Why do bicycles have such big wheels?

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Offline Geezer

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Why do bicycles have such big wheels?
« on: 05/09/2009 17:05:52 »
Well, most bicycles anyway. Is it a macho thing? "Hey, look at the size of my wheels!" or do big wheels serve a useful purpose?

(Lights blue touchpaper and retires.)
There ain'ta no sanity clause, and there ain'ta no centrifugal force Šther.

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Offline RD

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Why do bicycles have such big wheels?
« Reply #1 on: 05/09/2009 17:25:40 »
Not all do ...

[attachment=9762]
http://www.treehugger.com/files/2007/11/marking_the_spo.php

Above smaller wheels have been used for portability.

The smaller tyres on smaller wheels will wear out quicker than larger tyres on larger wheels.
(ditto for hubs and spindles).
« Last Edit: 05/09/2009 17:36:30 by RD »

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Offline syhprum

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Why do bicycles have such big wheels?
« Reply #2 on: 05/09/2009 17:29:20 »
Cyclists like to operate in a near vertical position so big wheels are needed to accommodate their legs also they have less rolling resistance and ride the bumps better.
syhprum

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Offline Geezer

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Why do bicycles have such big wheels?
« Reply #3 on: 05/09/2009 17:33:26 »
Not all do ...

[attachment=9762]
http://www.treehugger.com/files/2007/11/marking_the_spo.php

Above smaller wheels have been used for portability.

The smaller tyres on smaller wheels will wear out quicker than larger on larger wheels.

Wow! I'd like to see one of them in the Tour de France.
There ain'ta no sanity clause, and there ain'ta no centrifugal force Šther.

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Offline RD

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Why do bicycles have such big wheels?
« Reply #4 on: 05/09/2009 17:52:42 »
The bigger the diameter of the wheel the less the cyclists energy is lost to friction,
 so for maximum speed/efficiency the bigger the better.

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Offline John Chapman

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Why do bicycles have such big wheels?
« Reply #5 on: 06/09/2009 02:45:13 »
Presumably the frictional benefits are in the spindles. Larger wheels spin at slower RPM to cover the same distance.

There are also tranmission benefits to having a large driving wheel. Otherwise the front (pedal) sprocket would have to be enormous to gain the same benefit.
 

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Offline lightarrow

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Why do bicycles have such big wheels?
« Reply #6 on: 06/09/2009 03:00:29 »
Presumably the frictional benefits are in the spindles. Larger wheels spin at slower RPM to cover the same distance.

There are also tranmission benefits to having a large driving wheel. Otherwise the front (pedal) sprocket would have to be enormous to gain the same benefit.
 
Also, the equilibrium on the bicycle is more difficult with little wheels because of little gyroscopic effect.

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Offline syhprum

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Why do bicycles have such big wheels?
« Reply #7 on: 06/09/2009 11:59:31 »
Do the gyroscopic effects play any part in the equilibrium of bicycles ?
I thought that tests had been made with bicycles fitted with contra rotating wheels that were found to be little if any more difficult to ride than regular ones.
syhprum

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lyner

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Why do bicycles have such big wheels?
« Reply #8 on: 06/09/2009 12:48:30 »
The Moment of Inertia of a bicycle wheel is pretty small (particularly a light weight racing wheel)  so is it really likely to have much effect?

Surely the biggest advantage of big wheels is that the footprint is longer for a given tyre pressure. This will mean that the distortion , and hence the losses / internal friction will be less. The better efficiency of the transmission with big wheels will also be a factor, natch, but tyre friction is sure to be a major factor.

btw, does anyone know of any work done cycling on rails / steel track, with steel wheels. (Obviously, stabilisers could be needed). There may be a world record out there for someone.

There is also the issue of ironing out bumps in the road but neither this nor any (possible?) balancing help would not be relevant for competitive cycling on high quality cycle tracks. Pros would cycle on 10cm wheels if there was a speed advantage.

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Offline LeeE

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Why do bicycles have such big wheels?
« Reply #9 on: 06/09/2009 17:55:24 »
The bigger the diameter of the wheel the less the cyclists energy is lost to friction,
 so for maximum speed/efficiency the bigger the better.

Hmm... on a hard surface, a smaller wheel will have a smaller area of contact than a larger wheel, so a smaller wheel should offer less rolling resistance than a larger wheel.

