We dont know how a bicycle works Really?

[DivergentSpectrum]

http://www.newstatesman.com/science/2013/08/we-still-don’t-really-know-how-bicycles-work
This is really kinda embarassing. Forget quantum mechanics our top scientists can't even figure out how a bicycle works. Is this true?

I always figured it wasnt so much the gyroscopic effect of the wheels, but more like the inertia of the entire bike.

 

[Astronuc]

http://www.newstatesman.com/science/2013/08/we-still-don’t-really-know-how-bicycles-work
This is really kinda embarassing. Forget quantum mechanics our top scientists can't even figure out how a bicycle works. Is this true?

I always figured it wasnt so much the gyroscopic effect of the wheels, but more like the inertia of the entire bike.

I think that the gyroscopic effect is part of it, but so is the balancing of the cyclist, i.e., keeping the center of mass/gravity over the line of points of contact of the tires/wheels with the solid surface - kind of like a tightrope walker. Skilled cyclists can stay upright at a stop (standstill).

 

[Tribo]

Reminds me of a Philip K. Dick novel. The undercover-junkie protagonist fails to understand something about how a bicycle works and the government assumes he's losing his mind.

 

[Doug Huffman]

David Gordon Wilson's Bicycling Science (2004 MIT) explains it perfectly. The analogic demonstration is balancing a broom in the palm of your hand. The gyroscope is well discredited by a device with a counter rotating front wheel.

 

[leroyjenkens]

I think that the gyroscopic effect is part of it, but so is the balancing of the cyclist, i.e., keeping the center of mass/gravity over the line of points of contact of the tires/wheels with the solid surface - kind of like a tightrope walker. Skilled cyclists can stay upright at a stop (standstill).

If they're going fast enough, they can stay upright without a rider.


So if there's no rider, and all the bike has is Newton's First Law to keep it upright, then obviously that must be enough to keep it upright.

 

[lavinia]

I'd be interested in an explanation especially why the gyroscopic effect is irrelevant.

On the surface it seems that any spinning wheel must be a gyroscope and that the gyroscope stabilizes the bike - this because a gyroscope resists a change in its plane of rotation. Additionally, precession of the the front wheel allows the rider to turn the bike. One needs to lean to one side in order to turn the bike so that the front wheel will precess. In some sense a bike does not really turn say the way one turns a tricycle does but rather precesses from the torque of gravity on the front wheel.

Why is that wrong?

 

[DivergentSpectrum]

as i understand the gyroscopic effect is just a manifestation of "an object in motion will continue in motion" So therefore its harder to tilt a spinning wheel than a stationary one, because youre changing the direction of the rotation/motion.
One thing about gyroscopes, is the wheels are intentionally heavy. This makes it require more force to change the trajectory.
Most of the time bicycle wheels are intentionally lightweight- just a few spokes and an aluminum rim.
So yes, the gyroscopic effect of a bicycle is negligible.

 

[leroyjenkens]

as i understand the gyroscopic effect is just a manifestation of "an object in motion will continue in motion" So therefore its harder to tilt a spinning wheel than a stationary one, because youre changing the direction of the rotation/motion.
One thing about gyroscopes, is the wheels are intentionally heavy. This makes it require more force to change the trajectory.
Most of the time bicycle wheels are intentionally lightweight- just a few spokes and an aluminum rim.
So yes, the gyroscopic effect of a bicycle is negligible.

Negligible compared to what? There has to be a main force that's keeping bikes upright if there's other forces that can be said to be negligible in accomplishing that task. How much gyroscopic force would be required to keep a bicycle upright? More than the wheels provide? Is there evidence of this?

 

[DivergentSpectrum]

well, I am not sure how they did it, but in the article they mentioned a device that cancels out the gyroscopic effect, and the bike remained stable. So I am guessing there must be some kinda other force at work. In my experience riderless bikes don't go very far, but apparently there must be something to it.
Definitely worthy of an (ig)noble prize.

 

[jtbell]

I've moved this thread to the General Physics forum. Over the years, there have been a number of threads here and in the Classical Physics forum, about bicycle stability. Use the forum search (top of the page) to find them. Simply searching for "bicycle" in the desired forum should do it.

 

[CWatters]

It's certainly worth looking at studies and experiments.. It seems NEITHER the trail or the gyro effect are mandatory (but may help)..

http://io9.com/5792341/engineers-overturn-physics-but-keep-a-bicycle-upright

Traditionally, two physics phenomena were considered necessary for keeping bicycles upright. Turns out neither of them are. And humans aren't necessary either. If anyone had gone up to their physics professors a few days ago and asked what keeps bicycles upright, they would have gotten two answers; gyroscopic stability and the trail.

Scientists have built a bike that has neither property, but not only does it stay upright, it stays upright without a rider.

