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This is a discussion on Foale Frame designs within the Technical forums, part of the Caferacer.net Forums category; Originally Posted by jordan That may be true, but you know as well i that every action has an equal and opposite reaction. You can ...

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  1. #31
    Senior Member XB33BSA's Avatar
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    Quote Originally Posted by jordan View Post
    That may be true, but you know as well i that every action has an equal and opposite reaction. You can angle the rear shock appropriately so that the upper shock mount and SA pivot are facing equal magnitude forces with opposing directions.
    So if the sa pivot is perfectly in line with the backbone, it is purely under tensile stress. The upper shock mount however will need to be placed higher up on the backbone and will be at a steeper angle to the spine.

    When you try to bend a pipe, do you push on it end to end to compress it until it bows out?
    No, you push at 90 degrees to it. Well here i want to minimize the flexing, so the forces must be directed along the spine.

    The goal is to align the forces along the spine as best as possible because would be a purely compression and tensile stress. This is not possible with a standard SA rear suspension as the shock mount of the SA would need to be as close as possible to the pivot, so compromises must be made.

    I never claimed i was aiming the forces from the rear to the steering head, i said i wanted to have them aimed as close to parallel with the spine as possible to minimize the strain of the spine.
    you could never bend that pipe you speak of the top shock mount is not pushing beyond the mount with any significant force
    you need to educate yourself the forces are strictly into the pivot
    you are not grasping the concept
    iyou have some weight transfer but not the huge leverage multiplied forces of suspension

  2. #32
    Senior Member XB33BSA's Avatar
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    for example lets take a 1981 yz465 the top shock mount way up by the steer tube
    if your horseshit theory was correct then any time the mx racer was ion the throttle hard ,front wheel not touching dirt much just skimming
    while attacking nasty whooped out bumps ,the goddam bumps ,each one that deeply activates rear suspension,would make the front end drop
    this iis not the case in fact the rider would feel it if so and that would have been a reason to never put tilkens design into production at allName:  1980 yz 465.jpg
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    Last edited by XB33BSA; 03-14-2018 at 10:50 AM.

  3. #33
    Member jordan's Avatar
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    Quote Originally Posted by XB33BSA View Post
    you could never bend that pipe you speak of the top shock mount is not pushing beyond the mount with any significant force
    you need to educate yourself the forces are strictly into the pivot
    you are not grasping the concept
    iyou have some weight transfer but not the huge leverage multiplied forces of suspension
    I must be missing something.

    Do you agree that when the frame a load such that the rear tire is under 450lbs of load, if the SA is a cantilever with a 2:1 length ratio, the rear shock is under 900lbs of load?

    Well what is the direction vector of that load? It points straight along the shock body, from sa mount to frame mount. So if this vector point perpendicular to the spine, there will be more bending force on spine from this than if it was pointing parallel to the spine.

    What point of this stick has the greatest load?


    This is the same situation as if the top hand is the sa pivot, inline with the neck.
    The shock pushes into the spine as opposed to pushing along it.

    The reason you want the pivot inline is that you also have the chain pull and other forces to worry about. So yes the pivot is the most important part, but the angle the shock is mounted at is also important to consider.

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  5. #34
    Member jordan's Avatar
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    Quote Originally Posted by XB33BSA View Post
    for example lets take a 1981 yz465 the top shock mount way up by the steer tube
    if your horseshit theory was correct then any time the mx racer was ion the throttle hard ,front wheel not touching dirt much just skimming
    while attacking nasty whooped out bumps ,the goddam bumps ,each one that deeply activates rear suspension,would make the front end drop
    this iis not the case in fact the rider would feel it if so and that would have been a reason to never put tilkens design into production at allName:  1980 yz 465.jpg
Views: 287
Size:  147.3 KB

    Look at the angle of that shock though
    It runs parallel to the imaginary line between the neck and pivot.

  6. #35
    jcw
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    Good discussion. Im learning stuff...

