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When I turn I usually shift my weight on the same side I'm turning to. To ease the discussion let's assume we are turning left, on a paved road (no mud/gravel/potholes/grease/oil/ice/door traps).

I would lower the right foot, move my bottom to the left and this is sufficient to tilt the bicycle to the left.

However I have seen other people doing different things (remember we are turning to the left):

  1. some people do the same as I do, but in addition they stick their left knee sideways. I tried (when nobody was watching) but don't see any advantage.
  2. Some other people instead move their bottom to the right and then (I think using their hands) they tilt the bicycle to the left.

Having given this context (but feel free to add more styles/methods) I wonder what's the best way to take a turn, primarily in terms of speed, but I'm also interested in safety, when I can activate my brain enough (sometimes happens).

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The reason to leave your bike more upright while leaning with your body is if you want to / have to continue pedalling. It removes the possibility of pedal strike. –  user973810 Oct 24 '12 at 3:02
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5 Answers

I would say that the knee thing won't give you any real advantage and is more or less some imitation of the "coolness" of motorcycle racers.

The second thing, where you push your bike down into the turn while keeping your body more upright has some advantages when used in the right situation. On a paved road -- as pointed out in your question -- it shouldn't make a difference. However, on loose or slippery ground pushing your bike down will give you more safety.

Think of riding through a sharp turn where in the "standard configuration" (bike and rider lean into the turn by the same angle) you have to lean into the turn quite far. If your tires lose grip now, you will slip away and dump like a bag of rice as you have quite little possibilities to react. However, if you push the bike down and keep your body more upright it will be easier to get a foot on the ground when your bike slips away. Therefore you can in best case regain control and drive on or in a worse case let your bike go and try to catch yourself safely.

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I am pretty sure from my own experience that cornering with my inside knee bent at speed gives me considerably more control. –  robthewolf Oct 23 '12 at 9:15
    
I don't think that putting out your knee will do any harm at least and as long as you feel more safe with it, it should be fine. I just doubt from the physicists point of view, that it will have a considerable huge effect at typical cycling speeds. –  Benedikt Bauer Oct 23 '12 at 9:34
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I don't see how "pushing your bike down" in slippery conditions gives you more safety. On the contrary, keeping the bike more upright while leaning your body into the turn makes it less likely that the wheels will pop out from under you. –  Daniel R Hicks Oct 23 '12 at 11:11
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@BenediktBauer -- Could be because I find it "hard to react" to anything. In a slide I'd never have time to get my foot down, eg, at least not with my shoes cleated. Better to minimize the chance of the slide in the first place. –  Daniel R Hicks Oct 23 '12 at 15:39
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@DanielRHicks I think the popular video of Cancellara's descent in the Tour de France counters your point of view here. If you watch closely, Cancellara leans his bike into sharper turns considerably more than he leans his body. You want as much downward force over the wheels as possible, and by leaning your body farther into the turn than your bike, you produce greater lateral force and reduce the downward force, increasing the chance of a slip. –  Stephen Touset Oct 25 '12 at 0:00
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I think this relates quite nicely to motorbikes where you corner at very high speeds and I'll give a run-down of the techniques, why they are useful and how they apply, and how they might apply to cycling.

So when turning left:

  • You shift your weight over on the seat and tilt the bike left. This allows the centre of gravity to be slightly lower, aiding in stability and keeps the contact patch of the tyre to road more central.

  • Sticking your left knee out also. This is done for two reasons, firstly (if you're wearing knee protection) it can give an indication of when you're leant over too far (your knee touches the floor). Secondly it works like an air brake (for additional braking) and also will start pulling you towards the direction you are going slightly. You have to be going pretty fast for this to be effective, but it can help if you're racing.

  • I can't see any benefit of tilting the bike over whilst staying more upright. Higher centre of gravity, less control. It might be a sub-conscious impulse to not falling over low-side.

Another tip for cornering fast:

  • When going into a corner and you feel like you may run wide, or want to tuck into the apex a little more, turn the handlebars in the opposite direction to the turn (right) just a little. This is called 'counter-steering' and will cause the bike to tip over more and turn in a little harder. This can be a life saving technique if there is a car coming the other way or is overtaking you. Don't over do it though ;)

Also, ultimate safety tip: never brake and turn at the same time.

Source: I've been riding high performance motorcycles for about 5 years.