 

[Doug Huffman]

John Forester addresses 'Steering and Handling' (and stability) in Chapter 3 of Effective Cycling (MIT 1993)

 

[SteamKing]

You shouldn't rely on a publication like the New Statesman for the latest news in what science does or does not know.

 

[A.T.]

The gyroscope is well discredited by a device with a counter rotating front wheel.

By this logic, the rider's steering inputs are discredited by self stable bikes.

 

[jedishrfu]

There is some discussion from a 1970 article on bicycle physics:

http://socrates.berkeley.edu/~fajans/Teaching/MoreBikeFiles/JonesBikeBW.pdf

from this web page:

http://socrates.berkeley.edu/~fajans/Teaching/bicycles.html

 

[rcgldr]

To summarize the previous threads on this, self stability of a bicycle relies on a geometry that steers the front tire into the direction of a lean. The conventional method for doing this is trail, where the extended steering axis intercepts the pavement in front of the contact patch. The alternative used in some exeperiemental bikes locates a mass ahead of and above the front tire, with the front tire mounted so it's free to rotate about it's steerring axis, resulting in a yawing torque on the frame when leaned, which ends up steering the front into the direction of lean, without requiring any trail or caster setup.

Gyroscopic forces are reactions to a change in lean angle, not to the amount of lean, so they dampen lean rate, but do not correct an existing lean.

There is a small gyroscopic steering reation due to the roll torque related to the lean angle of a bike (gravity effectively pulls down at the center of mass, pavement pushes up at the contact patches), but generally it's insufficient to result in self stability, and the main effect is that gyroscopic forces dampen the lean rate.

There also is a tiny corrective roll torque in response to the rate of yaw while a bike is turning, but it's also insufficient to result in self stability.

 

[A.T.]

Gyroscopic forces are reactions to a change in lean angle, not to the amount of lean,

The front wheel reacts to roll torques, by steering into the lean that the roll torque tries to achieve.

so they dampen lean rate, but do not correct an existing lean.

Which is a crucial element of a good control mechanism:
http://en.wikipedia.org/wiki/PID_controller

There is a tiny corrective roll torque in response to the rate of yaw while a bike is turning, but it's insufficient to result in self stability.

The key is front wheel response the roll.

 

[rcgldr]

The front wheel reacts to roll torques, by steering into the lean that the roll torque tries to achieve ...

Take the case where the bike just happens to end up in a coordinated turn, the lean angle combined with the speed and steering angle of the front tire resulting in zero net torque about the roll axis, so no gyroscopic related tendency to return to a vertcial orientation. In the same circumstance, trail or other self correcting steering geometry would steer the front tire further inwards, resulting in a correction to a vertical orientation (but in a new direction).

Generally the gyroscopic reaction to roll torque is insufficient for self-stability. Even with sufficient trail geometry for self-correction to vertical orientation within a range of speed, if the speed of a bike exceeds what is called "capsize" speed, then the combined effect of gyroscopic reaction and trail results in the bike falling inwards at an extremely slow (virtually imperceptable in real world examples) rate due to the dampening of lean rate, and in the case of racing motorcycles, the sense that a rider gets from a racing motorcycle at high speeds is that the bike tends to hold it's current lean angle as opposed to tending either fall inwards or return to a vertical orientation. At these speeds, it takes the same amount of counter steering effort to return a bike to a vertical orientation as it does to lean the bike from a vertical orientation.

 

[sophiecentaur]

This is all pretty well understood (not by everyone, though - as happens with the Moon Landings Conspiracy and other bits of non-Science). There are several different factors at work that keep a bicycle from falling over. Proponents of each factor get very precious about it and claim it's the only relevant one, But you can arrange a counter rotating wheel, turn the front forks the other way round, put an incompetent rider on the bike etc. etc. It will fall over. You could improve just one of those parameters and the bike would stand a chance of not falling over. However, most bikes have all factors working in their favour and they usually don't fall over.
Angular Momentum = Magic for many people, which accounts for a lot of the misunderstandings.

 

[A.T.]

Generally the gyroscopic reaction to roll torque is insufficient for self-stability.

Yes, just like generally the derivative term alone, doesn't make a good controller.

 

[DaveC426913]

Rider balance is by far the major factor.

Consider:
Take a good rider, get him going up to some nice speed, enough to convince the gyroscopists that the wheels are keeping the bike upright.
Now drop the bike's wheels into a streetcar track.
Wheels are still spinning nice and fast, but the rider cannot balance.
How long will he remain upright? A dozen yards? then Wham!

 

[sophiecentaur]

Rider balance is by far the major factor.

Consider:
Take a good rider, get him going up to some nice speed, enough to convince the gyroscopists that the wheels are keeping the bike upright.
Now drop the bike's wheels into a streetcar track.
Wheels are still spinning nice and fast, but the rider cannot balance.
How long will he remain upright? A dozen yards? then Wham!

