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Author Topic: Why do bicycles have such big wheels?  (Read 28056 times)

Offline Bored chemist

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Why do bicycles have such big wheels?
« Reply #25 on: 07/09/2009 18:43:09 »
I have a recollection that someone with nothing better to do produced a bike with a contra rotating gyroscope to cancel out the gyro effect of the wheels- it didn't make much difference.
 

Offline lightarrow

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Why do bicycles have such big wheels?
« Reply #26 on: 07/09/2009 20:04:52 »
To LeeE and Geezer: if you remove unessential things from the physical model of the problem, that is air friction ecc, the only difference between a moving and a not-moving bicycle is the fact wheels spin, in the first case. So if you want to find a cause of the different equilibrium, you have to look for here.
 

Offline lightarrow

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Why do bicycles have such big wheels?
« Reply #27 on: 07/09/2009 20:12:58 »
I have a recollection that someone with nothing better to do produced a bike with a contra rotating gyroscope to cancel out the gyro effect of the wheels- it didn't make much difference.
Don't know what to say, because I really have difficulties to believe it.
 

Offline Geezer

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Why do bicycles have such big wheels?
« Reply #28 on: 07/09/2009 20:32:22 »
To LeeE and Geezer: if you remove unessential things from the physical model of the problem, that is air friction ecc, the only difference between a moving and a not-moving bicycle is the fact wheels spin, in the first case. So if you want to find a cause of the different equilibrium, you have to look for here.

Lightarrow - As we might say, baloney! (You might prefer Balogna of course ;D)

Please refer to the second post in this thread. How much stabilization would wheels that size produce? Yet I'm sure that bicycle actually worked.

The human operator (cyclist) and the bicycle become a control system. While the bicycle is moving, the operator makes exquisite movements of the steering and his/her body mass to maintain equilibrium. We are so good at it, we hardly even know we are doing it, although we do have to learn how to do it. If the bicycle was self stabilizing, why would we need to learn how to balance it?

When a bicycle is stationary, the operator has lost a very important method of adjusting the system (the steering) so it becomes almost impossible to maintain equilibrium.

 

Offline Bored chemist

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Why do bicycles have such big wheels?
« Reply #29 on: 07/09/2009 20:50:55 »
I have a recollection that someone with nothing better to do produced a bike with a contra rotating gyroscope to cancel out the gyro effect of the wheels- it didn't make much difference.
Don't know what to say, because I really have difficulties to believe it.
Seeing is believing.
http://www.rainbowtrainers.com/default.aspx?Lev=2&ID=34
"Zero-Gyroscopic Bike I is a clever and yet simple experiment that dispels once and for all the centuries old conventional wisdom that a bike stays upright primarily due to the gyroscopic action of the two rotating tires. "
 

Offline lightarrow

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Why do bicycles have such big wheels?
« Reply #30 on: 07/09/2009 21:04:40 »
When a bicycle is stationary, the operator has lost a very important method of adjusting the system (the steering)
Can you pleas help me to understand this? I really don't see which limitations he has in steering when stationary.
 

Offline lightarrow

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Why do bicycles have such big wheels?
« Reply #31 on: 07/09/2009 21:07:00 »
I have a recollection that someone with nothing better to do produced a bike with a contra rotating gyroscope to cancel out the gyro effect of the wheels- it didn't make much difference.
Don't know what to say, because I really have difficulties to believe it.
Seeing is believing.
http://www.rainbowtrainers.com/default.aspx?Lev=2&ID=34
"Zero-Gyroscopic Bike I is a clever and yet simple experiment that dispels once and for all the centuries old conventional wisdom that a bike stays upright primarily due to the gyroscopic action of the two rotating tires. "
With wheels put in that way, you certainly don't have exactly zero gyroscopic effect.
 

Offline Geezer

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Why do bicycles have such big wheels?
« Reply #32 on: 07/09/2009 21:39:02 »
When a bicycle is stationary, the operator has lost a very important method of adjusting the system (the steering)
Can you pleas help me to understand this? I really don't see which limitations he has in steering when stationary.
I'll try (sorry about the baloney comment - I could not resist it.)

Imagine you are cycling in a straight line at constant speed. Your body mass will tend to continue in a straight line. Now, the path of the tires deviates slightly so that it is no longer directly beneath your center of mass. This causes you, and the bicycle to tilt very slightly. I'm not sure exactly how the brain detects this tilt, but somehow it does. Because our brain is "trained" to maintain the bike in balance, we make a slight adjustment of the steering so that the path of the tires moves back under our center of mass.

