Why does a bike turn

[OldernYZer]

All good stuff. Your points regarding angular precession are accurate, but don't really disagree with the statement that AP does not initiate a turn, and doesn't really help the process at all. It resists it. Someone back a little way said that the gyro effects had been "engineered out". Sorry, but you can reduce them, and sometimes counter them, but you can't engineer out physics.

Regarding roll:

It's easy to see that gravity is a minor reason why countersteering causes a bike to roll. Consider the following numbers, which come from simple trigonometry. (I used sine of the angle from vertical, but I didn't spend much time on the diagram and wouldn't rule out tan as the correct relation, but the numbers come out basically the same at these angles.) When the bike is upright, 100% of it's weight acts to squash the tires. At 5 degrees of lean, 8.7% of the weight acts to roll the bike, the rest is still squashing the tires. Use that 5 degree value as the standard. At 10 degrees twice as much of the weight acts to roll the bike. At 15 degrees it's almost three times as much. If gravity were the main reason a bike rolls, it should roll either much faster or much more easily as the lean angle increases. Gravity clearly does help the roll, but it's such a minor effect that it's hard to notice. What does happen is that when you want to turn right and countersteer left, the bike starts to turn left and centrifugal force leans it to the right.

...I won't argue with your calculation regarding the amount of weight that factors into various lean angles, but accepting them as accurate, you still cannot make the leap that one may subtract that much weight from the pressure bearing on the tires, either. Remember that the bike is now turning, which involves lateral acceleration and the centripetal forces generated by that. At the fringes of traction with modern tires, cornering forces could conceivably exceed 0.8 G's.

And it is still important to understand the difference between rolling the bike on its axis as opposed to swinging the whole mass left or right over a static center line under the wheels.

 

[OldernYZer]

I might also add, going back to the matter of how tire wear affects handling, that tire wear, both external and internal, affects the amount of rolling friction and drag the tire contact produces. As the bike leans, the contact patch moves to the inside (towards the turn) of the steering axis, and a change in drag either more or less (usually more as the tire gets older) will affect how much the tire is "dragged in" to the turn.

 

[Northwoods Snowman]

OldernYZer wrote a long post starting with a claim that no one else understands the issues. Then he proceeded to write what would be a pretty good post (great if not for some genuine errors) if the title of the thread was "why does a bike feel the way it does while it is being leaned?" or "why does a bike feel the way it does while it is turning?" Nothing in his post tells us what force he thinks causes a bike to turn. Clearly he thinks no one else knows, so it would be great if he told us his opinion of why a bike turns.

I read this whole thing, and there’s some truth here and there throughout, along with quite a bit of stuff that appears to have been made up on the fly because it sounds good. The post by Northwoods Snowman comes closest to painting the whole picture, but doesn’t quite cover the wall on the first coat. So I’m going to take a shot at it.

I don't know how you can interpret what you are quoting OldernYZer say to mean that no one understands the issues. To say that is completely twisting his words. And he DOES explain the WHY of how a bike turns (extremely well in my opinion), albeit in an advanced engineering textbook kind of way that you need to have an engineering or physics background to fully comprehend and is not particularly friendly to someone without said background.

First, I want to be clear with anyone who bothers to read this on some terms I’m going to use. First there is the center of gravity, or “CG”. That speaks to the point in a 3 dimensional object around which it will rotate if it were to be tossed into the air and spun in any direction, and the point at which it would be balanced end for end, side to side, top to bottom if it could be perched on that single point. It’s found up in the middle of a motorcycle somewhere, and reflects the weight distribution in any of those directions. The “X axis” is the fore and aft center line running through the CG, and when the bike rotates on this axis, it is said to “roll”. The “Y axis” is a vertical line through the CG, around which the bike “yaws”. The “Z axis” runs from left to right through the CG, and the bike “pitches” around this one.

OldernYZer, I think your 'first' point adds an unnecessary complexity to the 'Why does a bike turn' discussions, and was quite confusing to me as to why you wanted to clarify the terms of "X,Y, Z axis" as I think it is has more relation to a vehicle such as an aircraft suspended in air or submersible such as a submarine suspended in water. In reference to normal operation, a motorcycle is suspended at ground level with the entire weight distribution on two tires at their ground contact points. A motorcycle jumped off both wheels, or a wheelie or stoppie I think should be left out of the discussion as those are exceptions to normal operation. So, in normal operation.......