But the gyroscopic action in question is not keeping the bike up directly.It is just causing the front wheel to turn into the fall, which causes a 'righting' moment. The necessary torque for this is a lit less than the rote needed to pull the bike and rider upright.

 

[A.T.]

There are several different factors at work that keep a bicycle from falling over.

Exactly. Claiming that one factor "has been discredited", because it can also work without that single one, doesn't make sense. It would mean that all of them have been "discredited".

 

[A.T.]

Rider balance is by far the major factor.

enough to convince the gyroscopists that the wheels are keeping the bike upright.

As sophiecentaur noted, you completely misunderstood the issue.

 

[rcgldr]

Turn the front forks the other way round.

Since most forks reduce trail (they curve forward), turning them around increases trail, and such a bike will be self-stable even at very slow speeds.

Exactly. Claiming that one factor "has been discredited", because it can also work without that single one, doesn't make sense. It would mean that all of them have been "discredited".

Gyroscopic precession isn't required, since a bike could be make with two rounded skate blade (non-rotating), and with sufficient trail would be self stable on ice (within a range of speeds). What is common to all self stable geometries, is some method to turn the front tire into the direction of lean (even if the turn is coordiated, in which case, there's no roll axis torque to invoke a gryoscopic reaction). The main method for conventional bikes is trail: the contact patch is "behind" the point where the extended steering pivot axis intercepts the pavement. An alternate method is to locate a weight above and in front of the front tire without trail or castor but free to rotate about a steering axis, with the weight inducing a yaw torque in response to a lean which steers the front tire into the turn. I'm not aware of any other self stable methods.

 

[DaveC426913]

As sophiecentaur noted, you completely misunderstood the issue.

I did not misunderstand the issue.

I said rider balance is the major factor. There is some gyroscopic effect but it is greatly superceded by rider balance.

Yes, an empty bicycle can stay upright for a short time without a rider. So what? It is not a meaningful example of a bike "working".

Put a 150lb deadweight on the bike, three feet above the ground. How long do you think it'll stay upright then?

 

[A.T.]

I did not misunderstand the issue.

You obviously did misunderstand the gyroscopic steering mechanism, as your misguided proposal of putting the bike's wheels into a streetcar track shows. Or what do you think that would demonstrate?

Put a 150lb deadweight on the bike, three feet above the ground. How long do you think it'll stay upright then?

If you provide it with propulsion, so it says within a certain speed range, it can stay upright indefinitely.

 

[A.T.]

Gyroscopic precession isn't required

And neither is trail, and neither is rider steering input. That's the point. None of them is, by itself required.

 

[rcgldr]

Gyroscopic precession isn't required ...

And neither is trail, and neither is rider steering input. That's the point. None of them is, by itself required.

From my previous post, what is required is "some method to turn the front tire into the direction of lean", sufficient enough to return the bike back to a vertical orientation (within a range of speeds, I'm not aware of any passive geometry that works at all speeds). Adding a weight high enough and forward enough on a bike without trail or caster works, at 1:30 into this particular video, a random lean occurs soon after release, but the bike recovers.



The test no trail bike in this video appears to be more stable at 9:20 into the next video. I'm not sure if any changes were made to the bike from the previous video to result in what appears to be a more stable bike.



One advantage of the caster effect that's part of the trail geometry (versus the weight mounted above and in front of the bike) is that reaction to a side force applied relatively low on the bike (so a relatively smaller roll torque versus a side force applied high on the bike), such as a gusting side wind on a faired motorcycle, results in the tire steering downwind, leaning the bike into the wind, which then results in the tire steering into the lean, reducing the amount of downwind drift due to the wind.

I have yet to see any test bike made stable relying only on gyroscopic reactions to roll and/or yaw torque(s).

TU Delft also did a study on a conventional bicycle, and did some treadmill tests. For some unexplained reason, although the mathematical model for this bicycle shows that it should be in capsize mode (falling inwards at a very slow rate) at 8.00 meters / second or faster, the actual bike is shown to be "very stable" at 8.33 meters / second (30 kph) in the last video on this web page:

http://tudelft.nl/nl/actueel/laatste-nieuws/artikel/detail/treadmill-measurements

The graph of the model for this particular bicycle is show on page 4 of this pdf file. At the time the pdf document was created, it appears that they had only tested the model up to 6 meters / second, and that the treadmill test at 8.33 meters / second was done later.

http://www.tudelft.nl/fileadmin/UD/MenC/Support/Internet/TU_Website/TU_Delft_portal/Onderzoek/Wetenschapsprojecten/Bicycle_Research/Dynamics_and_Stability/doc/Koo06.pdf

 

[Wes Tausend]

...If you provide it with propulsion, so it says within a certain speed range, it can stay upright indefinitely.

That seems like a pretty good insight A.T.

