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  1. #101
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    Tweed Run

    I've been looking at pictures of the Tweed Run. Some of you know what I'm talking about. It seems Brits ride more upright than many of us. More upright means higher CG, and they all seem to be having a splendid good time.

  2. #102
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    Quote Originally Posted by Ze_Zaskar View Post
    A simple way to explain this issue is with the following concepts:
    -High CG- greater range of stability (angular amplitude) but a stronger and slower input is required to correct it.
    -Low CG- smaller range of stability (angular amplitude) but weaker and faster inputs are required to correct it.

    From a mechanical engineer's point of view, this pretty much sums it
    I am fine with that definition apart from the concept of stronger or weaker inputs being required in order to correct losses of stability.

    Remember that the corrective input mechanism is the moving of the front wheel from one side of the COG to the other. Because a tall bike falls more slowly the bike will lean less in a given time and so less corrective wheel movement is required. A rider of a less tall bike, reacting in the same time will require more movement. The magnitude of correction required also varies with the speed of the bike and how far back the COG is from the front wheel.
    File:Bike weaving.gif - Wikipedia, the free encyclopedia

    The notion that it is an input force applied by the bike that corrects the lean is misleading as the force that corrects the bike is that of gravity pulling downwards. The front wheel moves sideways whilst the COG travels in a smooth line. There is a small force that moves the front wheel from side to side but this force does not directly re balance the bike.

    To summarize:

    -High CG- greater range of stability (angular amplitude), with a slower rate of lean that can be countered more slowly. If countered quickly less front wheel movement will be required.
    -Low CG- smaller range of stability (angular amplitude), with a faster rate of lean that needs to be countered more quickly. If countered slowly larger front wheel movement will be required.

  3. #103
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    Quote Originally Posted by I'm suba View Post
    I've been looking at pictures of the Tweed Run. Some of you know what I'm talking about. It seems Brits ride more upright than many of us. More upright means higher CG, and they all seem to be having a splendid good time.
    They do appear to have a good time. Some commentators say that Geoff was well ahead of the modern tweed wearing trend as this 1984 photo shows. Cleland: The original big wheeled off-road bicycle?-4321632293_01fcfb3fa1_b.jpg

    The tweed run is not typical of British cycling as most UK riders ride in a forward position. The main upright cyclists in Europe are the Dutch, Germans, Belgians and Danes. Though there bikes, unlike Clelands, have slack frame angles, longer wheelbases and low bottom brackets.

  4. #104
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    Quote Originally Posted by smilinsteve View Post
    Funny, in the video Graham just posted the guy balances a golf club with head up and head down and says its easier with head down, (COG closer to fulcrum, ie. finger)
    Take another look.

    He actually says it's easier with the head upwards so that the center of gravity is higher.

  5. #105
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    Quote Originally Posted by I'm suba View Post
    I've been looking at pictures of the Tweed Run. Some of you know what I'm talking about. It seems Brits ride more upright than many of us. More upright means higher CG, and they all seem to be having a splendid good time.
    I don't look like I'm having much fun, but I'm concentrating I suppose, those rocks are slippy.
    Attached Thumbnails Attached Thumbnails Cleland: The original big wheeled off-road bicycle?-tweed001.jpg  


  6. #106
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    Quote Originally Posted by GrahamWallace View Post
    Take another look.

    He actually says it's easier with the head upwards so that the center of gravity is higher.
    I watched it again. He says the club head is more easily balanced when it is near your finger. It is shown at time 4:15 to 4:40 in the video.

    Edit: But I think he mis-spoke. And looking at the video, he seems to be having more trouble with the club head down.
    Last edited by smilinsteve; 09-26-2012 at 12:57 PM.

  7. #107
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    Quote Originally Posted by GeoffApps View Post
    I don't look like I'm having much fun, but I'm concentrating I suppose, those rocks are slippy.
    Look like a BAMF to me. I really want to try out one of those bikes!

  8. #108
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    Quote Originally Posted by smilinsteve View Post
    I watched it again. He says the club head is more easily balanced when it is near your finger. It is shown at time 4:15 to 4:40 in the video.

    Edit: But I think he mis-spoke. And looking at the video, he seems to be having more trouble with the club head down.
    You are right.

    He does in fact say "it seems to me that the club head is more easily balanced with the club head, near!"

    I had the sound turned down and thought he said "club head, here!

    I eventually find a video that explains the principle and but the demonstrator gets a key word wrong?
    But you can see from the other demonstrations that a mass further away from the center of rotation moves more slowly. i.e. it is harder to move.

  9. #109
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    Here's a web page that explains it:

    Exhibit Cross Reference - Balancing Stick

  10. #110
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    Bamf?