If both tires are contacting the ground, absorbing the full weight distribution of the bike, the 'X' axis is no where near the middle of the bike, rather at ground level. That is why we refer to the rotation as 'lean' rather than 'roll'. The bike will lean left and right from that ground contact point, not at the CG of a suspended bike.

Because of this tire contact on the surface, considering the surface is flat, as most of our streets, the bike is not going to rotate around the 'Y' axis, you reference as somewhere near the middle of the bike, rather not rotate at all, other than a small tire compression difference between front and rear dependant on the trust/brake loading. The frame, due to suspension would create more of a rotation as the front/rear loading changed, the the point of rotation would again be at ground level, not through the point of CG.

The only axis I think may be relative in the explanation is the 'Z' axis. This should be somewhere in the center, between the front and rear tire contact points, but will not change due to the CG loading. For example, the CG will change from solo riding to 2-up riding, position of the rider(s) sitting upright or leaning forward, or somewhat by the level of fuel on board, yet the axis point will never change as it is fixed by ground contact of the tires. OK, may be a very small bit dependant on the trail and angle of steering deflection.

Anyway, my response here isn't to drive off topic, rather I thought others may be confused as I was to the value of the relationship you used in this 'first' point. On the other hand, perhaps I'm still confused!!



For those confused on CG bit I'll try to translate a bit. In engineering you use static force diagrams to calculate loads on objects or system. The CG of an object or system (system being a motorcycle comprised of many parts, or a motorcycle and rider(s) and luggage) is the point at which gravity will act on the system equally. Think of it as the location where you could hang the bike from and no matter the orientation (right side up, upside down, sideways and backward, angled etc.) the bike would stay stationary and not rotate. When you balance a tire, you're balancing it so the CG of the tire and wheel assembly (system) is at the center of the axle. You can represent the entire system on a static force diagram with the CG. You can break down individual forces into the vertical and horizontal components and also calculate moments (or rotational forces) acting on the system. From all the input forces the resulting forces can be calculated to determine the resulting behavior of the system in the real world. In our case, the CG of the bike (with riders/luggage as applicable) is at a point somewhere in the middle of the total system. This doesn't change because the tires are on the ground. It simply means that when all the forces are summed, because the tires are a fixed point, the bike will fall over and pivot about the tires

I do know (at least I think I know!), if the steering head of our FJR's are locked, the bike will not turn and becomes unrideable. I know using counter steering is an expedient method of inducing a lean, which counters the centrifugal force of a turning bike. I also know the explanation often used such as simply, "push on the right grip to turn right" will almost always dump the bike! If we push forward on the right grip (to induce a lean to the right) and don't follow-up with turning the steering head into the turn, the bike will turn left, throw the weight to the right and without the rider moving his position adjusting the CG (works at parking lot speeds, not highway speeds), the bike will simply fall over to the ground.

I love it when people get super detailed and over analyze things like I often do! In this case, the bike naturally takes over and turns the steering head into the direction of the turn once it is initiated by countersteering and it's not something that must be done consciously so instructors don't (and don't really need to) tell people to then turn the steering head into the direction of the turn. I know I can "press right to turn right" with only my right palm on the bars and I don't need to do anything else until I want to straighten out. I keep pressure with my right hand, but as the bike leans right and begins to turn my right hand naturally comes back toward me. Also, once you are in a turn, if you want to turn tighter, you press more right, and if you want to straighten up (turn left essentially but stopping before you actually go left) you press on the left bar.

Despite the relationship and effect of centrifugal force, gyro force, rake and trail, CG, contact patch, slipping and friction, etc., if I don't steer the front tire, after inducing the lean, back in the direction of the turn, I'm going down and not having a good day.

I'm not a scientist, nor engineer, and have never written and published a research paper, so I may fall into the category of, "along with quite a bit of stuff that appears to have been made up on the fly because it sounds good"........if so, I'm good with that.