I suspect, when we cycle in a straight line the tires actually trace a very small amplitude sine wave while our bodies in fact do travel in almost a straight line. This would be the same behaviour that a servo control system would exhibit. There is always a small error that it tries to cancel out.

The speed we travel at has some effect on the "gain" of the system - in other words, how quickly a certain steering input repositions the tire path under our center of mass. I suspect the "gain" increases with speed, but that might be baloney! If it does, when the speed is zero, the system has no gain.

Either way, you can see that if the bicycle is stationary, moving the steering is not going to be able to reposition the path of the tires relative to the center of mass of the rider (and bicycle).
« Last Edit: 07/09/2009 21:42:10 by Geezer »
 

Offline JimBob

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Why do bicycles have such big wheels?
« Reply #33 on: 07/09/2009 21:58:44 »
Just an observation - a bicycle will not function without a rider. The rides needs to push the pedals to alternative sides of the center of gravity to make the thing move (only "center" if it were still.) Thus to BE a functional bicycle, it must be moving. I watched part of the Tour de France this summer. The front wheel had to move on every one of the bicycles.
« Last Edit: 08/09/2009 16:43:24 by JimBob »
 

Offline RD

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Why do bicycles have such big wheels?
« Reply #34 on: 07/09/2009 22:06:55 »
Just an observation - a bicycle will not function without a rider.

As someone who has dismounted a bicycle at high speed by grabbing onto a rope swing I can say the above is incorrect:
 my bicycle continued riderless for at least 50 meters.
 

Offline Geezer

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Why do bicycles have such big wheels?
« Reply #35 on: 07/09/2009 22:20:26 »
JimBob - Good point. The bicycle and the cyclist become a single "unit", and it does get a lot more complicated when the rider is working hard. Going round bends adds further complications.

The case I was describing is possibly the simplest where the cyclist is cruising down a slight gradient in a straight line at constant speed with no, or almost no, force applied to the pedals. That case is hard enough to understand. When you start adding all the other variables it gets really complicated. The interesting thing is that we can figure all this out when we are about four years old, and we don't even know we are doing it!

Wonder if anyone ever made a control system, or robot, that can "go" a two wheeler? Sounds like an interesting AI challenge project. Anyone got a million quid to spare for the prize?

EDIT Neil's pretty "pally" with that Branson geezer. He should be good for a few quid, don't you think?
« Last Edit: 07/09/2009 22:25:18 by Geezer »
 

Offline LeeE

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Why do bicycles have such big wheels?
« Reply #36 on: 07/09/2009 22:59:50 »
I think we need to go back and remember the difference between static and dynamic stability.

A box sitting on the ground will be statically stable; as long as its CoG remains within the plane of contact it won't topple over.  When we walk or cycle anywhere though, we rely upon dynamic stability: in either case, while we are moving we are not statically stable, and if we were to suddenly 'freeze' we'd fall over, whether walking or riding a bicycle, but because we're moving and anticipating where we will be, even as we start to move to that point, we're already subconsciously planning how we will react to ensure that we don't fall over when we get there.  Cycling, just as with walking, is really more a case of being in a state of controlled falling rather than being in a state of stability.

If someone were to make a bicycle with fixed handlebars and forks, so that the bicycle could only go in a straight line, and someone was catapulted away on it, they'd fall over sideways just as quickly as if they were standing still.  Unless the wheels had significant mass, or were spinning at unfeasibly high speeds, the gyroscope effect would be insignificant.
 

Offline RD

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Why do bicycles have such big wheels?
« Reply #37 on: 07/09/2009 23:42:23 »
Wonder if anyone ever made a ... robot, that can "go" a two wheeler?


« Last Edit: 07/09/2009 23:50:22 by RD »
 

Offline Geezer

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Why do bicycles have such big wheels?
« Reply #38 on: 08/09/2009 02:06:25 »
Wonder if that's RD doing his famous robot impersonation?

Edit: If the gyroscope effect on a bicycle was quite large, what might happen when you actually tried to change direction?

Edit: RD, are you up for a small experiment? LeeE has something in mind involving a bike with no steering and a steam catapult. I think the idea is to launch a volunteer off the deck of an aircraft carrier or something, and I thought you were well qualified based on your past experience.

