Riding in the wind, a snippet from the IAM newsletter
 

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!

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 ?

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

mmmm
 

Sounds a bit iffy to me.

Bikes aint spoon shaped.

The wind pressure has the same effect as counter steer, so the bike "leans" into it. This is a gyroscopic effect - nothing to do with aerodynamics.

What I do know is that the less you interfere and thew more you relax into the bike the better.

Please elaborate?

Counter steer is to do with the shape of the tyre and the 'rolling cone effect' causing over correction, counter steer is there is because the Camber thrust is too great (causing the bike to oversteer) for the Corner Force Demand. In other words you provide negative slip that matches the difference between the camber thrust and the corner force demand.

As opposed to a cars steering, where you cannot use negative slip, and you have to use slip force and thrust to equal the cornwer force demand.

Gyroscopic effect is an urban myth, nothing to do with how a motorcycle handles, otherwise you'd never get the bike to lean over in a bend.

It's very simple A level physics.

My 16Yr old daughter understands it.........

Urban Myth? I'd say Gyroscopic effort is very real. Just grab a spindle of a spinning wheel and try and turn it - it's hard and the heavier or faster the wheel's turning the more force needed..

The larger diameter and/or greater weight or speed, require a bigger input to move and counteract against.

Lightweight wheels are a big deal and is why race teams spend so much and manufacturers of performance bikes put effort into lightweight wheels/discs/tyres and then there's wheel diameters. The change from a 16" front wheel being too easy to turn to a 16.5" GP front being right to a 17" being slow is all a result.. As for a heavy 21" with high windage and you need more force to make the thing move or turn.

GULG - You sound like a man who knows and just try a 17" front on the Ten' and feel the difference. That's gyroscopics..

There are many different effects all of which have their part in how and why a motorcycle behaves as it does. It is certainly not correct to isolate one effect to explain cause and effect.

Certainly there is a rolling cone effect to be considered, certainly the wind and aerodynamic properties of bike and rider will provide input into the mix. Centrifugal/centepetal forces acting against the frictional resistances provided by the surface. Gyroscopic effect is part of this - clearly demonstrable and certainly not a myth.

What I always do, with strong sidewinds and passing a large truck: next to the truck hold the throttle, the moment I pass the truck open it up when the wind comes in full speed. It always works.
Greetz, Hans.

Sorry, maybe I need to elaborate a little, in terms of counterster and the upright ability of a bike, which is ofen explained by centripitle or gyroscopic forces, is a myth. Of course these forces are present and are many many more, but in such small quantities that they do not have a noticable impact to the handling of a normal road bike. It's relaitive, In fact, as you are all so interesed I'll post up the full explanation of the forces on the front wheel of a motorcycle during a turn, but the OP was about wind effect and to be honest turning forces (and those involved in a change of direction) have little to do with the effects of a headwind, my intial diversion into turning forces was to explain the error of one reply.

I beg your pardon?

That's a very bold statement. It's also, how can I put it...wrong. Yeah that's the word ;)

I await your explanation with interest.

It is when you take it out of context. The discusion is about the effects of a side wind on the motorcycle, so yes Gyroscopic effect is an urban myth in this respect. So no, the statement is not wrong.

if the statement was made regarding the general handling of a motorcycle then yes, you are correct, but that isn't the topic, is it.

Ok then, I'll be a bit more constructive than my last post. But I don't think I'm taking this out of context...

I struggle to read that as being specific to cross-winds and not talking about morotcycle handling in general. And yes the person who first mentioned countersteering was wrong in saying it had anything to do with leaning into the wind. But you didn't *correct* him, because what you said wasn't correct.

Camber thrust, or what you refer to as 'rolling cone effect' is very real, and is what causes a motorbike which is leant over to follow an arc rather than a straight line.

However, in order to generate camber thrust, you need to get the bike leant over, and you absolutely, definitively cannot change the lean angle of a bike* without making use of gyroscopic forces.

