist, I just have a basic grounding in the subject. I've also been windsurfing since I was about 10 (I'm 27 now), so I've got a bit of experience in making sailing craft go upwind :D

and in English: Bla bla bla blubberbabbella bla bluoooberlybla <yawn> bla bla

Whatever floats your boat,



Room 001100101 anyone ??

You set the bar very high now...... I must 'adjust ' my expectations....

I love this thead!!!!

I do colouring in.

sounds like ******** to me.


the higher pressure is on the windward side. If you push something from, say the RH side it goes to the left, because there is a higher pressure.

The lean is caused by the rider countering the effect, ie leaning into the wind.

Which way do sailing boats lean then? and why doesn't the same principle apply. ****** I say!

So... can I stick my knee out to counter cross winds?