EDIT (again!!): I think I may be full of it (chorus of "We already new that!"). When you get moving at any reasonable speed, I don't think you really turn the handle bars at all. (What - Geezer's lost it.) I think you actually transfer your weight which causes the handle bars to turn. It's a subtle distiction I know, but here's the proof. You can easily cycle great distances without actually touching the handle bars. Look mum, no hands!
« Last Edit: 08/09/2009 06:11:52 by Geezer »
 

Offline lightarrow

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Why do bicycles have such big wheels?
« Reply #39 on: 08/09/2009 07:46:43 »
When a bicycle is stationary, the operator has lost a very important method of adjusting the system (the steering)
Can you pleas help me to understand this? I really don't see which limitations he has in steering when stationary.
I'll try (sorry about the baloney comment - I could not resist it.)

Imagine you are cycling in a straight line at constant speed. Your body mass will tend to continue in a straight line. Now, the path of the tires deviates slightly so that it is no longer directly beneath your center of mass. This causes you, and the bicycle to tilt very slightly. I'm not sure exactly how the brain detects this tilt, but somehow it does. Because our brain is "trained" to maintain the bike in balance, we make a slight adjustment of the steering so that the path of the tires moves back under our center of mass.

I suspect, when we cycle in a straight line the tires actually trace a very small amplitude sine wave while our bodies in fact do travel in almost a straight line. This would be the same behaviour that a servo control system would exhibit. There is always a small error that it tries to cancel out.

The speed we travel at has some effect on the "gain" of the system - in other words, how quickly a certain steering input repositions the tire path under our center of mass. I suspect the "gain" increases with speed, but that might be baloney! If it does, when the speed is zero, the system has no gain.

Either way, you can see that if the bicycle is stationary, moving the steering is not going to be able to reposition the path of the tires relative to the center of mass of the rider (and bicycle).
Geezer, I still can't understand why all that you wrote is impossible to do when the bicycle is stationary. First answer me this question: which is the *only* difference between stationary and moving bicycle? Don't tell me that it is the bicycle speed or linear momentum because you can always take the frame of reference where the bicycle is not moving and nothing must change, for what we are considering here.
 

Offline syhprum

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Why do bicycles have such big wheels?
« Reply #40 on: 08/09/2009 09:27:22 »
Geezer

When you cycle without your hands on the handlebars what you do is push the saddle from side to side with your bottom which moves the cycle from the vertical position which in turn rotates the steering due to the castor angle.
Try riding just standing on the pedals no hands you will find it near impossible.
 

lyner

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Why do bicycles have such big wheels?
« Reply #41 on: 08/09/2009 09:49:48 »
lightarrow
I think you are ignoring the couple which acts to pull the bike upright when it is following a curve and leaning in. It cannot be ignored at speed (see speed skating - no wheels).  The castor angle causes the bike to lean in the appropriate direction, producing the same required couple.
This requires movement - as does the gyroscopic action.

Edit - added skating thing
« Last Edit: 08/09/2009 09:53:49 by sophiecentaur »
 

Offline RD

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Why do bicycles have such big wheels?
« Reply #42 on: 08/09/2009 12:26:00 »
RD, are you up for a small experiment? LeeE has something in mind involving a bike with no steering and a steam catapult.
 I think the idea is to launch a volunteer off the deck of an aircraft carrier or something


No thanks. We need a stuntman like Evel Knieval* to prove a bike does not need a rider ...



http://www.firebox.com/product/1430/Evel-Knievel-Stunt-Set?itc=30&src_t=nwt&src_id=119

[* the plastic toy version]

Trials bikers and BMX riders can balance on two wheels without going forward by rapidly shifting their body, so the gyroscopic effect of the wheels is not necessary for balance, but the gyroscopic force does exist  particularly at high speed.


« Last Edit: 08/09/2009 18:38:27 by RD »
 

lyner

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Why do bicycles have such big wheels?
« Reply #43 on: 08/09/2009 13:22:21 »
The trucks on skateboards must have a similar self-righting effect but the skill of actually staying upright is so great that the effect is probably not noticed by anyone who can actually ride on one. (There can hardly be any significant gyroscopic effect)
 

lyner

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Why do bicycles have such big wheels?
« Reply #44 on: 08/09/2009 13:29:22 »
RD, your last comment made me realise - the effect which keeps you up once the bike starts to tilt and then turn due to the castor action is also, effectively, a gyroscopic / angular momentum vector effect. Will not changing the angular momentum vector by turning from a straight path involve a couple at right angles, which will tend to rotate the bike 'upwards'? I am not referring to the wheels - I refer to the curved path taken by the bike and the associated angular momentum.
 