  11. #111
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    Well done Steve!

    Perseverance is the key to understanding counter-intuitive concepts like this.

    Bit by bit they start to make sense!

    Here's another one to ponder?
    When riding up a steep slope you need to keep the weight over the rear wheel to maintain traction. But you also need to keep your weight over the front wheel in order to hold it down.

    But how can the weight be over both the front and back wheels at the same time?
    Theoretically it can if the weight is an infinite height above the bike.

    By the same logic a taller bike should be able to climb a steeper hill than a short one

    This also explains why bikes with shorter wheel-bases climb best.

  12. #112
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    Quote Originally Posted by GeoffApps View Post
    Bamf?
    Bad. Bottomed. Mother. Thingy?

  13. #113
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    I remember a scene from the seminal film 'On Any Sunday' about the hillclimb called 'The Widowmaker'.
    Intuition told those attempting it, year after year, that a huge engine in a mega long wheelbase motorbike with a low low saddle was the way to get to the top.
    If I remember rightly, few, if any, succeeded.
    Then a chap on a small-engined short wheelbase trials bike, standing on the pegs, slowly tiggled his way up to the top, turned round and came back down.

  14. #114
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    Quote Originally Posted by GrahamWallace View Post
    Bad. Bottomed. Mother. Thingy?
    Cor, is that a compliment?

  15. #115
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    Quote Originally Posted by GeoffApps View Post
    Cor, is that a compliment?
    The meaning of BAMF

    Yea, it's a compliment here in the colonies.

    Is this the hill climb you are talking about: Hill climb ride up (from movie - On Any Sunday) - YouTube

  16. #116
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    Yes, that's the one.
    Clearly, my memory of the film is wrong.
    Mind you, the last time I saw the film was in 1970.
    Thanks for that link ~ I must get the DVD, it was a most influential film for me.

  17. #117
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    BAMF - yes big compliment.

    Geoff and Graham,

    Your attitude and responses to the posts in this thread display qualities of intelligent thinking by enthusiastic riders/innovators. Keep up the good work!

    As a chiropractor I find Geoff's take on spinal health to be on the money!

  18. #118
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    Quote Originally Posted by BacDoc View Post

    Your attitude and responses to the posts in this thread display qualities of intelligent thinking by enthusiastic riders/innovators. Keep up the good work!
    It's the English thing. They're farther along on the evolutionary chain.

  19. #119
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    Quote Originally Posted by BacDoc View Post
    BAMF - yes big compliment.

    Geoff and Graham,

    Your attitude and responses to the posts in this thread display qualities of intelligent thinking by enthusiastic riders/innovators. Keep up the good work!

    As a chiropractor I find Geoff's take on spinal health to be on the money!
    Thanks for that.
    I was talking to one of the chiropractors in a group practice which had a stand set up at a cycle show.
    He agreed with me that the common 'stretched' posture on a bicycle is bad bad news for most of the body, particularly the spine.
    However, if he were to say anyting along these lines to a conventional cyclist he had discovered through experience that, because he was not also a cyclst, he would alienate his clients; not good for them, not good for his income.
    So, traditional cycling wisdom rules.

    I assume you've looked at this

  20. #120
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    Quote Originally Posted by GrahamWallace View Post
    But how can the weight be over both the front and back wheels at the same time?
    Theoretically it can if the weight is an infinite height above the bike.
    Apparently I need educating on this one, as I do not believe it is correct. Could you explain your logic?
    Weight distribution is independent of COG height.

    [QUOTE=GrahamWallace;9725280By the same logic a taller bike should be able to climb a steeper hill than a short one[/QUOTE]
    As the height of the COG increases, so does the rider's need to move his position relative to the axles to account for it. Thus, height will at some point limit the grade that can be climbed without the bike tipping.
    Quote Originally Posted by pvd
    Time to stop believing the hype and start doing some science.
    29er Tire Weight Database

  21. #121
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    Quote Originally Posted by meltingfeather View Post
    Apparently I need educating on this one, as I do not believe it is correct. Could you explain your logic?
    Weight distribution is independent of COG height.
    In theory, you can draw a line from the COG to the front or the rear axle, and as you raise the COG that line becomes more vertical. At infinitity, the line is vertical.
    But if you want to distribute weight evenly between front and rear, there are easier ways than putting your seat up in the sky. You just center your weight.

    Climbing steep hills does require a careful balance between front and rear weight distribution, but 50/50 is not necessarily the best distribution. I think longer top tubes allow for more rider adjustment to move weight back for traction, or to the front for stability, as needed.

