# The Naked Scientists Forum

### Author Topic: Why do bicycles have such big wheels?  (Read 27940 times)

#### Karen W.

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##### Why do bicycles have such big wheels?
« Reply #50 on: 08/09/2009 20:00:13 »
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.

Don't you have to do more rotations with the pedal to cover the same area, and therefore expend more energy in doing so, at least for getting from point A to point B?

(Increased friction losses) am I mistaken, that means there are definitely losses of energy from pedaling moreas you stated, correct.. So how does pedaling more not require more energy, as you would have to increase the rate at which you made each rotation in order to keep up with a bigger wheeled bike...?
Am I missing something?

#### lightarrow

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##### Why do bicycles have such big wheels?
« Reply #51 on: 08/09/2009 20:15:41 »
Why not, or, at least, why isn't the remaining gyro effect so small as to be unnoticable?
Because the axis of rotation don't coincide.

#### lightarrow

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##### Why do bicycles have such big wheels?
« Reply #52 on: 08/09/2009 20:39:31 »
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,
I have to admit that I don't see it (my problem...) but if this effect is not _also_ gyroscopic (but of course I claim it is), then you must have it even when the bicycle is stationary.

Quote

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).
I reminded you in my previous post that you cannot invoke linear momentum, since the same effect must be present in a frame of reference where the bicycle is stationary.

#### Geezer

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##### Why do bicycles have such big wheels?
« Reply #53 on: 08/09/2009 21:00:44 »
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,
I have to admit that I don't see it (my problem...) but if this effect is not _also_ gyroscopic (but of course I claim it is), then you must have it even when the bicycle is stationary.

Quote

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).
I reminded you in my previous post that you cannot invoke linear momentum, since the same effect must be present in a frame of reference where the bicycle is stationary.
Lightarrow - I gave you an example of the effect when the frame of reference IS stationary and there is no linear momentum. Did you not understand it?

#### syhprum

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##### Why do bicycles have such big wheels?
« Reply #54 on: 08/09/2009 21:21:38 »
Karen

Why you get more friction losses with small wheels is because with a shorter contact area with the road they follow the minor hills and dales of the road with more squishing of the tyre.
On a dead smooth road there would be no difference but we don't have them.
I am amazed how much correspondence this trivial subject has provoked I thought the gyroscope myth had been laid to rest long ago.

#### Geezer

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##### Why do bicycles have such big wheels?
« Reply #55 on: 08/09/2009 21:59:45 »
Like all good myths, it's not quite dead yet.

#### Bored chemist

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##### Why do bicycles have such big wheels?
« Reply #56 on: 08/09/2009 22:26:10 »
Why not, or, at least, why isn't the remaining gyro effect so small as to be unnoticable?
Because the axis of rotation don't coincide.
Both are fixed to a rigid frame so (I think) the only effect of the gyro effect is that you put some forces on the frame.
Angular momentum is a vector. The direction is parallel to the axis. The sum of the two momenta of the two wheels is zero.

#### lyner

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##### Why do bicycles have such big wheels?
« Reply #57 on: 08/09/2009 22:30:59 »
Karen
There is theory and there is practice and the two may give different answers but you need to identify which is which.
You would need to use different (higher) gearing, natch, to go at the same speed but, in the end, the speed of the periphery of any size wheel needs to be the same as the road speed. Gears will just multiply the actual speed your feet move and achieve the road speed you want. I am ignoring any effect of friction in the gearing system, which may be a bit worse (in practice) for a higher gear. Not very thorough, I know but right in principle. The actual power delivered to the road wheels is force times speed and that will be the same, whatever the wheel size. The actual losses are very low in modern chains and sprockets. It would be details of the tyres etc that would make all the difference to the friction losses, through contact with the road and internally in the tyres, I'm pretty sure.

#### lyner

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##### Why do bicycles have such big wheels?
« Reply #58 on: 08/09/2009 22:50:11 »
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.