If it is entirely true, we could theoretically experiment with a riderless electric bicycle to prove your statement holds. An option would be to RC (radio control) control the riderless experimental bike to guide it within a reasonable "outdoor laboratory" range. This scenario would not greatly differ from RC controlled toys such as this demo:
.

The actual observable operation begins around 2 minutes. These toys do use a gyro assist, perhaps as low speed necessity, but I do not believe it provides all the amazing inherent stability.

The "head angle" seems to me to make the most amount of difference in the bike handling, i.e. loss or gain of self-righting degree. One can exaggerate this "fork angle" in ones mind to imagine different effects. For example, reversing the head angle will cause the instability of a bike rolling backwards. Kicking the head angle out (raking the fork) seems to me to more-or-less vary the tendency of self-steer correction at differing speeds.

I have a couple of older off-road bikes that are contrasts in purpose. One is designed for slow nimble Trials riding with a quick-steering, steep fork angle. The other is designed for stable, fast high-speed desert riding, with generous head rake and trail. Manufacturers still tinker with the various settings of new models today. It must still be more of an art than science. The geometry understanding is mostly science now, but the remaining art skill must be estimating the average forces encountered on the various tracks to be used. Yet it seems tracks could be easily measured with the right dynamic bike sensors during practice, and become all science, as are stability systems on modern autos.

Wes
...

 

[A.T.]

If it is entirely true, we could theoretically experiment with a riderless electric bicycle to prove your statement holds. An option would be to RC (radio control) control the riderless experimental bike to guide it within a reasonable "outdoor laboratory" range.

All control you need is speed. The rest can be left to the bike. But I'm afraid no length of stable run will convince people, who believe that some rider input is required.

This video shows not only a long stable run, but also how the bike recovers from a significant perturbation:

 

[sophiecentaur]

I don't see why that comment is worth making. We all realize that it's a mechanism with many different factors so why should it be at all remarkable that there are limits to some of the parameters? A given design of aircraft only flies within a certain range of speeds but that is 'obvious' and the fact is not used to discredit the standard treatment of an aerofoil.
The whole business of what keeps a bike upright is very interesting but the topic always seems to trigger dissension rather than co operation between the proponents (experts?) of certain factors. More than one person can be right here without others being necessarily 'wrong'.

HAHA- my auto spell-correct just replaced "bike" with "bile". The iMac made my point.

 

[A.T.]

The whole business of what keeps a bike upright is very interesting but the topic always seems to trigger dissension rather than co operation between the proponents (experts?) of certain factors.

I'm certainly a proponent of a combination of many different factors.

More than one person can be right here without others being necessarily 'wrong'.

Claiming that a bike cannot be bike self stable and needs a balancing rider, as Dave seems to suggest, is almost certainly wrong, given all the existing theoretical and practical evidence to the contrary.

 

[rcgldr]

The whole business of what keeps a bike upright is very interesting but the topic always seems to trigger dissension rather than co operation between the proponents (experts?) of certain factors.

The only dissension here is whether gyroscopic reactions help or hurt self-stability (defined as a tendency to return to a vertical orientation). Take the case of a bike with zero trail: without going into the math details, note the rate of precession along the steering axis of the front wheel is proportional to the roll torque, but as the front wheel precesses inwards, the roll torque is reduced, and the roll torque becomes zero once the fornt tire steers inwards enough to produce a coordinated turn (assuming that this even occurs), leaving the bike leaned over (in a coordianted turn) as opposed to steering inwards enough to return to a vertical angle. Take the case of a bike with a conventional amount of trail. Once the front tire steers inwards enough to start correcting the lean, an "outwards" roll torque is produced, and gyroscopic reaction would tend to oppose the inwards steering needed to correct the lean angle.

It's my belief that the gryoscopic reaction is always opposing the corrective action of trail in most circumstances, since the trail geometry attempts to steer the front tire inwards at a greater rate than the precession rate related to roll torque during the lean in stage, and this is why bikes go into capsize mode (fall inwards at a very slow, almost imperceptible, rate) above some critical speed where the gyroscopic reactions dominate over the trail reactions.

 

[A.T.]

The only dissension here is whether gyroscopic reactions help or hurt self-stability (defined as a tendency to return to a vertical orientation).

A better definition of self-stability is "a tendency to return to a vertical orientation, an stay there, not oscillate around it."

It's my belief that the gryoscopic reaction is always opposing the corrective action of trail in most circumstances,

That is the point of "damping", or the derivative term in control theory: preventing over-correction and oscillation around the optimum.

 

[rcgldr]

The only dissension here is whether gyroscopic reactions help or hurt self-stability (defined as a tendency to return to a vertical orientation).

A better definition of self-stability is "a tendency to return to a vertical orientation, an stay there, not oscillate around it." That is the point of "damping", or the derivative term in control theory: preventing over-correction and oscillation around the optimum.

My concern is that gryoscopic reactions are sometimes described as correcting factors as opposed to dampening factors.