A smaller wheel would impose a higher pressure upon the surface it's traveling across though, so if you're traveling across an unmetalled surface you'll sink further into it with a smaller wheel than with a larger wheel, which will raise the rolling resistance.  The larger wheel, with it's larger contact area, will sink less on a soft surface, so it's rolling resistance will be more constant across hard and soft surfaces.  This was an issue when the first bicycles were invented, as many 'roads' were unmetalled.

Smaller wheels are also more susceptible to bumps, irregularities and even stones on the surface.  Obviously, a wheel cannot roll over an obstacle unless the height of the obstacle is less than the radius of the wheel, so larger wheels can roll over larger obstacles than smaller wheels, but even when the obstacle is small enough that both large and small wheels can roll over it, the larger wheel, with its reduced curve, will intercept the obstacle sooner than a small wheel and result in a lower vertical acceleration as the wheel passes over it; in short, a larger wheel is more comfortable than a smaller wheel.
...And its claws are as big as cups, and for some reason it's got a tremendous fear of stamps! And Mrs Doyle was telling me it's got magnets on its tail, so if you're made out of metal it can attach itself to you! And instead of a mouth it's got four arses!

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Offline Geezer

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Why do bicycles have such big wheels?
« Reply #10 on: 06/09/2009 18:02:44 »
btw, does anyone know of any work done cycling on rails / steel track, with steel wheels. (Obviously, stabilisers could be needed). There may be a world record out there for someone.

No data. But these guys might have some. (Note very small front wheel). There are several kits available to convert a regular bike into a rail bike.

http://www.railbike.com/buying.htm
There ain'ta no sanity clause, and there ain'ta no centrifugal force Šther.

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Offline lightarrow

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Why do bicycles have such big wheels?
« Reply #11 on: 06/09/2009 18:25:08 »
Do the gyroscopic effects play any part in the equilibrium of bicycles ?
I thought that tests had been made with bicycles fitted with contra rotating wheels that were found to be little if any more difficult to ride than regular ones.
I don't know of those tests; anyway gyroscopic effect should play some part: it's not easy to stay in equilibrium on a bicycle which is not moving, but as soon as it moves it's much more easy; why?

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Offline LeeE

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Why do bicycles have such big wheels?
« Reply #12 on: 06/09/2009 19:06:44 »
Do the gyroscopic effects play any part in the equilibrium of bicycles ?
I thought that tests had been made with bicycles fitted with contra rotating wheels that were found to be little if any more difficult to ride than regular ones.
I don't know of those tests; anyway gyroscopic effect should play some part: it's not easy to stay in equilibrium on a bicycle which is not moving, but as soon as it moves it's much more easy; why?

When a bicycle is stationary you can only keep it in balance by moving your weight from side to side to keep your CoG over the wheels.  This isn't an ideal solution because you're moving the larger of the two masses instead of the smaller to maintain balance.  Moving the larger of the two masses also requires more force to be used.  When the bicycle is moving though, this is reversed and instead you steer the smaller mass of the bicycle to keep it beneath the larger mass of yourself, with consequently less force being required to do so.

The steering process is also more progressive than than simply moving your body from side to side because the bicycle follows the sum of the forward and steering vectors, instead of just the sideways vector, giving a finer degree of control.
...And its claws are as big as cups, and for some reason it's got a tremendous fear of stamps! And Mrs Doyle was telling me it's got magnets on its tail, so if you're made out of metal it can attach itself to you! And instead of a mouth it's got four arses!

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Offline John Chapman

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Why do bicycles have such big wheels?
« Reply #13 on: 06/09/2009 19:36:48 »
That was well explained LeeE.

You'd make a good teacher  [:)]

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lyner

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Why do bicycles have such big wheels?
« Reply #14 on: 06/09/2009 20:14:50 »
Do the gyroscopic effects play any part in the equilibrium of bicycles ?
I thought that tests had been made with bicycles fitted with contra rotating wheels that were found to be little if any more difficult to ride than regular ones.