There is a major difference because of the interaction with the ground. Stationary, the bike tips and there is no inherent restoring force.
Moving, there will be a force against the direction of motion because the wheel has turned itself (castor action, as I keep repeating). This force produces a couple because it does not act through the cm of the bike and rider. The couple will tend to return the bike upright.

The temporary equilibrium which a trick cyclist (and a wire walker) achieves is a very different matter and must rely on using skill to change the moment of inertia and twisting body / frame. With experience, all cyclists get to have a bit of skill with this but it isn't necessary on a moving bike.
How can this not be relevant?

SO  - the cyclist doesn't make it happen, the gyroscopic effect can be reduced as much as you like and the bike still stays upright so what else is there but the castor effect and friction with the road? (It wouldn't work with a bike on ice even with massive wheels - would it?)

#### Geezer

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##### Why do bicycles have such big wheels?
« Reply #59 on: 08/09/2009 23:58:22 »
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.

There is a major difference because of the interaction with the ground. Stationary, the bike tips and there is no inherent restoring force.
Moving, there will be a force against the direction of motion because the wheel has turned itself (castor action, as I keep repeating). This force produces a couple because it does not act through the cm of the bike and rider. The couple will tend to return the bike upright.

The temporary equilibrium which a trick cyclist (and a wire walker) achieves is a very different matter and must rely on using skill to change the moment of inertia and twisting body / frame. With experience, all cyclists get to have a bit of skill with this but it isn't necessary on a moving bike.
How can this not be relevant?

SO  - the cyclist doesn't make it happen, the gyroscopic effect can be reduced as much as you like and the bike still stays upright so what else is there but the castor effect and friction with the road? (It wouldn't work with a bike on ice even with massive wheels - would it?)

SC - Please refer to my latest post to Lightarrow which does include the importance of castor action. Castor action might also be described in the context of negative feedback which is, intrinsically, a stabilizing mechanism. Further, you might note that I have described a rather simple experiment that might dispel the confusion regarding bicycle stability once and for all.

#### lyner

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##### Why do bicycles have such big wheels?
« Reply #60 on: 09/09/2009 11:36:47 »
Yes, Geezer, I agree but two points need amplifying.
1. I don't think enough has been said regarding the mistaken assertion that changing the frame of reference gives you a stationary bike with no outside influence. The fact is that, if you take the bike as the reference, you have the road coming towards it and it can  produce a force on the bike. Either way the force is the same.
2. Gyroscopic action is not a resilience effect - like a spring. It is the equivalent of a mass(a reactive effective) , merely slowing the bike's fall but doing nothing to actually return the bike upwards. A simple toy gyroscope never goes back up again - it just sags as it loses angular momentum to the surroundings.
But syphrum's comment  is right. This has run its course, and has run the same course several times. I think it demonstrates how a problem with so many degrees of freedom (particularly involving angular momentum) is beyond intuition. 'Opinions' start to take over from Mechanics.
It's still magic how you can, so easily, ride with no hands and no concentration. Hands in pockets and whistling are essential, too.

#### Geezer

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##### Why do bicycles have such big wheels?
« Reply #61 on: 09/09/2009 16:27:12 »
SC - I like LeeE's observation about "being in a controlled state of falling". It seems that the combination of castor action (with motion) and the the rider's ability to transfer weight, provide a near perfect mechanism for the rider to "prevent falling". As you point out, about the only way you can "get surprised" is if you encounter a low friction surface like ice or gravel (I think I still have some of the scars on my knees, but not enough to prevent me wearing my kilt  )

It's remarkable when you consider how old the invention is (the bike that is).

#### lightarrow

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##### Why do bicycles have such big wheels?
« Reply #62 on: 09/09/2009 19:51:03 »
Lightarrow - I gave you an example of the effect when the frame of reference IS stationary and there is no linear momentum. Did you not understand it?
No, I didn't, sorry, however now I'm beginning to understand. Thank you for your patience...  [:I]
« Last Edit: 09/09/2009 20:06:19 by lightarrow »

#### lightarrow

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##### Why do bicycles have such big wheels?
« Reply #63 on: 09/09/2009 20:04:24 »
There is a major difference because of the interaction with the ground. Stationary, the bike tips and there is no inherent restoring force.
Moving, there will be a force against the direction of motion
That's right. Now (finally!) I understand what is "castor action" [:I]....