Offline Karen W.

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Why do bicycles have such big wheels?
« Reply #45 on: 08/09/2009 15:39:25 »
Hey besides all the reasons that have been mentioned in the two pages here I would simply like to add my observation.

Is it quite possible that the smaller wheels cover less ground per rotation, and thus, take more effort to reach a required distance due to size when larger wheels actually cover more ground surface and distance per rotation then smaller wheels, and would that not make them more efficient simply because of distance covered per rotation...?
« Last Edit: 08/09/2009 15:43:20 by Karen W. »
 

lyner

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Why do bicycles have such big wheels?
« Reply #46 on: 08/09/2009 17:12:51 »
Karen
We did mention the increased frictional losses from small wheels, earlier. There is no, inherent, difference between the energy needed for large or small wheels - 'just' the frictional effects. If you use the right, lossless, gearing, there is no difference in the work needed to be done on the pedals for large or small wheels to, say, go up a given hill.
 

Offline Geezer

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Why do bicycles have such big wheels?
« Reply #47 on: 08/09/2009 18:20:21 »
So far, as Turveyd pointed out, the main advantage of large wheels (apart from the obvious machismo impact of course  :D) seems to be that they really do reduce the likelyhood of "wedging" the front wheel on rocks and causing an A over T while riding off-road.
 

Offline Bored chemist

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Why do bicycles have such big wheels?
« Reply #48 on: 08/09/2009 18:54:51 »
I have a recollection that someone with nothing better to do produced a bike with a contra rotating gyroscope to cancel out the gyro effect of the wheels- it didn't make much difference.
Don't know what to say, because I really have difficulties to believe it.
Seeing is believing.
http://www.rainbowtrainers.com/default.aspx?Lev=2&ID=34
"Zero-Gyroscopic Bike I is a clever and yet simple experiment that dispels once and for all the centuries old conventional wisdom that a bike stays upright primarily due to the gyroscopic action of the two rotating tires. "
With wheels put in that way, you certainly don't have exactly zero gyroscopic effect.

Why not, or, at least, why isn't the remaining gyro effect so small as to be unnoticable?
« Last Edit: 08/09/2009 19:01:56 by Bored chemist »
 

Offline Geezer

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Why do bicycles have such big wheels?
« Reply #49 on: 08/09/2009 19:19:54 »
Geezer, I still can't understand why all that you wrote is impossible to do when the bicycle is stationary. First answer me this question: which is the *only* difference between stationary and moving bicycle? Don't tell me that it is the bicycle speed or linear momentum because you can always take the frame of reference where the bicycle is not moving and nothing must change, for what we are considering here.
OK - Let me try again.
Same conditions as before - constant speed, no pedalling etc. (I'm changing my story slightly!) Let's say the rider transfers weight slightly to the right. This weight transfer exerts a turning moment in the steering because of the castor angle, and the steering turns slightly right. The tires follow a path to the right, but the combined center of mass of the rider and bicycle tends to continue in a straight line - (think inverted pendulum). Because the path of the tires moved to the right relative to the center of mass of the rider/bicycle, the turning moment in the steering now reverses and the path of the tires now swings to the left, etc. etc.

Now, when the ensemble is stationary and the rider transfers weight to one side or the other it has no effect on the points where the tires contact the road relative to the rider/cycle center of mass. To stay upright, he must rapidly transfer weigh to the other side. He gets no assistance from the bicycle to stay upright. In the stationary case, the cyclist has lost the ability to alter his contact point with the road surface.

Here's a possible experiment. Set the bicycle and rider on a platform that can move left and right relative to the rider. Lock the bicycle's wheels. Drive the platform with a servo that is controlled by the attitude of the rider. When he leans right, the platform moves right. When he leans left, the platform moves left.

I suspect that, after a little training, the rider will be able to keep the bicycle upright indefinitely.

Should be "as easy as riding a bike!"
  
« Last Edit: 08/09/2009 19:32:36 by Geezer »
 

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Why do bicycles have such big wheels?
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