  7. #36
    Senior Member XB33BSA's Avatar
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    Quote Originally Posted by XB33BSA View Post
    you could never bend that pipe you speak of the top shock mount is not pushing beyond the mount with any significant force
    you need to educate yourself the forces are strictly into the pivot
    you are not grasping the concept
    iyou have some weight transfer but not the huge leverage multiplied forces of suspension
    the load goes into the pivot on a more verticasl shock as well
    the fulcrum is bottom shock mount and it is levraging the swinger pivot point in a direction opposite of shock
    iot is tryingn to rip then frame apart between top shock mount and swinger pivot
    on a more verticasl shock ther load is downward at pivot
    the stlyle like foale and yamaha used is basuicallyrotated close to 90 degrees for the other style
    hence the load at swinger pivot more rearwards
    that i describe above IS for every action there is a reaction

  8. #37
    Senior Member Teazer's Avatar
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    Quote Originally Posted by XB33BSA View Post
    for example lets take a 1981 yz465 the top shock mount way up by the steer tube
    if your horseshit theory was correct then any time the mx racer was ion the throttle hard ,front wheel not touching dirt much just skimming
    while attacking nasty whooped out bumps ,the goddam bumps ,each one that deeply activates rear suspension,would make the front end drop
    this iis not the case in fact the rider would feel it if so and that would have been a reason to never put tilkens design into production at allName:  1980 yz 465.jpg
Views: 287
Size:  147.3 KB
    Hang on a minute. If a force is applied to the rear wheel on a bump, it will tend to push the front wheel down even though most/much of the force is dissipated in the shock. If the shock upper mount is free to move as when the front wheel is in the air, it must move. Yes? There are all sorts of other factors at play including inertia, but it must move.
    woodsman likes this.

  9. #38
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    “on a more verticasl shock ther load is downward at pivot”
    Yes, and depending on the geometry, a 2:1 leverage ratio of wheel to shock travel means the pivot sees the same (but opposite) force as the wheel does.

    “the stlyle like foale and yamaha used is basuicallyrotated close to 90 degrees for the other style”

    Kind of, except that with the same 2:1 ratio the rear pivot sees 2.25x the force on the rear wheel.
    Say you have force F pushing up the wheel,
    You have then 2F at the shock.
    The resulting force vector has a magnitude such that F(pivot)^2=2F^2+F^2
    So F(pivot)^2= 5F
    F(pivot)=2.25


    “the fulcrum is bottom shock mount and it is levraging the swinger pivot point in a direction opposite of shock”

    Yes and no. It depends on the angle of the shock. You can angle the shock downwards towards the front wheel, depending on the leverage ratio of the SA (2:1 means 30* off horizontal) you can balance out the vertical load of the sa pivot so that the load is straight back. This is not practical however as you transfer that load to the spine in a direction so the frame is under the same stresses. In your statement you seem to be implying that the fulcrum of a lever sees no forces, this is incorrect. If the pivot is under load, this load must have an equal and opposite load somewhere else on the SA. Part of it is transmitted by the wheel, but the rest comes through the shock. But what holds the shock? The frame. Thus the magnitude of the load on the SA pivot is geometricly related to the load on the frames rear shock mount.

  10. #39
    Member jordan's Avatar
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    Quote Originally Posted by Teazer View Post
    Hang on a minute. If a force is applied to the rear wheel on a bump, it will tend to push the front wheel down even though most/much of the force is dissipated in the shock. If the shock upper mount is free to move as when the front wheel is in the air, it must move. Yes? There are all sorts of other factors at play including inertia, but it must move.
    Exactly
    I would love to see a rear suspension setup that doesn’t impart a force downwards on the front wheel.
    The best way to picture it is to replace the shock with a solid strut and ask yourself “what’s going to happen now?”

  11. #40
    jcw
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    Quote Originally Posted by Teazer View Post
    Hang on a minute. If a force is applied to the rear wheel on a bump, it will tend to push the front wheel down even though most/much of the force is dissipated in the shock. If the shock upper mount is free to move as when the front wheel is in the air, it must move. Yes? There are all sorts of other factors at play including inertia, but it must move.
    We needs a physics teacher and a force vector diagram...


    Actually, I think solidworks or autocad's FEA program might be able to calculate how much load in each direction.

    Edit- But seriously, there are definitely forces transmitted through the frame.

    Remember Honda's unit pro link rear suspension? No? google it. Rossi won on the Honda with it and it finally made it to production on the 600rr in 2010 (?)
    Last edited by jcw; 03-14-2018 at 02:28 PM.

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