Seatpost Question

Discuss light weight issues concerning road bikes & parts.
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Birch
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Location: Chicago, IL USA

by Birch

With the new fad being an integrated post, I was thinking of the following.

I have a CAAD9 currently, and the only post I had laying about was a Thomson Elite 410mm. So right now, I've probably got 10CM that I could chop off it for weight savings.

Here's the question. Does having a longer post like that produce the same effect of having an ISP type frameset??
In desperate moments, on the rivet, opening up my giant suitcase of courage, definitely not a climber but a big strong man..

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Zak
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by Zak

Maybe dumb question, but what effect are you talking about?

Birch
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Location: Chicago, IL USA

by Birch

Not dumb at all. I thought one of the benefits of this was to produce a stiffer platform... or not so much?
In desperate moments, on the rivet, opening up my giant suitcase of courage, definitely not a climber but a big strong man..

mattr
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by mattr

The stiffness of the frame/post interface is only affected by the actual bit of the post which touches the inside of the seat tube, probably about 8-10cm in most double butted or CF frames. If you can be bothered you can probably see/feel where the step change in internal diameter is. Any post sticking out below this point is useless. Some frames/posts with poorer tolerancing may only contact properly over 1/2 of this length as the clamp will not take up the gap between the ID and OD all the way down to the diameter change.

So no, having a longer than needed seatpin won't give you the effect you think you'll get.

Birch
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Joined: Fri Jul 25, 2008 4:12 pm
Location: Chicago, IL USA

by Birch

Ah. That makes sense. I didn't even think of the fact that the tube lengths change.

Now... where did I put my hacksaw. That thing's gettin' chopped today!

Thanks for the replies!
In desperate moments, on the rivet, opening up my giant suitcase of courage, definitely not a climber but a big strong man..

mattr
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Joined: Fri May 25, 2007 6:43 pm
Location: The Grim North.

by mattr

Careful you don't chop too much or you will end up with below minimum insertion and end up with a snapped/cracked frame.

Measure 26 times, check a further 5 or 6, sleep on it, measure a couple more times, then cut.

Birch
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Joined: Fri Jul 25, 2008 4:12 pm
Location: Chicago, IL USA

by Birch

Thanks...

I was going to measure the minimum insertion line from the bottom of the post to the line and leave one cm more than that distance when I chop.
In desperate moments, on the rivet, opening up my giant suitcase of courage, definitely not a climber but a big strong man..

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Frankie - B
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by Frankie - B

A very good rule of thumb is to let the post extend one inch past the underside of the toptube and then cut what is left.
'Tape was made to wrap your GF's gifts, NOT hold a freakin tire on.'

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