Wheels and tyres are pretty heavy things, particularly on production bikes. They also have most of their mass concentrated out around the rim/tyre. Which means they have a large moment of inertia about the wheel spindle. And they are spinning pretty fast, so there is a lot of gyroscopic effect going on. If you think otherwise, ask why so much is spent on lightweight wheels for racing.

This means that a motorcycle with both wheels pointing straight ahead wants to stay upright and carry on in a straight line. This is quite a useful thing, as it means we can get beyond the end of our streets without falling off. People have built bicycles with contra-rotating flywheels to cancel out the inertia of the wheels, and they're pretty unrideable.

Where you go wrong is in your assumption that because you can get a motorcycle to turn, the gyroscopic effects are insignificant. In doing this, you are ignoring the fact that you can turn the front wheel, and a fun little phenomenon called gyroscopic precession.

Anyone that's been to a science museum on a school trip will have experienced this (and this is way before A-level age). The old experiment of putting someone in a swivel chair, holding a spinning bicycle wheel out in front of them by the spindle, and observing that tilting it to the left or right causes the person, and chair to revolve. You're doing exactly the same thing to change the lean angle of a bike, except this time you are rotating the wheel around a (near) vertical axis - the steering head - which is causing the bike to revolve around a horizontal axis (the line between the tyre contact patches). To make the bike tilt left, you steer right, and vice versa. This is what is referred to as counter-steering, and like it or not, unless you only ride at walking pace, you *are* doing it.

To summarise:

- Gyroscopic effects allow you to change the lean angle of a motorcycle.
- Camber thrust causes a leant over motorcycle to follow an arc rather than a straight line.
- Both are significant.


As to the point of your OP...I'm still digesting that. There's some things in there which jar a bit with me, but I'm trying to figure out whether the problem is with the physics, the explanation, or the ignoring of other factors.


*at a significant rate. You can get a bike to change its lean angle by energetic use of your bodyweight, but not quickly.

That's better, I see now where you are coming from, and yes you explanation agrees with my understanding.

The post you reffered to was (in hindsight a mistake) a brief and broad attempt to direct the terms used, into the correct area of discussion, not a full description of the forces that act upon a bike during a manouver.

-------

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?

It is the same principle (no I'm not suggesting that a motorcycle has a mast or sails, or is indeed amphibious, has gills, scales or rope), and this is very simplistic (as I don't fully understand the mechanics of it, but I do understant the principle - Maybe SteveD can add to this?) in that you fill a sail with air, and this forms a dome on one side and a shorter surface on the other, the trick is then to get the wind to pass along the sail (not blowing it along like an old can of coke) which causes the air to pass over the curved surface faster than the air passing over the 'flat' or short surface (Technically it's not flat as it curves inward due to sails being made of fabric/plastic/thin stuff). The speed differential creates lower pressure on the curved surface with normal pressuer on the 'flat' surface. The net result is that the sail (and the attached water craft) will move from the high pressure area to the lower pressure area propelling the boat forward, while the wind direction is coming from the side. In the same way the wing of an aircraft creats lift, or a 'spoiler' on a car (racing, F1 or otherwise) uses the same principle to creat downward force to press the car onto the road.

In the case of the OP, and in particular Toms comment, I would suggest that the OP is correct to a point, and that point would be as the cross wind speed approaches the head wind speed. At which point the effects of pressure would be overcome by the pure directional force of the wind over the available grip and mass of the motorcycle. And thus you would become the tumbling coke can.... or rolling truck, tree or building (clearly your bike would not turn into a 4 storey block of flats, or a 40ton truck, or grow branches, or develope a sugary fruit-ish flavoured drink inside it's engine)

I'm happy to debate these points, not least as it helps me solidify my understanding but if people are going to make statements that contain no explanation as to why they disagree then I have to ask what the point of it is ? (the statement), and how it can benifit others understanding, or substantiate the point they are making, especially those that are reading this as new understanding or learning. Questions are fine, and good, and help everyone.

