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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Modified Steering Stem
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<blockquote data-quote="hadronuk" data-source="post: 75947" data-attributes="member: 1866"><p><img src="http://i1308.photobucket.com/albums/s615/hadronuk/Fork%20forces092_zpsunizp1rr.jpg" alt="" class="fr-fic fr-dii fr-draggable " style="" /></p><p></p><p></p><p>This is my model of opposing vertical forces in the forks. I have not thought about this in detail for a while, so I hope I have not made any howlers.</p><p></p><p>To the right of the point where the two force lines intersect, lift forces exceed downward forces and the forks will probably go into vertical oscillation or lock up. I have called this the “watershed position”.</p><p>As I have said, I cannot see that the angle of the bottom link is the sole determinant of this watershed position.</p><p></p><p>Some observations:</p><p></p><ul> <li data-xf-list-type="ul">This is at constant braking force. Note that the braking force magnitude effects both up and down forces. But we know from experience that harder braking increases up forces more than down. So the watershed point moves leftwards if braking is increased.</li> <li data-xf-list-type="ul">Plainly increasing the ride height setting means that the forks spend more time in closer proximity to the watershed point and the probability of problems increases. Without knowing the exact shape of the force lines, how close is too close cannot be judged.<br /> </li> <li data-xf-list-type="ul">Decreasing spring preload moves the whole lift curve downwards, so the watershed moves to the right, away from the ride height fork position. David’s short stiff springs and my degressive springs both lower preload of course.</li> <li data-xf-list-type="ul">The JE geometry will still generate some lift I think, but testing clearly indicates that either the watershed point is far enough to the right to perhaps hardly ever be reached, or even that lift never exceeds weight so there is no force curve intersection point.</li> <li data-xf-list-type="ul">I have no clear idea of the shape of the lift force line!!!!!! It is certainly not a straight line. I did get my model working, and it was broadly of the form shown, but the lift forces were so high my force vector analysis was clearly wrong, in detail at least.</li> <li data-xf-list-type="ul">I think that when the forks are heavily compressed the vertical forces generated by the links may become downwards. But whether this can exceed the spring forces to produce a net downward force that adds to the other downwards forces I don’t know. I suspect it does not matter.</li> <li data-xf-list-type="ul">As we have discussed, braking is not the only cause of a rearward force on the forks. I think an out of line wheel touching down must produce a sizeable rearwards kick in addition to the sharp turning moment impulse on the forks. The necessity with the standard geometry forks in an extended position for the wheel to move forwards in order to ride up over a bump has also been mentioned. Undoubtedly a bump generates a rearward impulse whatever the fork geometry. But the magnitude must largely depend on speed and the ratio of bump height to wheel diameter. If you were to ride over a kerb at 60 miles an hour, this is going to be a sizeable impulse force. But for more normal sized and more sloping bumps, I can’t see that the force is large enough for the difference in fork geometry to have a significant effect. I may be totally wrong of course. But the need for the wheel to move forward by an additional 0.5 inch when at 60 mph it is already moving forwards at 1056 inches per second doesn’t seem important.</li> </ul></blockquote><p></p>
[QUOTE="hadronuk, post: 75947, member: 1866"] [IMG]http://i1308.photobucket.com/albums/s615/hadronuk/Fork%20forces092_zpsunizp1rr.jpg[/IMG] This is my model of opposing vertical forces in the forks. I have not thought about this in detail for a while, so I hope I have not made any howlers. To the right of the point where the two force lines intersect, lift forces exceed downward forces and the forks will probably go into vertical oscillation or lock up. I have called this the “watershed position”. As I have said, I cannot see that the angle of the bottom link is the sole determinant of this watershed position. Some observations: [LIST] [*]This is at constant braking force. Note that the braking force magnitude effects both up and down forces. But we know from experience that harder braking increases up forces more than down. So the watershed point moves leftwards if braking is increased. [*]Plainly increasing the ride height setting means that the forks spend more time in closer proximity to the watershed point and the probability of problems increases. Without knowing the exact shape of the force lines, how close is too close cannot be judged. [*]Decreasing spring preload moves the whole lift curve downwards, so the watershed moves to the right, away from the ride height fork position. David’s short stiff springs and my degressive springs both lower preload of course. [*]The JE geometry will still generate some lift I think, but testing clearly indicates that either the watershed point is far enough to the right to perhaps hardly ever be reached, or even that lift never exceeds weight so there is no force curve intersection point. [*]I have no clear idea of the shape of the lift force line!!!!!! It is certainly not a straight line. I did get my model working, and it was broadly of the form shown, but the lift forces were so high my force vector analysis was clearly wrong, in detail at least. [*]I think that when the forks are heavily compressed the vertical forces generated by the links may become downwards. But whether this can exceed the spring forces to produce a net downward force that adds to the other downwards forces I don’t know. I suspect it does not matter. [*]As we have discussed, braking is not the only cause of a rearward force on the forks. I think an out of line wheel touching down must produce a sizeable rearwards kick in addition to the sharp turning moment impulse on the forks. The necessity with the standard geometry forks in an extended position for the wheel to move forwards in order to ride up over a bump has also been mentioned. Undoubtedly a bump generates a rearward impulse whatever the fork geometry. But the magnitude must largely depend on speed and the ratio of bump height to wheel diameter. If you were to ride over a kerb at 60 miles an hour, this is going to be a sizeable impulse force. But for more normal sized and more sloping bumps, I can’t see that the force is large enough for the difference in fork geometry to have a significant effect. I may be totally wrong of course. But the need for the wheel to move forward by an additional 0.5 inch when at 60 mph it is already moving forwards at 1056 inches per second doesn’t seem important. [/LIST] [/QUOTE]
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Tech. Advice: Series 'B' / 'C' 500cc/1000cc Bikes
Modified Steering Stem
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