I wasn't going to get into an argument with OldernYZer, but there's one pretty important error that many motorcycle journalists also make and I think it's worth discussing so that anyone reading this far gets a different point of view. It's in this paragraph of post #121:
Vehicles with two inline wheels steer, or turn, using a combination of dynamics that blend to varying degrees, very well or poorly, depending on the overall chassis setup. First, since it’s been brought up so many times, let’s examine counter steering. Counter steering can be used to initiate a turn by encouraging the motorcycle, bicycle, or scooter to bank, or lean into a corner. Essentially, it is a way to force the bike to roll on the X axis in spite of the angular momentum (gyro effect) of the wheels which resists its doing so. The concept is rather like that used to balance a hammer or broom on one’s finger tip with the heavy end up. Balance is maintained by shifting the low end of the mass in any direction on the X-Z plane, causing the object to roll or pitch in such a way as to remain upright and counteract the tendency to fall one way or other. When a bike running straight and upright is counter steered to the left, the contact points of the wheels are moved to the left under the CG, and the bike rolls, or falls, toward the right. Note that it is also true that one can simply shift the rider weight to the right over the CG and cause the bike to roll right in response to that without counter steering, so counter steering is not an essential element of turning, just a more effective way of initiating a turn more quickly and with less motion than shifting the rider weight.

It's easy to see that gravity is a minor reason why countersteering causes a bike to roll. Consider the following numbers, which come from simple trigonometry. (I used sine of the angle from vertical, but I didn't spend much time on the diagram and wouldn't rule out tan as the correct relation, but the numbers come out basically the same at these angles.) When the bike is upright, 100% of it's weight acts to squash the tires. At 5 degrees of lean, 8.7% of the weight acts to roll the bike, the rest is still squashing the tires. Use that 5 degree value as the standard. At 10 degrees twice as much of the weight acts to roll the bike. At 15 degrees it's almost three times as much. If gravity were the main reason a bike rolls, it should roll either much faster or much more easily as the lean angle increases. Gravity clearly does help the roll, but it's such a minor effect that it's hard to notice. What does happen is that when you want to turn right and countersteer left, the bike starts to turn left and centrifugal force leans it to the right.

You're on the right track though your interpretation of the results in incorrect. At any given moment all forces on the bike must be in equilibrium around the CG (center of gravity) of the bike in order for it to be stationary (side to side, I'm not talking about forward motion). You can divide the forces into the vertical and horizontal planes. In the vertical plane you have the weight of the bike due to gravity and the force the ground pushes up against it. This is constant whether the bike is upright, on the kick stand, leaning through a turn, or dumped on it's side; the weight of the bike (and rider/luggage if you want to include that) at a single moment in time is constant and the sum of the weight and reaction force of the ground is zero. Now, as the bike leans a certain percentage of the weight is applied to the horizontal plane. Say the bike is leaning at 45 degrees for simplicity. In the triangle the horizontal and vertical legs are equal. What you describe above is incorrectly interpreted as 50% of the weight is vertical and 50% is horizontal. In reality, the weight of the bike is still the same so the vertical leg of the triangle is always the full weight of the bike no matter the lean angle. Because the proportions of horizontal and vertical forces are equal at a 45 deg lean angle the horizontal force applied by the bike to the ground is now equal to the weight of the bike!

Here's where the CG comes into play. As I mentioned all forces acting on the bike must be in equilibrium for the bike to maintain the lean angle. Think of the CG as a pivot point in the middle of the bike somewhere. The only forces acting on that CG are from the contact point of the tires and ground when nothing else (kickstand, rider's foot etc.) is touching the ground. Because the bike is leaning (lets say to the right, with us looking at the bike from the rear) the CG is not directly over the contact points on the tires but off to the right. The vertical force up on the tires creates a clockwise moment (torque or rotation force) about the CG (or you could look at it as the CG creates a clockwise moment about the tire contact patch too but I'll describe this as rotating about the CG) which is being pulled down by gravity. In order to counteract this moment and keep the bike stationary, you need an equal and opposite moment. This is where the centrifugal force come into play. As the bike is turning right, the centrifugal force is pushing the bike to the left and is calculated at the CG. The reaction force in the horizontal plane is the reaction force of the ground on the tires pushing back to the right. Because the tires are pushing right horizontally below the CG which is being pulled left this creates a counterclockwise moment that cancels the clockwise moment created by gravity. The net force exerted on the tires is the hypotenuse of the triangle or 1.41 times the weight of the bike and is the sum (combination not the algebraic sum) of the horizontal and vertical forces.