    As the height of the COG increases, so does the rider's need to move his position relative to the axles to account for it. Thus, height will at some point limit the grade that can be climbed without the bike tipping.
    Good point. As the grade gets steeper, the COG moves backwards to a greater degree the higher it is.

  22. #122
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    Quote Originally Posted by smilinsteve View Post
    In theory, you can draw a line from the COG to the front or the rear axle, and as you raise the COG that line becomes more vertical. At infinitity, the line is vertical.
    Parallel lines do no get closer together (what would be required for even weight distribution) no matter how long you make them. Also, for weight distribution, the horizontal distance from COG to axle is relevant. Height of the COG is irrelevant, unless I'm missing something.
    Quote Originally Posted by pvd
    Time to stop believing the hype and start doing some science.
    29er Tire Weight Database

  23. #123
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    Quote Originally Posted by meltingfeather View Post
    Parallel lines do no get closer together (what would be required for even weight distribution) no matter how long you make them. Also, for weight distribution, the horizontal distance from COG to axle is relevant. Height of the COG is irrelevant, unless I'm missing something.
    Not parallel. Picture you draw a line from the COG to the rear axle, and from the COG to the front axle. So you have a triangle, with a line between the axles as the base. As you raise the COG (at the point of the triangle) the angle at the peak of the triangle gets smaller and smaller...

    You never get to parallel, but theoretically you can get that angle as close to zero as you want.

    Not that I think this exercise has any practical use. The difference in COG height is too minor in real bike design to be a player in the weight distribution problem. Like you said, its horizontal position that determines it in real life.

  24. #124
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    Quote Originally Posted by meltingfeather View Post
    Apparently I need educating on this one, as I do not believe it is correct. Could you explain your logic?
    Weight distribution is independent of COG height.


    As the height of the COG increases, so does the rider's need to move his position relative to the axles to account for it. Thus, height will at some point limit the grade that can be climbed without the bike tipping.

    To repeat my original assertion:

    When riding up a steep slope you need to keep the weight over the rear wheel to maintain traction. But you also need to keep your weight over the front wheel in order to hold it down.

    But how can the weight be over both the front and back wheels at the same time?
    Theoretically it can if the weight is an infinite height above the bike.

    By the same logic a taller bike should be able to climb a steeper hill than a short one"

    This also explains why bikes with shorter wheel-bases climb best.

    Here is my explanation:

    The force that causes the front wheel to lift is an equal and opposite reaction to the torque being applied by the rotation of the rear wheel.

    In this instance a bike can be considered to be a third class lever with its pivot at the center of the rear axle, the input effort is the torque reaction from the rotation of the rear wheel as applied at the outside of the axle, and the load being the riders weight levered is at the COG. Bear in mind that with a third class lever the input force is always larger than the force applied to its load. Therefore, the further away the COG from the rear axle the smaller the reaction force applied to the COG. And at an infinite distance the force applied at the COG would be zero. In order for the bicycle not to tip forwards or backwards the COG needs to remain within the wheelbase so the only place it can be located at infinity is above the bike.

    Of course in reality you can't locate the COG at infinity. Even if you could its inertia would cause the bike to tip backwards as the bike went forwards. However moving the COG as far away from the rear axle whilst keeping it within the wheel-base will reduce the magnitude of the torque reaction. The best place to put the COM is high above the front wheel. Remember that the front wheel is further up the hill than the rear wheel so the wheel-base will have shortened relative to the horizontal. Due to the inertia of the COG any acceleration force applied to the will increase rear wheel traction but can also cause the bike to tip backwards. So smooth pedaling is essential. This can be aided by the use of elliptical gears that also allow the cadence to be lowered whilst reducing the chances of rear wheel slip.

    Its never fails to amaze me just how complicated is the physics of the simple machine that we call the bicycle.

    To help with the key concepts here are two videos:

    In this one of unicyclists cycling uphill you can see how unicyclists have to lean forvard as they pedal in order to counter the combined effect of the torque reaction and the inertia of their COG.
    How Levers Work - YouTube

    And towards the end of this this one, the Third Class Lever is explained.
    How Levers Work - YouTube
    Last edited by GrahamWallace; 09-27-2012 at 12:33 PM.

  25. #125
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    Quote Originally Posted by smilinsteve View Post
    Climbing steep hills does require a careful balance between front and rear weight distribution, but 50/50 is not necessarily the best distribution. I think longer top tubes allow for more rider adjustment to move weight back for traction, or to the front for stability, as needed.
    With a longer top tube your upper body is stretched in order to reach the handlebar.
    Your upper body is what you use to transfer weight fore and aft.
    A stretched upper body has only limited capacity for moving forwards and backwards.
    Therefore, a shorter toptube and reach does allow the flexibility.

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