The caster aspect of trail is a dampening factor, and increasing trail can reduce or eliminate speed wobble. In the case of motorcycles, this was done on the Honda CBR900RR (1990's) which increased trail by reducing the triple clamp offset used on the early versions of that bike, initially an aftermarke part before Honda made the same change in later versions. Early radio control motorcycles used a lot of trail (like fork tubes located behind the pivot axis), but the newer ones rely more on gyros and/or active control.

Other dampening methods called steering dampers are used on some motorcycles. These can be friction oriented, small shocks linked to frame and triple clamp, or electronically controlled.

The test bikes made with counter rotating wheels don't seem to have an oscillation problem with self stability, but the test speeds may not have been fast enough to result in speed wobble.

I'd like to see an actual two skate bike gliding on ice to show that gryoscopic reactions are not required for correcting or dampening lean angle.

Speed wobble can also be caused by too much flex in the frame and/or swing arm. I don't know if trail and/or steering dampers can compensate for this.

 

[A.T.]

My concern is that gryoscopic reactions are sometimes described as correcting factors as opposed to dampening factors.

If you think that the two are mutually exclusive, then maybe the term "damping" is misleading here. Its better to see the derivative correction as a predictive correction. It's based not on the current error, but the projected error, based on it's current change.

 

[rcgldr]

My concern is that gryoscopic reactions are sometimes described as correcting factors as opposed to dampening factors.

If you think that the two are mutually exclusive, then maybe the term "damping" is misleading here.

My point is the potential misconception that gyroscopic reactions alone (without any trail or other corrective steering geometry) would result in a self stable (one that tends towards a vertical orientation) bike. In order to correct a lean, the front tire must be steered inwards enough to produce an outwards roll torque, which gyroscopic reaction alone will not do.

However trail without gyroscopic reaction can result in a self stable bike without oscillation if the caster effect related to trail is large enough (within a range of speeds). In the Delft test bike without gyro or trail effects, I'm not sure what prevents oscillation. The front steering assembly has weights attached to the sides, but I'm not sure if this is used to dampen the reaction. Friction in the steering bearing could be sufficient to dampen oscillation.

 

[Wes Tausend]

If you think that the two are mutually exclusive, then maybe the term "damping" is misleading here. Its better to see the derivative correction as a predictive correction. It's based not on the current error, but the projected error, based on it's current change.

I agree, "predictive" is better word. Damping can only slow a process, not correct it.

I also agree with , rcgldr when he said in post #34, "...that the gyroscopic reaction is always opposing the corrective action..." . There are times when the wheel must automatically freely self-turn to drive back under the bike's own center of gravity, and gyroscopic forces could only be detrimental.

I believe David E.H. Jones nailed the primary self-stability principle on page four (mid-right column) of his pdf (given us earlier in post #15 by jedishrfu - thank you).
Jones said:
"Why does steering geometry matter? One obvious effect is seen by wheeling a bicycle along, holding it only by the saddle. It is easy to steer the machine by tilting the frame , when the front wheel automatically steers into the lean. This is not a gyroscope effect, because it occurs even if the bike is stationary. A little study shows that it occurs because the center of gravity of a tilted bicycle can fall if the wheel twists out of line. So here was a new theory of bicycle stability--the steering is so angled that as the bike leans, the front wheel steers into the lean to minimise the machine's gravitational potential energy. To check this theory I had to examine the implications of steering geometry very seriously indeed."

So basically, because of castor (steering-head) angle and subsequent trail, the weight of the bike geometrically seeks to settle to a lower gravitational level which is found by the wheel self-turning (allowing frame falling) whenever the bike tilts. This self-evident observation is consistant with the wheel self-turning whenever we may place a stationary bike on it's side-kickstand, whereby the bike leans onto the stand and the wheel prefers to turn into the lean. This self-turn is also designed to be the correct direction the falling bike must take to cause a corrective, uprighting force when rolling. As the bike then "uprights", the self-steering event tapers to naught and the rotating frame only overshoots if the uprighting momentum is too great. In this case the same self-correction events tip it back upright from an alternate lean. The well designed bike may exhibit a momentary diminishing wobble as it thus oscillates, but soon recovers to a smooth roll.

Intuitively, I think the speed (distance per time) of forward bike motion with which the events unfold successfully with a particular geometry depend on the distance per time of gravity. IOW, the bike-spec ideal speed range would change with gravity or inertia. This entire principle could account for high speed motorbike wobble, sometimes appearing during acceleration or deceleration. Of course the easiest fix is a friction damper or geometry change. But even then, during increased centrifigal forces against a dirt berm or banked track, wobble could still suddenly rear it's ugly head under additonal inertial "gravity".

