# Slightly to small seatpost a problem?



## phazer (Oct 5, 2005)

I have a cannondale system six with a thomson elite seatpost the thomson
is slightly to small for the frame. As in it freely falls in, I have to hold it the right
height and then tighten the clamp quite a bit which then push that bit of fram under the clamp againts the post keeping it in. There is a little cut down the down tube in the back
that bends which allows the frame to touch the post. Do I run a risk of cracking the frame
at that point? Should I just get a shim? Or is it ok the way it is at the moment?


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## Uncle Grumpy (Jul 25, 2005)

Gees, just go crazy and buy the right size seatpost. Sell the smaller one to recoup some of the cost.

I wouldn't take the risk of cracking the frame. <controversy> It's a Cannondale, it don't take much to crack 'em. </controversy>

Grumps


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## FBinNY (Jan 24, 2009)

If by "slightly too small" you mean not by an amount where the next size post would correct, but only that it's a free slip fit in the frame, then you're OK. One way to test is if the post can rock in the frame, which would mean it's the wrong post. If the size is one where the next size is 0.2mm larger, you might see if that one fits better.

If the post is the right size, meaning that the next size up is too big, then a bit of pinch at the top of the slot is OK, as long as the ears don't touch in the back. You only tighten this a few times in the life of the frame, so it's not going to get flexed a lot. 

The problem with undersized posts is that they're hard to clamp tight enough and tend to creak causing folks to overtighten the binder bolt cracking the frame.


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## PlatyPius (Feb 1, 2009)

What size is the seatpost?

Thomson doesn't have a lot of different sizes, so I would guess that the post is the correct size, but your BIKE is not.


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## tihsepa (Nov 27, 2008)

Get a new frame to go with your Thomson.
I match all of my frames to whatever seatposts I have lying around.


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## PlatyPius (Feb 1, 2009)

A from Il said:


> Get a new frame to go with your Thomson.
> I match all of my frames to whatever seatposts I have lying around.


lol

The sad thing is, a Thomson seatpost CAN cost more than a frame, sometimes. (Think Nashbar)


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## tihsepa (Nov 27, 2008)

PlatyPius said:


> lol
> 
> The sad thing is, a Thomson seatpost CAN cost more than a frame, sometimes. (*Think **Nashbar*)


No! and you cant make me!!!


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## phazer (Oct 5, 2005)

FBinNY said:


> If by "slightly too small" you mean not by an amount where the next size post would correct, but only that it's a free slip fit in the frame, then you're OK. One way to test is if the post can rock in the frame, which would mean it's the wrong post. If the size is one where the next size is 0.2mm larger, you might see if that one fits better.
> 
> If the post is the right size, meaning that the next size up is too big, then a bit of pinch at the top of the slot is OK, as long as the ears don't touch in the back. You only tighten this a few times in the life of the frame, so it's not going to get flexed a lot.
> 
> The problem with undersized posts is that they're hard to clamp tight enough and tend to creak causing folks to overtighten the binder bolt cracking the frame.


so basically if the ears don't touch it should be ok? I have a campy record post that
fits but it's SOOOO tight I almost have to hammer it in, plus the thomson goes
nicely with my thomson stem 

that said I can see a small gap between the post and the frame at the bottom of the
"cut" in the frame.


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## Creakyknees (Sep 21, 2003)

For the price of a (cheap) seatpost or shim, you won't risk ruining your expensive frame.

Buy or borrow a pair of calipers (under $25 for a basic set) and find the correct size. Then buy it.


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## David Kirk (Mar 6, 2005)

As odd as it sounds not all 27.2 posts are the same size and none are actually 27.2 mm.

The post size is nominal and not actual. I'll use the standard 27.2 mm post as an example since I don't know what size post your Cdale uses. A 27.2 post is not 27.2 in diameter but it is smaller than that and is designed to fit into a 27.2 hole. Most 27.2 posts are about 27.15 in actual diameter but there is no standard and some are smaller and some are larger - yet all are labeled as the same size.

Some brands tend to run small (Thompson) and some run larger (Campy) so if you have to correct size post and the fit is loose you might try some different brands to see if changing brands will give you a better fit.

It's also at least remotely possible that the frame was reamed slightly oversize so that even a 'big' post might fit loose. If that is the case then you will need to do some experimenting with a larger nominal size to get the perfect fit.

Good luck.

Dave


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## batman1425 (Nov 29, 2005)

+1 To what Dave said. I think your best bet is to measure the inside diameter of your frame first to make sure you have the correct size post to begin with. If you have the "correct size", like in Dave's example, you might consider calling Thomson and asking if they can pick out a post with a variance on the larger side for you from the assembly line for a tighter fit. I seem to recall reading a post here where someone did that for a similar problem.


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## phazer (Oct 5, 2005)

Managed to fix my problem with a coke can. LBS did not have a thin
enough shim to fit between my post and frame so the mechanic suggested
I cut a strip of metal out of a coke can and wrap it once around my post. Gave
it a go and it works like a charm, seatpost fits perfectly now


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## Dinosaur (Jan 29, 2004)

You can make a shim out of a pop or beer can. Cut it 3x1 inch (experiment with the size). Round out the corners. You might have to put a light (note light) coat of grease on both sides of the shim. The trick is to get the seatpost to slide down the seat tube and not let the shim fall all the way down. And not to have a little bit of the shim exposed at the top. I had the same problem with my Thomson. Mark the seatpost with a felt tip pen or put a piece of electrical tape around it and check for slippage. it should work, if not just buy a different seatpost. I have two Thomson and they both slipped. I eventually went to a different seatpost.


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## Argentius (Aug 26, 2004)

Sounds like this is what the OP did, pop can is ~0.1mm thick, and it seems like most parts and frames are mfr'd to about that tolerance. I've seen plenty of bikes soda-can-sleeved. 



Dinosaur said:


> You can make a shim out of a pop or beer can. Cut it 3x1 inch (experiment with the size). Round out the corners. You might have to put a light (note light) coat of grease on both sides of the shim. The trick is to get the seatpost to slide down the seat tube and not let the shim fall all the way down. And not to have a little bit of the shim exposed at the top. I had the same problem with my Thomson. Mark the seatpost with a felt tip pen or put a piece of electrical tape around it and check for slippage. it should work, if not just buy a different seatpost. I have two Thomson and they both slipped. I eventually went to a different seatpost.


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