Ah, what the heck, while I'm at it I'll point out the error in the next paragraph:
Gyroscopic precession does not initiate or maintain a turn at all, and in fact works specifically against any rotation of the axle in any direction from its position on the Z axis. It is a highly stabilizing influence on the chassis, owing to the mass of the rotating tire and rim assembly. The strength of the effect depends the mass involved, and on the diametrical location of the rotating mass, the farther from the center, the more effective it is. It resists leaning the bike in either direction, and that is the main reason that counter steering is used to more forcibly cause the lean than by shifting the weight of the rider.

Well, the gyroscopic force does affect leaning (rolling), but its effect is indirect. When you're countersteering to the left and leaning to the right, the gyroscopic force acts to turn the handlebars to the right. So it directly counteracts your countersteering efforts and thus increases the force you have to put into the bars. The net effect is the same, but the picture of what's happening is different. This isn't just my opinion. It's stated explicitly in the book I cited above.

You're not saying anything different here! They are both correct aspects and work together. They do not oppose each other as you're suggesting. The gyroscopic effect resists the turning of the handle bars as well as the leaning of the bike. Hence why it's harder to quickly turn the bike at high speed (think swerving).

Oh, and by the way you can't shift your weight to the side without having the bike move to the other side (Newton's laws and all that). So something has to resist the bike's motion to the other side if you are to move the centre of gravity by moving yourself sideways on the bike. That does happen - in effect the bike countersteers for you. So it's not really correct to say there is no countersteering when you lean the bike by shifting your weight sideways. It's just that you aren't countersteering.

So... OldernYZer is correct, and your theory is correct also, but you draw the wrong conclusion. You can steer by changing your weight, but the bike is NOT countersteering for you. As you mentioned, you have to lean the bike to maintain the CG over the tires as you shift your weight, but what happens when you lean the bike? As the bike leans steering geometry takes over and wants to turn the bike in the direction of the lean. You can experience this by trying to ride the bike without hands on the handle bars.

 

[lnewlf]

 

[festar]

forehead slap
How does anything have NO forward momentum if it is NOT stationary?

Going backwards

 

[wil780]

Northwoods, I think you've overdoing your interpretation of my criticism of OldernYZer's posts. I thought I owed him something for reminding me to think about why a coin turns. That was the only thing he said about why a bike turns (as opposed to why a bike leans or why it feels the way it does), and it's not completely correct. But one of the two mechanisms that cause a bike to turn can be understood by realizing that the turning force arises because a bike can't turn the way a coin does.

A few years ago I gave up disagreeing with people on the internet, and I've saved myself a lot of time and aggravation. However, I couldn't stop myself completely on this topic because I'm so passionately interested in it. Because of my debt to OldernYZer and because most of what he says about bikes is correct (just irrelevant to why a bike actually turns) I thought I would help him out by trying to explain most of what I see as a handful of errors he made. Over the years I've read at least a dozen threads on this topic with over a hundred posts each on several forums, and his posts are substantially different from 99% of them, including yours, because I don't disagree with just about everything he said.

 

[HotRodZilla]

OMG, a bunch of guys trying to write something smarter than the guy before him, and no one really knowing what he's talking about. Bored much?

 

[wil780]

OMG, a bunch of guys trying to write something smarter than the guy before him, and no one really knowing what he's talking about. Bored much?

From the person whose posts are mainly about showing how clever he thinks he is. Know the meaning of the word hypocrisy?

 

[HotRodZilla]

OMG, a bunch of guys trying to write something smarter than the guy before him, and no one really knowing what he's talking about. Bored much?

From the person whose posts are mainly about showing how clever he thinks he is. Know the meaning of the word hypocrisy?

