Why does a bicycle tilt to its side when making a turn? What are the forces at play here?
When a bike is going in a straight line, you need to keep the center of gravity of the bike/rider system directly over the point of tire-to-road contact to keep from falling off to the side.
Going around a turn adds a second, sideways force in addition to the downward force of gravity. When a bicycle turns, it has to exert a force perpendicular to a tangent of the turn to change direction, to counteract the bike’s inertia (tendency to proceed in a straight line). This is experienced by the rider as “centrifugal force.” When this happens it is equivalent of a force pushing the bike sideways. So now the center of gravity of the system has to align along the net force vector (gravity straight down plus sideways force) to the contact point.
Interestingly, when you corner on a bicycle (or a motorbike), you actually start by turning the handlebars in the opposite way from the one you want to go in. If you’re not aware of it, it’s hardly perceptible, but once you realise it you can actually exaggerate it to improve manoeverability.
Say you want to turn right.
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Almost imperceptably, you jog the bars to the left. (It is this movement and the gyroscopic effect of the wheel that makes the bike lean, not shifting your weight).
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When you’ve reached the desired angle you turn the bars to the right, actually overcompensating a little to check the bike’s leaning.
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Straighten up.
This is also why it takes a relatively long time to learn to balance a bicycle, you instinctively try to turn the bars the same way you want to turn, which, if you manage it, causes the bike to fall over in the opposite direction to the turn.
(If you can already ride a bicyle, the best way to experience this again is to try one of those trick bicycles with the steering geared up to work in reverse - i.e. bars left, wheel turns right.)
Actually the gyroscopic forces are not that significant on a bicycle. (I don’t have any motorcycle experience, but I belive it is different)
This is come up before. some will find the link. I saw an episode of Newton’s Apple that had wheels mounted on it that were spun in reverse and the bike was quite rideable. What is more importinant is what I believe is called “track”. The bike automatically leans into the turn. Take an old bike and give it a good shove and watch it do it’s thing.
Also, one can ride a bike and steer without using the handlebar at all–obviously leaning causes the bike to turn.
Of couse it is also possible to turn without leaning–example: tricycles.
Brian
trabi, as an experienced bike rider I believe your post is incorrect and I would like to see some proof. Sounds like pure UL or BS to me.
Sailor, try this:
Go get on your bicycle and get up to a decent cruising speed. Now take your hands off of the handlebars and very gently pust the left grip forward (steering right). The bike will turn left! Now do the same thing with the right grip. Bike turns right doesn’t it. The principal is called counter steering and is the only way to turn a 2 wheeled vehicle (inline wheels) at anything above a slow speed.
Look here http://www.google.com/search?sourceid=navclient&q=counter+steering for a bunch of explinations on the physics on counter steering.
There is always a huge discussion, bordering on argument, whenever counter steering is brought up with motorcyclists.
The effect is much more noticable on a motorcycle, but it is still easy to see on a bicycle. Get on your bike and ride no handed, put your hand behind the right hadlebar and push gently forward, the front wheel will turn left and the bike will lean and turn right.
No it’s true, although not an exhaustive description of what happens when you corner. (for example, all that CookingWithGas wrote is also true and relevant).
The evidence, m’lud?
First off, go and take a ride round the block in the light of what I wrote, concentrating on the movements you make with the handlebars and then exaggerating them a bit for effect. You’ll be surprised at what you discover, even after so many years of riding. It’s a sub-conscious process. You learnt to balance a bike at a very early age, and it’s become second nature, like standing or walking etc, so you don’t tend to think about it much.
And/or, see what a google search on “bike counter steering” turns up. 
Also, you didn’t mention whether you are an experienced cyclist or a motorcyclist. I think more motorcyclists are generally familiar with the concept than cyclists.
Oops! My previous post was directed at sailor, but by the time I pressed submit I’d been pipped to the post. My apologies.
Countersteering is definitely possible on a bicycle, and any book on bicycling (e.g. Effective Cycling by Forester) will tell you that countersteer is an extremely useful technique to learn. I think there’s still a debate about whether it’s the only way to turn a bicycle, and whether we are doing it unconsciously all the time. Jobst Brandt (a controvarsial figure on the online bicycling community, but also an author of the highly respected book: The Bicycle Wheel) holds the same view as trabi, as expressed here.
I’ve been wondering about this for a long time. I’ve tried to observe myself subjectively (which of course is impossible but I tried anyway) and I seem to use countersteer most of the time. But I also tried turning without countersteer and it seemed to be possible. You just shift your upper body, or balance asymmetrically (i.e. allow yourself to tilt to one side).
Ok, having read the above posts now I know for a fact it is BS. The “experiment” only proves one thing and that is that the people doing it do not understand physics or the rules of logic reasoning either.