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+1 for never break + turn. Really good answer! –  John Doucette Oct 23 '12 at 11:02
    
These answers seem a bit at odds with how I turn, especially at speed. Perhaps I'm just not reading the answer right? This video more or less describes how I turn: youtube.com/watch?v=q9wq160yYdw ...weight on the outside, not inside. –  Ken Hiatt Oct 24 '12 at 19:35
    
You might also want to mention putting as much weight as possible on your outside foot. –  Stephen Touset Oct 24 '12 at 23:51
    
I'm trying different solutions, I might also write an answer with my experience later. For now, I want to share with you that brake+turn is pretty nasty. I tried it this morning and it was like one moment the sky, one moment later the earth... :) –  astabada Oct 25 '12 at 7:57
    
I played with this quite a bit myself and found my turning to be more consistent accurate and stable even in extremely "laid over" turns with high speed if I am inline with the bike. There are exceptions particularly when trying to avoid pedal strike where leaning more than the bike is helpful. Sheldon brown agrees: sheldonbrown.com/brakturn.html. I think though that it is ultimately a personal preference if there are no major personal downsides to your preferred technique. –  Glenn Oct 26 '12 at 21:38
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When it comes to turning on a bicycle, the two most important things you can do are:

  1. Keep your weight as close to the bike as possible. Simply put, your center of gravity should be as close to your bike as possible. If your turning left and sitting straight up with your torso haning off to the right of the bike the turn is going to be much more difficult to negotiate. You don't have to hunch down with your nose to the bar like a PRO, but you should do your best to keep your weight close to your bike.
  2. Look where you want to go. This may seem silly, but beginner cyclists almost never do this. Before entering a turn look through the turn to where you want to end up and keep your head and eyes on that spot. You'll be amazed how your body and bike will follow your eyes.

Try it out at slower speeds in a low (or no) traffic area to get a feel for it. There's no shame in going to a local office park in the evening (when everyone has gone home for the day) and putting out some cones to practice going around.

Most importantly be safe...going into a turn at 30mph is no time to start experimenting with cornering technique.

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Your question is based on an incorrect assumption about how a bicycle makes turns. You say

"...move my bottom to the left and this is sufficient to tilt the bicycle to the left."

Unfortunately, what you are stating above is simply impossible in practice.

A bicycle can be thought of as a vehicle that is "attached" to the surrounding environment by an imaginary "hinge" at its very bottom point. I.e. the tire-ground contact areas are the only points of contact between the bicycle and anything stationary ("the world"). A bicycle riding in a straight line is essentially hinged (like a door) by these two contact points.

How, your body is the heaviest thing on the bicycle. Your body's center of gravity is located relatively high, i.e. relatively far from these hinge points. In this configuration trying to make your bike to lean to the left by shifting your body to the left is virtually impossible. In fact, if you attempt to shift your weight to the left, the bicycle itself will reactively compensate by leaning to the right (!). To the right, not to the left. Try it sometime on a stationary bicycle (ask someone to gently hold it for you) and you will see what I mean.

(Trying to make a bike to lean to the left by shifting your body weight to the left is no different that trying to lift yourself up by pulling on your own shoelaces. It won't work.)

The mechanics of the sustained turn on a bicycle is actually completely different. In order to make a left turn you actually turn your handlebars (and the front wheel) to the right for a relatively short period of time. This makes the tire path of your bicycle to gradually shift to the right from under your body. How quickly it tracks to the right depends on the amplitude of the right turn of the handlebars. Meanwhile, the inertial properties of your body make sure that it doesn't shift anywhere. Your body mass continues to "fly" in a straight line. The combination of these two motions (body - straight, contact path - to the right) results in a left lean of the bicycle frame.

This is a key moment many bicyclists fail to understand: you initiate a left-leaning turn not by shifting your weight to the left (which, as I said above, is simply impossible), but rather by shifting the bicycle itself to the right. Your body does not shift anywhere. It is the bicycle that "slides" from under you by some controlled amount, thus creating a left lean and allowing you to enter the properly balanced left-leaning configuration for a left turn.

What you do with the specific configuration of your body itself does not make much of a difference (aerodynamic issues aside). Some riders believe that they have to keep their bodies upright in any turn (i.e the "lean your bike, not your body" rule). Some riders believe that the body should stay in the plane of the bike, i.e. that you should lean with the bike. Some riders might even prefer to "overlean" into the turn, although I see no good purpose in that.

P.S. The above mechanics of the left turn is actually why it is difficult (or even impossible) to initiate a left turn when you are riding very close to the right edge of the road. As I described above, in order to initiate the left lean you have to force your contact path to track to the right. However, if you have no extra room on the right-hand side, it becomes impossible: the wheels will either hit the curb or slide off the pavement. And this is actually what often happens to cyclists who find themselves in a situation when they have to make an emergency turn, and yet have no road available on the other side of the bicycle.