I suppose there is always a danger in attempting to explain things by a one way conversation (like a forum) as things get mis-understood, mis-read, mis-quoted, but this shouldn't stop people from being bold and attempting to spread the knowledge. If you are not interested then you don't have to read it. Some folk are happy to just get on the bike and ride, others like to add shiney bits and polish, some like to use them to get to work, other like to learn a bit......

this thread seems to be a bit like herding cats :D bet you glad you offered the snippet in the 1st place eh Daz :P
I liked it by the way and cant wait to try the knee out tip.

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?

no u cant,selina needs to get by http://www.xt660.com/images/icons/icon14.gif

Good point :sleep01:

I used to use the trick of sticking out my windward knee in high winds on the motorway and can confirm that it does actually help. Reading the OP was helpful in understanding why and where it did not.

With my knee out it helped in how much the bike lean towards the wind (or how much I countered depending on how you want to view it) which meant I felt a little bit more in control than I would otherwise. You still need to correct the bike from veering away form the wind however (ie getting blown across the carriageway) but overall the riding experience is improved.

Forecast 50mph gusts were the limit I would ride in (about 30mph constant winds) and it was these gusts than sticking my wind-side knee out helped most with (tree lined motorways rarely have constant strong winds but I suspect it would help lots there too)

Now here in Guernsey riding more in built up areas than a motorway this is much less useful as the direction of wind can change far too quickly (and I'm not going over 35mph legally anyway).

Tried this theory a few years back when I had the Africa twin. Like many theories, it sounds good but for me at least, I just increased my fuel consumption and I got a cold knee! But hey, if it works for someone then stick with it.:laughing7:

all these headwinds, sidewinds, crosswinds and technical bla bla went right through my helmit and gave me a bj deluxe...
i tried sticking my leg out and i hit a car ...
gb.

Runs to toilet in floods of tears

So to recap, if my cat is running at 50 mph following a good scalding and it has a 30 mph crosswind it can stop itself falling over by acting like a dog having a pee..

Or do I just lean my bike into the wind to stop it being pushed into the bushes ?

You need to also consider forward movement. It's all there in the earlier posts.

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.

I'm waiting for a windy day - my gut feeling is still that a bike *doesn't* lean into the wind without any input from the rider. I'm also not sure you can sit on a bike in a crosswind without either falling off the top of it, or putting some steering input through the bars or pegs.

I'm not going to try and explain something I don't believe happens, until I know it happens ;)

Back to your flow diagrams...I don't think that's how the air flows. You mentioned having some trouble figuring out the relationship of headwind to sidewind...

The thing you need to figure out is what frame of reference (point of view) you are operating in, and stick to it. The obvious one is to sit yourself on the bike, as that is what we are interested in.

The bike doesn't know that it's moving. Take the cross wind out of the equation, and all the bike knows is there is a 27m/s air flow coming from straight ahead.

Alternately, just considering the cross-wind, all the bike knows is there is a 9m/s air flow coming from 90deg to the way it's facing.

Combine the two, and you have a wind direction which is the resultant of those two components. There is no difference* between a bike moving at 27m/s with a 9m/s cross wind, and a stationary bike in a 28.5m/s wind which is coming from 18.5degrees off straight ahead.

(or if you're feeling clever, a bike in a two wheel drift at the speed and angle above on a perfectly still day).

Which brings us back to this flow diagram...

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

with alpha in this case being 18.5deg. And that will, undoubtably result in 'lift' towards the downwind side.

Your point about what happens when the cross wind starts to approach or exceed the headwind is an interesting one. Aerofoils will 'stall' and stop producing lift if the angle of attack is too great, so if you take a bike which is a fairly efficient foil (not many of them) then there might be a 'worst' speed to ride at for a given cross-wind speed - speed up or slow down from that speed and you'd be affected less. Hmm.


*well...aerodynamically. And strictly speaking the stationary bike would have to be on some sort of treadmill with both wheels spinning at 27m/s, because wheels do affect airflow.