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Old 02-20-08, 05:12 PM   #16
jur
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Join Date: May 2005
Location: Melbourne
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Quote:
Originally Posted by invisiblehand View Post
Were you close to the max extension?

I made a telescoping seat post and do not experience the bobbing. However, neither of the two components of the seat post are close to their minimum insertion point.
I'm actually about an inch beyond max extension. I checked the end of the post where it sits with respect to the frame joints, and it is still below the top tube (or is that downtube?). So I don't think I will damage the frame. The post is another matter. That may bend. I have a piece of very tight-fitting 1" ally tube inserted inside the seatpost for added strength and stiffness, but obviously it is not stiff enough. I think the bounciness is therefore caused by the springiness of the post, not by minimum insertion issue. The post's srping plus my upper body mass gives a particular resonant frequency coinciding with 100rpm. A stiffer post would shift that frequency up, say to 120rpm, and since I don't pedal there, that would solve the problem.

A telescoping post by itself may not do much for me, since most of the flexing happens near the point where the post leaves the frame. It is in that region that I have to increase stiffness more than any other. Which is why I think a CF post (MUST be 400mm to work) and a telescoping setup may be an improvement: The CF post's bottom end would be below the seat tube's top, and so form a homogenous portion with the alloy sleeve in the critical region. It would combine its stiffness with that of the sleeve, since both would be clamped down by the seat tube's QR.
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