P.P.S It appears that the "lean your bike, not your body" rule helps some riders to gain more confidence while gong through "scary" turns. Another variation of that rule is "keep most of your weight on the outside pedal" and it serves the same purpose. It is intuitively less scary to perform a risky high-speed turn while following these rules. However, I personally don't believe it achieves anything beyond some immaterial feeling of greater safety and comfort.

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Right! The brief turn in the "wrong" direction is something that occurs unconsciously (and probably the most difficult cycling skill for a novice to learn). (Also, it occurs to me, a possible reason that some folks are "attracted" to cycling hazards as they consciously try to avoid them.) Only when avoiding a very near (and small) road hazard would you actually steer left to turn left. –  Daniel R Hicks Oct 26 '12 at 21:08
    
Of course, you want to keep the outside pedal down rounding a curve to avoid having your pedal drag. And this posture makes you naturally put more weight on the outside leg, since the straight leg can support weight without effort. In fact, this would be a reason for a slight lean to the outside, to put more weight on the outside leg and less on the inside one, allowing one to rest and relax more than would be the case with weight balanced between legs. –  Daniel R Hicks Oct 26 '12 at 22:58
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We need to introduce some basic physics here...

The "angle of thrust" -- the angle between the ground and the line running from the tire contact point up to the center of gravity of bike+rider -- is determined solely by the speed and the radius of the turn. The downward pressure at the tire will, on a reasonably bump-free road, be precisely the weight of bike+rider (divided between the two tires, of course), while the outward pressure -- the tendency to skid out, as it were -- will be determined by the laws of centrifugal force. If you know those two forces you can use the squaw on the hippopotamus to compute the "thrust" along the center-of-gravity line, and you can use trig to figure out what the angle to the horizontal is.

So whether the cyclist leans in or out makes zero difference in the thrust (and tendency to want to skid out) -- it's purely determined by speed and turn radius.

With an essentially cylindrical tire cross-section, the amount of friction (to resist skidding out) will be primarily determined by the coefficient of friction of the tire material and the downward weight on the tire. Since a road tire has essentially the same coefficient of friction over its entire usable surface, the "lean" of the tire has little effect on traction.

There will be some degree of effect on traction based on how close to the rim one is, and how the tire is deforming -- it could be reasonably argued that as a tire is leaned and deforms sideways more it will "squirm" more, resulting in some loss of traction. But this effect would be slight on high-pressure road tires.

The two major effects to consider here are more mechanical. One is the way the geometry of the bike+rider changes as the bike traverses minor bumps, and the other is the way that steering behaves.

With regard to the bumps, consider two cases: 1) The bike is essentially vertical, with the rider leaning into the turn to achieve the right angle of thrust. 2) The rider attempts to stay vertical while leaning the bike (and necessarily the lower portion of his body) into the turn.

In the first case, when a bump is encountered, the bike will be pushed upward, with the "pivot" of the rider's body bending to absorb the shock. There will be little change in the overall "geometry" of the "system" (though there may be some change in the geometry of the rider's back, requiring chiropractic services to correct). In the second case, the rider will remain relatively motionless while the angle of bike to road changes dramatically. I think it's clear that, other factors being equal, the second case will result in a less stable behavior.

With regard to steering behavior, consider how much change in direction occurs for a minor change in steering angle. With the bike essentially vertical the radius of the turn is determined almost completely by the steering angle. It takes a relatively major change in steering angle to effect a change in turn radius.

On the other hand, with the bike leaning the radius of the turn is affected by the curve of the bike tire -- as the steering angle increases the point at which the tire touches the road moves forward along the wheel diameter, so that a minor change in steering angle produces a much more pronounced change in turn radius. But an interesting side-effect of this is that as the bike leans more it tends to turn more sharply, and turning more sharply increases outward thrust, tending to right the bike. This results in a relatively stable steering configuration.

The net-net of this to me is that on a relatively smooth road you'd want to lean the bike "naturally", to achieve optimal stability (not only for speed/safety but also to reduce rider fatigue). However, on a less ideal surface one might not want to lean so far. (Of course, a relatively prudent rider would not ride as fast on poor surfaces anyway, so naturally there would be less leaning.)

But I suspect that a lot of how people ride (including the rider in that video) has to do more with body mechanics than bicycle mechanics. On a long downhill the rider is taking the opportunity to rest, but also having to be ultra-alert to avoid wiping out. Certain body configurations will permit more relaxation/recovery of the major muscles while at the same time optimizing control and the ability to comfortably "ride out" the shocks that one experiences at high speeds even on a "smooth" road.

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