Quote
because the wheel has turned itself (castor action, as I keep repeating). This force produces a couple because it does not act through the cm of the bike and rider. The couple will tend to return the bike upright.
...but I don't understand this. Let's say the the wheel has turned right, then shouldn't the couple be such to rotate the system to the right? Or do you mean that not only the horizontal component of the friction on the front wheel increased, but the vertical too? I can't see how  the couple is oriented.

Quote
The temporary equilibrium which a trick cyclist (and a wire walker) achieves is a very different matter and must rely on using skill to change the moment of inertia and twisting body / frame. With experience, all cyclists get to have a bit of skill with this but it isn't necessary on a moving bike.
How can this not be relevant?

SO  - the cyclist doesn't make it happen, the gyroscopic effect can be reduced as much as you like and the bike still stays upright so what else is there but the castor effect and friction with the road? (It wouldn't work with a bike on ice even with massive wheels - would it?)
Good question.

#### Geezer

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##### Why do bicycles have such big wheels?
« Reply #64 on: 09/09/2009 20:39:40 »
Lightarrow - I gave you an example of the effect when the frame of reference IS stationary and there is no linear momentum. Did you not understand it?
No, I didn't, sorry, however now I'm beginning to understand. Thank you for your patience...  [:I]

No problem Lightarrow. Here's another experiment we could try (if we had lots of money!)

Get a bicycle with the smallest possible wheels and a "volunteer" cyclist (a little brother would be good if you have one.)
Get volunteer to pedal bicycle on rolling road at 20km/hour.
Supply refreshing drinks to volunteer.

I think the volunteer will be able to stay balanced, even though he has no linear momentum. However, he can easily adjust the point of contact with the rolling road relative to his center of mass while the road is rolling. When the road stops rolling he loses that ability and falls over. So he should wear a helmet

* treadmill - Ornaments frequently purchased by Americans to decorate their houses.

#### Geezer

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##### Why do bicycles have such big wheels?
« Reply #65 on: 09/09/2009 21:03:16 »
Here's a link to a nearby waterpark. It's possible (for some people anyway) to surf and stay upright, even without forward motion, and apparently, no gyroscopic action at all.

http://www.silvermt.com/Waterpark/default.aspx?page=WP-Surf-Club

#### lyner

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##### Why do bicycles have such big wheels?
« Reply #66 on: 09/09/2009 22:09:49 »
Lightarrow
The axis is through the two points of contact with the ground. The force, I guess, is the centripetal force, due tu the curve followed by the mass. This is in the appropriate sense to pull you upright.

#### lyner

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##### Why do bicycles have such big wheels?
« Reply #67 on: 10/09/2009 08:58:12 »
Ahh. I looked at your post again, lightarrow. The bike does deviate from it's course so there wll be a moment, initially, about a vertical axis.

#### lightarrow

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##### Why do bicycles have such big wheels?
« Reply #68 on: 11/09/2009 11:46:10 »
Here's a link to a nearby waterpark. It's possible (for some people anyway) to surf and stay upright, even without forward motion, and apparently, no gyroscopic action at all.

http://www.silvermt.com/Waterpark/default.aspx?page=WP-Surf-Club
It's a rather different situation, isnt'it?

#### syhprum

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##### Why do bicycles have such big wheels?
« Reply #69 on: 11/09/2009 12:54:32 »
Staying upright requires similar skills to balancing on a slack wire, as there is virtually no friction between the board and the water with training the bodies automatic balancing reflex learns to keep the board beneath ones center of gravity.
Rather like balancing a broom on ones hand (robots can easily be programed to do this).

#### Geezer

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##### Why do bicycles have such big wheels?
« Reply #70 on: 11/09/2009 22:59:05 »
I love the broom balancing analogy. Wait a minute, I think I used that about twenty posts ago.

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##### Why do bicycles have such big wheels?
« Reply #70 on: 11/09/2009 22:59:05 »