Since this all involves suspension science, I believe there is another way to look at the situation. The ideal chosen geometry when moving, seems to project a virtual center of gravity significantly higher than the bikes actual stationary center of gravity. Thus the rolling bike acts as though it is guided by a tightrope strung somewhere above it. If the seat were to be removed and a vertical pole mounted in it's place, one could theoretically attach a laser pointer to said pole, aiming straight ahead parallel with the frame. In spite of minor bike directional disturbances, the point of light should continue to subscribe within a limited circle on a building directly ahead of the direction of travel. One might be able to more simply imagine this by observing a riderless bike seemingly attempting to swing from such an invisible overhead guiderope.

Wes
...

 

[DaveC426913]

Point of order : before we beat to death the factors that make a bike work, are we sure.we are all.agreed on what it means for a bike to "work"?

To my mind an empty bike is fundamentally different than an occupied bike. The forces that keep a riderless bike vertical even while it follows an unguided trajectory.to nowhere are almost entirely irrelevant to the forces that keep an occuped bike vertical, stable AND going.in a USEFUL direction.

seems to me that is a critical point of contentin we must resolve BEFORE.deciding we have.proven any point about it.

Please. Convince me that a riderless bike is synonymous with a purposefully driven ridered bike.

 

[Paradox101]

Point of order : before we beat to death the factors that make a bi

ke work, are we sure.we are all.agreed on what it means for a bike to "work"?

To my mind an empty bike is fundamentally different than an occupied bike. The forces that keep a riderless bike vertical even while it follows an unguided trajectory.to nowhere are almost entirely irrelevant to the forces that keep an occuped bike vertical, stable AND going.in a USEFUL direction.

seems to me that is a critical point of contentin we must resolve BEFORE.deciding we have.proven any point about it.

Precisely. If one refers to an empty bike, it can be clearly explained because of the angle of the fork and yes, as everyone said, gyroscopic precession and equalized torques(duh).However, when a rider sits on a bike, we can't try to explain how it works in the very manner we would attempt to explain an empty bike,as there are a lot of factors(weight of biker,center of gravity,horizontal weight distribution,etc).

 

[Wes Tausend]

Point of order : before we beat to death the factors that make a bike work, are we sure.we are all.agreed on what it means for a bike to "work"?

To my mind an empty bike is fundamentally different than an occupied bike. The forces that keep a riderless bike vertical even while it follows an unguided trajectory.to nowhere are almost entirely irrelevant to the forces that keep an occuped bike vertical, stable AND going.in a USEFUL direction.

seems to me that is a critical point of contentin we must resolve BEFORE.deciding we have.proven any point about it.

Please. Convince me that a riderless bike is synonymous with a purposefully driven ridered bike.

Dave,

For what it's worth, I never had a problem with your original point that a bike would tip, with and without a rider, if it's wheels were caught in a streetcar track. I thought it a valid insight. But if the bike can, it will steer itself to remain upright, even without a rider.

The forces that keep a riderless bike vertical even while it follows an unguided trajectory.to nowhere are relevant to the forces that keep an occuped bike vertical, stable. Let me try to explain.

I agree, a riderless bike is not synonymous with a purposefully driven ridered bike, but it acts synonymous to a large degree. The difference is only that a change of direction cannot be normally chosen by a riderless bike. As an example, Jones (in his pdf), allowed a riderless bike to roll after wetting the tires. The tires therefore wrote their path on the pavement (figure 7, bottom). One can see that the rolling riderless bike initially attempts to stably continue in the same direction when undisturbed.

When Jones bumps the handlebar, he steers the bike in a "permanent" new direction as though he is momentarily the controlling rider although he is not aboard as normal. The wobbly bike then automatically seeks to geometrically straighten it's new "permanent" path. In my opinion, the initial wobble is characteristic of an instability that forms from gyroscopic reaction forces to steering input, but the bike geometry still automatically recovers on it's own. Because of castor, rake and trail geometry, the bike literally automatically steers under it's own center of gravity when leaned, aka "when falling". It does this when leaning because the frame can drop to a lower level.

One can more readily imagine the frame dropping when the wheel turns into the lean, by exaggerating the situation. Imagine the bike rake at rest has an extended fork, so extended the fork is closer to horizontal rather than vertical. When the bike is leaned in the least degree, the fork will automatically turn into the lean and the front wheel will nearly lay on its hub as the frame drops. At a lesser rake, the same frame drop occurs more subtly. The frame always naturally seeks the lowest gravity possible automatically. That is the secret, IMO.

In a different perspective, when rolling ahead, the bike geometry behaves as though it's dynamic center of gravity has moved above it, and it "hangs" from this invisible, virtual guidewire. A rider in continuous control can alter the bike path (alter the virtual guidewire) innumerable times, but as long as it rolls within a certain speed range, the bike itself (and load) remains automatically stable, seeks upright and favors the latest straight ahead path nearly regardless of loaded weight.

The Jones "geometry theory" I quoted in post #39 pretty much solves the primary moving stability of a ridered, or riderless bike, IMO.

