We dont know how a bicycle works Really?

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  • #2
Astronuc
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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 cant 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).
 
  • #3
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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.
 
  • #4
Doug Huffman
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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.
 
  • #5
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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.
 
  • #6
lavinia
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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?
 
  • #7
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.
 
  • #8
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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?
 
  • #9
well, im 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 im guessing there must be some kinda other force at work. In my experience riderless bikes dont go very far, but apparently there must be something to it.
Definitely worthy of an (ig)noble prize.
 
  • #10
jtbell
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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.
 
  • #11
CWatters
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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.
 
  • #12
Doug Huffman
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John Forester addresses 'Steering and Handling' (and stability) in Chapter 3 of Effective Cycling (MIT 1993)
 
  • #13
SteamKing
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You shouldn't rely on a publication like the New Statesman for the latest news in what science does or does not know.
 
  • #14
A.T.
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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.
 
  • #16
rcgldr
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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.
 
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  • #17
A.T.
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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.
 
  • #18
rcgldr
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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.
 
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  • #19
sophiecentaur
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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.
 
  • #20
A.T.
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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.
 
  • #21
DaveC426913
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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!
 
  • #22
sophiecentaur
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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.
 
  • #23
A.T.
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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".
 
  • #24
A.T.
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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.
 
  • #25
rcgldr
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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.
 

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