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Set-back seatposts...originated?
Last Post 12/19/2013 12:16 AM by 79 pmooney. 1 Replies.
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12/18/2013 10:39 PM
So I've been studying bike geometry for a while now and have stumbled upon something that seems to be done only because that's how it's always been done. Set-back seatposts. If you go back 40 years and strike a straight line from center of BB to center of saddle clamp, you'll find that the line travels through the rear wheel. Since steel is fairly difficult to manipulate into complex shapes, my guess is that this is where set-back seatposts were born. It allowed for the rider to sit back enough from the BB to generate power. Now take today's composite materials that can be molded into just about any shape and I have to wonder why set-back seatposts even exist. The "seat-tube" can effectively wrap around the rear wheel and then travel up to the saddle clamp. If the saddle clamp is perched directly above the center-line of the seat-tube, it should be exactly 73 deg. (or whatever the geometry states on your particular frame.) But if you use a set-back seatpost, you're effectively changing the angle from 73 to something closer to 71 or 70. Maybe I'm missing something here.


12/19/2013 12:16 AM
Two thoughts. 1) whatever gets you and your wheels in the proper place will work. Both bent seattubes and big setback posts have been done over the years and both work for the riders that need them. My two ti bikes have roughly 75 seat angles and 60mm setback posts. Both disappear under me. (I do that so the rear wheel can be far enough forward that the weight distribution is good on my geared bike and so I can run a very long dropout on my fixie, eliminating the need to mess with the chain when changing cogs. Also so I can run 25c tires and fenders on both bikes.)

2) I suspect there is a small amount more vertical compliance from a large setback post because in addition to the flex of the post clamp itself (which I would hope not be much) there could be flex in the seatpost itself as it it flexed from the lever that is the setback post. A zero setback post would nullify this completely.

I just opened the drawing of my geared custom. A seattube of 69.6 degrees would halve the seat rails. The centerline of that tube, if straight, would run 1/4" off a 25c tire. Allowing for fenders would require some real bends. (In ti, that could be a lot of fun. But seeing my $300 custom post? Maybe cheaper to bend.)

Doesn't UCI have something to say about bent seattubes?

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