The stable speed range of a bike appears to be in between too slow to steer-compensate for gravitational "fall", and too fast whereby the steering feedback corrects too fast (overcorrects) and oscillation (headshake) occurs. In addition, this principle seems to be the most likely candidate for the reason that some grocery cart castor wheels shimmy too.

The cart wheels are simply unstable when driven above the speed range afforded by their worn geometry. With a loose axle and/or "kingpin" (head bearing), the "leaning" cart wheel begans to oversteer from center (shimmy). I haven't thought this much about bikes geometry before, but I have long wondered why the cart wheels shimmy. The speeds are too slow for significant gyro forces. Having worked part-time in a grocery store while in college, I know oil didn't help, but rather made it worse.

Wes
...

 

[FactChecker]

We have to be careful here. Just because other designs do not require rider balance, trail, or gyroscopic effect does not mean that the common design does not need it. Also, examples where the tire is on a slippery surface may defeat all those mechanisms, so I don't think it proves anything. There are probably multiple modes that depend on different stability feedbacks. Clearly, a fast moving bicycle is stable for a short time without a rider but not for a long time. Just as clearly, there are situations where a bicycle is so unstable in a short period mode that no rider can keep it up. (I have been on a motorcycle on streetcar tracks that flipped so fast that no one could control it, Same for tank slap.) For the common design, I am confident that rake, trail, rider balance, and gyroscopic forces are all major factors in certain situations.

To answer the original question, I believe that we have the capability to model the equations of motion of a bicycle and know how much force is coming from every possible feature of the design at every speed and condition. If it hasn't been done, it is because no one needed to do it. I bet it has been done for motorcycles. But the stability is probably not so simple that a blanket statement can be made. Nothing is simple.

 

[Dylan Cram]

Hello All, I am new here, and have been an avid cyclist for 12 years (not including the usual stint in childhood that much of us probably share). I have long been interested in this topic, because bicycles are truly fascinating to me.

Anyway, I have an idea about what the root cause of bicycle stability is. There are many factors involved in the operation of a bicycle, but overall, the way they turn into a fall without a rider is the key. Now, the task is to explain WHY they turn into a fall. The reason isn't gyroscopic, or dependent on trail, although those factors affect the performance of a bike. For example: the longer trail of a touring bike has a less twitchy feel than the shorter trail of a track bike. I have both, and a track bike feels like an f-16 compared to a b-52 tourer.

As to the why. I will use an analogy. The reason bicycles self correct is the very same reason that trains stay on their tracks. Trains are not bound to their tracks in any way. They have beveled wheels, such that the outermost part has a smaller diameter than the inner part. So, if a train moves laterally on the tracks (perpendicular to it's direction of motion), on one side the diameter at the contact surface of that wheel will be smaller, and the other side will have an increase in diameter. That asymmetry will cause a corrective turn toward the lower rotational velocity, and the train continuously self-corrects and stays on the tracks.

Now, imagine a bicycle perfectly upright. It's tires have a round cross section. The weight of the bike, or bike plus rider, ensures that the contact area of the tire to the ground is a non-zero area. Across the contact patch, from left to right of the bike, there are different rotational velocities at play. The outer parts of the patch are different than a dead-center abstract point. That's okay~ it's balanced on both sides. If the bike starts to fall over, which it obviously would, the contact patch remains pretty much the same, but is shifted to one side of the tire. When that happens, the side of the patch 'inside' the lean has a slower rotational velocity than the part 'outside' the lean. What we have here analogously, is both wheels of a train contained within the contact area of the bike tire. What will happen, is that the front wheel will turn about it's steering axis toward the lean, because of the higher rotational velocity outside of the lean. The true key to why a bicycle stays upright is a combination of two things: the ability of the front wheel to rotate about a steering axis relative to the rest of the bike, and the cross-sectional shape of the tires. The bike referred to in earlier posts~ with the trail and gyroscopic effects nullified, still had wheels with a rounded cross section. That is the key. I'm actually kind of baffled as to why they didn't see it.

There was a previous poster whom asserted that rider balance is the most important factor. There is no truth to that. If I stop at a traffic light, I can balance pretty good, such that I might not have to put a foot down, but once the bike is stationary, that's all on me, and I have to do a track stand, which has nothing to do with why a bike stays up in motion. Once the bike is moving, there is no effort to balance. You can tell adept bicycle riders from the less skilled by how much of a line they can hold. If a rider can keep their tires inside a white line on the side of the road (a few inches) at high speed, they are good, but it doesn't mean that rider balance plays much of a part, just that fine motor control of the handlebars is good. Good riders know how to work WITH the bike. They just intuitively know how it responds.

A good experimental angle to pursue, is different contact area cross sections.