I don't know of those tests; anyway gyroscopic effect should play some part: it's not easy to stay in equilibrium on a bicycle which is not moving, but as soon as it moves it's much more easy; why?
I thought we'd been here before. You can make a bike which is impossible to ride by having the front forks so that the line of the top bracket hitting the ground behind the point of contact with the wheels. Gyroscopic action is the same in both cases. It has to be the castor action which steers the front wheel to produce a moment to push you upright. I have done diagrams which satisfy me but they wouldn't work on a post here cos I need to wave arms at the same time! [;)]

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Offline syhprum

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Why do bicycles have such big wheels?
« Reply #15 on: 06/09/2009 20:37:49 »
Research has also been done on this matter, bicycles fitted with gearing that causes the forks to rotate in the reverse manner to the handlebar's can be ridden with practice but it is not as instinctive as riding a normal machine.
With a zero castor angle the steering would be heavy but could be learnt.
As a boy I practiced riding a bicycle with ropes tied to the handlebar's like reins, by crossing the ropes you could produce the same effect as the reverse geared steering.
syhprum

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Offline Geezer

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Why do bicycles have such big wheels?
« Reply #16 on: 06/09/2009 20:48:58 »
Wasn't there something called the Moulton bicycle produced in the 60's? It had much smaller wheels. I think Raleigh produced something similar, but it had wider tires. As I seem to recall, they actually worked quite well, certainly on well paved surfaces.

(I just Googled Moulton - still seems to be going strong.)

http://www.moultonbicycles.co.uk/index.html
There ain'ta no sanity clause, and there ain'ta no centrifugal force Šther.

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lyner

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Why do bicycles have such big wheels?
« Reply #17 on: 06/09/2009 22:17:07 »
syphrum
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Offline that mad man

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Why do bicycles have such big wheels?
« Reply #18 on: 06/09/2009 22:24:00 »
Sir Clive Sinclair developed a portable folding bike with small wheels called the A-Bike and was described by some as a bit "wobbly" because of the size of the wheels.

I remember a science lesson at school when we had several sized bicycle wheels each mounted on a small axle that we could hold in outstretched arms. We sat on a desk type chair (one on wheels) and held the bicycle wheel out in front of us. Someone then spun the wheel and then we were asked to tilt the wheel from the vertical and also simulate it turning like a bikes front wheel.

The outcome was the chair, with you in it, spun round a bit. The different sized wheels produced different sized effects with the larger wheels producing greater effects. Unfortunately that was years ago and I cant remember exactly what that lesson was about but I think it was something to do with the gyroscope effect. [:I]


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Offline Turveyd

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Why do bicycles have such big wheels?
« Reply #19 on: 06/09/2009 23:40:33 »

As a Mountain Biker who's messed with wheel sizes and has been running a 29" wheel on the front of a standard normal 26" wheel for 2years,  I can safely say the main advantage is it takes longer from contact to highest spot therefore the peak bump force is lower,  therefore they ride smoother over rough ground which makes them more efficent as they stall less,  which leads to going over the bars and pain and maybe even a broken kneck!!

There is a slight drop in rolling resistance,  the foot print becomes slightly longer but narrorow and the narrower part helps in short,  for the same air pressure though the overall size is unchanged.


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Offline Geezer

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« Reply #20 on: 06/09/2009 23:59:24 »
29"? Holy crap! That's bigger than the tires on my truck.

Have you considered getting a penny-farthing? It would probably be the ultimate mountain bike.
There ain'ta no sanity clause, and there ain'ta no centrifugal force Šther.

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Offline lightarrow

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« Reply #21 on: 07/09/2009 12:42:08 »
I don't know of those tests; anyway gyroscopic effect should play some part: it's not easy to stay in equilibrium on a bicycle which is not moving, but as soon as it moves it's much more easy; why?
When a bicycle is stationary you can only keep it in balance by moving your weight from side to side to keep your CoG over the wheels.  This isn't an ideal solution because you're moving the larger of the two masses instead of the smaller to maintain balance.  Moving the larger of the two masses also requires more force to be used.  When the bicycle is moving though, this is reversed and instead you steer the smaller mass of the bicycle to keep it beneath the larger mass of yourself, with consequently less force being required to do so.

The steering process is also more progressive than than simply moving your body from side to side because the bicycle follows the sum of the forward and steering vectors, instead of just the sideways vector, giving a finer degree of control.
And you cannot steer when the bicycle is not moving?
With *not moving* I intended that you are staionary in the same point of terrain, not that you can't move the bicycle at all. Even if you steer, it's more difficult to stay in equilibrium, in comparison to when you are going at a minimum speed. Why?
« Last Edit: 07/09/2009 12:47:37 by lightarrow »

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Offline lightarrow

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« Reply #22 on: 07/09/2009 12:49:50 »
Sir Clive Sinclair developed a portable folding bike with small wheels called the A-Bike and was described by some as a bit "wobbly" because of the size of the wheels.