First off: I ride bycicles, scooters and motorcycles often and i have never steered in the opposite direction.
Second the experiment is just misunderstood. If you turn the front wheel right and do not lean in of course you will fall left. That’s why you lean in! Duh!
Third: from the above it does not follow by logic that you turn the wheel left when you want to turn right. What does follow is that you need to lean in at the same time.
Fourth: anyone with a basic understanding of mechanics can tell you that if the wheel turns right the vehicle will turn right and can tell you exactly the radius of the turn. The vehicle turns with a radius centered at the point where the axis of the wheels cross. Always That is why in cars the inner wheel has to turn more than the outer wheel.
Sailor, have you counted the wheels? If there are more than two, you may have been riding a tricycle. 
sailor, we are talking about how to initiate a turn, not what happens during a turn. In order to start a left turn you must create a situation where the center of mass is to the left of the contact patches. You could try to do this by moving your upper body to the left, but you have nothing to push against so your mid- or lower-body will move to the right and the center of mass does not move very far. But if you turn the handlebar to the right even a tiny bit, your contact pathces move quickly to the right and you are now set up for a left turn. You then turn the handlebar to the left and keep it that way until you complete the turn.
Also sailor, if you have a bicycle handy I suggest you try it yourself. Compare the two situations:
[ul]
[li]Start riding in a straight line, then turn right as sharply as you can. Make sure your handlebar is never turned to the left.[/li][li]Start riding in a straight line, then turn the handlebar to the left slightly to break your balance. Then follow through with a tight right turn[/li][/ul]
You will find that the second method allows a much quicker turn. At least that’s what I’ve found.
10K+ posts and still a closed mind.
Yes you have. I promise.
No, you lean in to lower the CG of the 2 wheeled vehicle and to provide more stability in the turn. Also, the more you lean in, the less the vehicle has to lean to make the same turn at the same speed (CG again). It doesn’t matter if you lean in at all, but it helps tremendously.
Again, no leaning is required. But it helps.
Anyone with a basic understanding of mechanics can tell you that 2 wheeled cornering and 4 wheeled cornering is completly different.
Here is a very simple experiment to show what happens when you turn a single tracked (inline 2 wheeled) vehicle. Go get a broom, mop or any other pole.
Balance in on the palm of your hand. Now what we are going to do is make the pole fall to the left (like turning our vehicle left). Which direction do you move your hand? You move it to the right. Now as the pole is falling left we want to make it stand upright again. Move your hand underneath the pole. It balances again. Same thing for a fall (turn) to the right.
Now when you are turning your single tracked vehicle in real life and you want to go left you would push forward on the left grip (or pull on the right). This causes the front wheel to turn right, upsetting the balance of the vehicle. The vehicle now wants to fall left due to physics and momentum. So the vehicle begins to move to the left. You then move the handle bars back to the left to maintain the arc you are traveling. To straighten up the vehicle you would push on the right (pull on the left) just like you did to get the vehicle to turn in, just in the opposite direction. Then you straighten out the handlebars and go about your merry way.
But don’t worry if you don’t believe me, or the truth. I have lost count of the number of motorcycle riders that don’t “believe” in counter steering, or claim to have never done it. It is a difficult uphill battle for me attempting to educate the unwashed masses. But the fact still remains that countersteering is a fact, and it is THE ONLY WAY TO STEER A SINGLE TRACKED VEHICLE!
If you still have doubts, take Keith Code’s riding school. He has a motorcycle that is setup to dispel the myth of body steering and teach about counter steering. Or search for Keith Code No BS
http://www.google.com/search?sourceid=navclient&q=keith+code+no+BS
Thank you, that is all.
Why sailor I’m surprised at this uncharacteristic ad honimen response.
Try this experiment. Take a wheel and an axle. Hold the axle and have someone get the wheel spinning. Try to turn the wheel to the right by pushing your left hand forward. It will, due to gyroscopic precession, lean left. The same thing happens on a bike or motorcycle. The effect is proportional to angular momentum, a function of how fast the wheel is spinning, how heavy it is, and how far away from the center of spin that the weight is distributed.
I’d like to see some slow motion footage of a motorcycle entering a curve to the right and steering left. I really would like to see that. Can I?
sailor, are you saying that there is no such thing as countersteer? You think all the bicycle and motorcycle references discussing countersteer as a high-speed turnining technique are dead wrong?
I’ll look for a video clip but if I fail to find one on the net, that doesn’t prove anything.
I’m almost positive I saw a Cecil column on this some time ago.
Sigh Oh all right then, if you promise to eat up all your vegetables.
Not slow motion I’m afraid. Does anyone know how to slow it down? Or is it worth bothering?