 

[rcgldr]

The stable speed range of a bike appears to be in between too slow to steer-compensate for gravitational "fall", and too fast whereby the steering feedback corrects too fast (overcorrects) and oscillation (headshake) occurs. In addition, this principle seems to be the most likely candidate for the reason that some grocery cart castor wheels shimmy too.

On most bicycles / motorcycles, if the speed is too fast, the gyroscopic dampening effect (which counters the corrective steering related to trail) is large enough to dominate and the bike doesn't correct (or wobble), but instead holds the current lean angle or falls inwards at an extremely slow rate, called "capsize" mode. Speed wobble on a bike is bike specific, factors include flexing within the frame work, too little trail, ... , and on some bikes, steering dampers were used to solve the issue.

It's tires have a round cross section. ... Across the contact patch, from left to right of the bike, there are different rotational velocities at play.

If this was true then a bike with the front tire locked straight ahead would steer if leaned but this doesn't happen. A pair of cone shaped "wheels", one in front of the other with parallel axis will move in a nearly straight line, with a lot of slippage on the surface of the cones. In the case of a bike in a turn, the cornering forces result in the contact patches being twisted a bit outwards.

 

[Dylan Cram]

If this was true then a bike with the front tire locked straight ahead would steer if leaned but this doesn't happen. A pair of cone shaped "wheels", one in front of the other with parallel axis will move in a nearly straight line, with a lot of slippage on the surface of the cones. In the case of a bike in a turn, the cornering forces result in the contact patches being twisted a bit outwards.

How would a bicycle 'steer' if leaned, if it's ability to steer is removed? A bicycle with a locked headset would not apparently turn if leaned, because there are two wheels involved on the same plane, and the contact area is tiny. You can do it yourself by standing on one side of the frame while coasting. The forward momentum of the bike would easily overcome the varying rotational speeds in the contact patch. Your example of come-shaped wheels is an extreme and thus an inaccurate analogy, yet kind of nullifies your point at the same time? I don't follow you there. The contact patches of a bicycle are very small, especially in the case of my road bike with 25 cm, 100psi tires.

With the train analogy, it is the limited freedom of motion between cars that allows the beveled wheels to do their job. If the entire train was a monolithic body, the train wouldn't be on the tracks for long.

When a bike is cornering, the contact patches move inwards, not outwards.

 

[cosmik debris]

Having read through all this it seems the original poster's suggestion, that no one know how a bicycle works, is true :-)

 

[rcgldr]

A pair of cone shaped "wheels", one in front of the other with parallel axis will move in a nearly straight line, with a lot of slippage on the surface of the cones. In the case of a bike in a turn, the cornering forces result in the contact patches being twisted a bit outwards.

How would a bicycle 'steer' if leaned, if it's ability to steer is removed?

I misunderstood what you were getting at. I thought you meant that the round profile of a tire would generate an inwards force, as opposed to generating a steering torque. What I should have posted is that the contact patch on a bicycle with thin tires is too small to generate a significant steering torque, yet such bicycles are very stable due to trail.

In the case of a bike in a turn, the cornering forces result in the contact patches being twisted a bit outwards.

When a bike is cornering, the contact patches move inwards, not outwards.

The contact patches shift laterally inwards, but twist (rotate) outwards due to the cornering loads. The path of a tire is a bit outwards of the direction of the tire due to this outwards twisting at the contact patches, known as slip angle, although actual slippage is not required to have a slip angle.

 

[Dylan Cram]

Having read through all this it seems the original poster's suggestion, that no one know how a bicycle works, is true :-)

Perhaps, but it seems we are in the end game now.

Hey rcgldr, looks like you edited a post :)

 

[rcgldr]

Hey rcgldr, looks like you edited a post.

I did a strike through, since I worded that badly. That should have been about lateral forces related to a leaned round tire, not a steering reaction.

Having read through all this it seems the original poster's suggestion, that no one know how a bicycle works, is true.

It's well understood that self-stability is related to having a geometry that causes the front tire to steer in the direction of lean sufficiently enough to tend to correct the lean and return to a vertical orientation (the direction of the path will change due to disturbance, but the bike will return to vertical). Depending on the geometry, there is a range of speed for that self stability. The conventional method for self-stability is trail / caster. One alternate method is to locate a weight above and in front of a front wheel with zero caster, so that when leaned, the yaw torque (about the vertical axis) results in a steering reaction by the front tire.

As posted before, although gyroscopic reactions seem like they would help, gyroscopic precession is a reaction to torque, not lean angle. In general, gyroscopic reaction dampens (resists) the self-correcting steering related to geometry.

The contact patches shift laterally inwards, but twist (rotate) outwards due to the cornering loads. The path of a tire is a bit outwards of the direction of the tire due to this outwards twisting at the contact patches, known as slip angle, although actual slippage is not required to have a slip angle.

Complicating matters is the relative slip angle at the front and rear tire (relative camber stiffness), similar to understeer versus oversteer and the effect on steering inputs.