No, please explain.

 

[Redfish Hunter]

A few years ago I gave up disagreeing with people on the internet

OMG, a bunch of guys trying to write something smarter than the guy before him, and no one really knowing what he's talking about. Bored much?

From the person whose posts are mainly about showing how clever he thinks he is. Know the meaning of the word hypocrisy?

I may not know the meaning of the word hypocrisy but I think this ^^^ is a fine example. It sure seems like he is disagreeing with Zilla and this is definitely the internet.

 

[SacramentoMike]

Zilla may not know the meaning of hypocrisy, in fact he may not know the meaning of lots of words--in fact he doesn't, But by god, he must know something. I'm almost sure.

What a jacked-up thread, though. He knows that, at least.

 

[Uncle Hud]

You know, the mechanics and physics of WHY a bike turns is not nearly as important as whether the pilot understands HOW to make it turn.

Like many things in this world -- continuously variable transmissions, dynamic shortest-path internet routing, cell phone text messages, photocopiers, creating a PDF -- it does not matter if I fully understand the principles that make it work. It only matters that I know how to use it for my purposes.

DIE, THREAD; PLEASE DIE !!!

 

[Northwoods Snowman]

You know, the mechanics and physics of WHY a bike turns is not nearly as important as whether the pilot understands HOW to make it turn.
Like many things in this world -- continuously variable transmissions, dynamic shortest-path internet routing, cell phone text messages, photocopiers, creating a PDF -- it does not matter if I fully understand the principles that make it work. It only matters that I know how to use it for my purposes.

DIE, THREAD; PLEASE DIE !!!

Yeah, I don't think there's much left of this horse to beat. You did suggest some great topics for the next "how it works" thread though. Eh, on second thought maybe not!

 

[wil780]

A few years ago I gave up disagreeing with people on the internet

OMG, a bunch of guys trying to write something smarter than the guy before him, and no one really knowing what he's talking about. Bored much?

From the person whose posts are mainly about showing how clever he thinks he is. Know the meaning of the word hypocrisy?

I may not know the meaning of the word hypocrisy but I think this ^^^ is a fine example. It sure seems like he is disagreeing with Zilla and this is definitely the internet.

A perceptive person might recognize there is a difference between responding to a personal attack ("no one really knowing what he's talking about") and criticizing an idea posted for general comment. Besides, I didn't agree or disagree with his comment. I implied it was hypocritical.

 

[Redfish Hunter]

I would argue with you on the interpretation...

But I recently gave up disagreeing with people on the internet.

 

[HotRodZilla]

Edit:

Wait a second, Personal Attacks? Someone who doesn't want to disagree with people on the internet has thin skin. Whaaaah.

A perceived personal attack is not a personal attack when it's true. This entire thread is a bunch of people pontificating their opinions on exactly what makes a bike move left or right. I could Google the subject and get similar answers.

Puking your opinion on the Internet does not mean you know what you're talking about. Hell, that's my opinion, so maybe someone here does, but from what I saw, it's nothing more than a bunch of mental masturbation brought on by either boredom, or the "Hey, look at me" syndrome. Some, like Fred, Phil, Ionbeam, NTX, the OP, and even our resident clown BanjoBoy have earned enough respect that I may laugh and point, but I'll also ponder what they said.

You, Mr, 23 posts, may be the smartest man on the planet, but so far, you're just some guy who read Lee Parks and tries to use big words...Oooooh, I'm so impressed. Or maybe I'm not. How's that for clever?

 

[Fred W]

All this jabber and nobody gave the right answer.

The bike doesn't turn. The earth turns under it.

You're welcome.

 

[HotRodZilla]

All this jabber and nobody gave the right answer.
The bike doesn't turn. The earth turns under it.

You're welcome.

Like Chuck Norris. OMG, my bike IS Chuck Norris!

 

[Uncle Hud]

Your bike is Chuck Norris? OK then, my bike is Wonder Woman. The Lynda Carter version. My 1300 cc, in-line 4 engine is supplemented by her invisible plane.

 

[Fred W]

Uncle Hud may be on to something there. I mean, the 3rd Gens do have a perky set of boobs on the dash.