I remember a science lesson at school when we had several sized bicycle wheels each mounted on a small axle that we could hold in outstretched arms. We sat on a desk type chair (one on wheels) and held the bicycle wheel out in front of us. Someone then spun the wheel and then we were asked to tilt the wheel from the vertical and also simulate it turning like a bikes front wheel.

The outcome was the chair, with you in it, spun round a bit. The different sized wheels produced different sized effects with the larger wheels producing greater effects. Unfortunately that was years ago and I cant remember exactly what that lesson was about but I think it was something to do with the gyroscope effect. [:I]
Yes, and about angular momentum conservation.

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Offline LeeE

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« Reply #23 on: 07/09/2009 15:38:48 »
Quote
And you cannot steer when the bicycle is not moving?

Well, you can waggle the handlebars about and, because of the castor angle of the front wheel, the front of the frame can pivot around the rear wheel but to keep balance you need to keep your CoG over the wheel/ground contact points.  It doesn't matter where the frame is or whether the frame is tilted, or not.  So if the bicycle is not moving the wheel/ground contact points won't change and you won't have steered anywhere.
...And its claws are as big as cups, and for some reason it's got a tremendous fear of stamps! And Mrs Doyle was telling me it's got magnets on its tail, so if you're made out of metal it can attach itself to you! And instead of a mouth it's got four arses!

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Offline Geezer

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Why do bicycles have such big wheels?
« Reply #24 on: 07/09/2009 18:41:16 »
With *not moving* I intended that you are stationary in the same point of terrain, not that you can't move the bicycle at all. Even if you steer, it's more difficult to stay in equilibrium, in comparison to when you are going at a minimum speed. Why?
I think the balancing effect is similar to balancing something like an inverted broom/brush. Put the broom handle on your palm with the brush end vertically above it. It is remarkably easy to balance the broom by only making small adjustments in the position of your hand. The large mass at the other end of the broom handle helps, because it has some inertia, so it takes a large hand movement to cause it to accelerate in any direction.

When a bicycle is moving, it's easy to continuously adjust the steering and make fine adjustments to the lateral position of the tires to maintain equilibrium against the other forces (similar to the hand movements with an inverted broom). When the bicycle is stationary the rider can only maintain equilibrium by transferring weight from side to side. I could never do it, but I knew people who could, and they did it by continually adjusting the steering angle (if I remember correctly).

I suppose they were taking advantage of the castor angle to make very small adjustments to their center of mass relative to the line joining the contact points of the two tires, but that might be another thread entirely  [:)]

EDIT: I think LeeE was perhaps making the same point re. balancing a stationary bike.
« Last Edit: 07/09/2009 19:37:23 by Geezer »
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Offline Bored chemist

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« 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.
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Offline lightarrow

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« 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.

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Offline lightarrow

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« 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.

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Offline Geezer

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« 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.

There ain'ta no sanity clause, and there ain'ta no centrifugal force Šther.

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Offline Bored chemist

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« 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. "
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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.

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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.

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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 »
There ain'ta no sanity clause, and there ain'ta no centrifugal force Šther.

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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 »
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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.

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Offline Geezer

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« 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 »
There ain'ta no sanity clause, and there ain'ta no centrifugal force Šther.

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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.
...And its claws are as big as cups, and for some reason it's got a tremendous fear of stamps! And Mrs Doyle was telling me it's got magnets on its tail, so if you're made out of metal it can attach itself to you! And instead of a mouth it's got four arses!

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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?

[attachment=9780]
http://www.youtube.com/watch?v=Srwk-i5aXRQ
« Last Edit: 07/09/2009 23:50:22 by RD »

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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 »
There ain'ta no sanity clause, and there ain'ta no centrifugal force Šther.

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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.

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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.
syhprum

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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 »

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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 ...

[attachment=9784]

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 »

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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)

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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.

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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. »

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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.

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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.
There ain'ta no sanity clause, and there ain'ta no centrifugal force Šther.

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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 »
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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 »
There ain'ta no sanity clause, and there ain'ta no centrifugal force Šther.