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 Post subject: Steel Seat Posts
PostPosted: Mon Oct 01, 2012 10:52 am 
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Joined: Tue Dec 21, 2010 6:31 pm
Posts: 1111
Hello,

Has anyone else had problems with (re-) installing steel seat posts?
Over the past year or so, I've only dealt with a few steel posts,
but had the following problems:

1) I undid, removed and regreased a steel post, then reinstalled it
but found I could not get the post clamped securely in the frame,
there was always a bit of movement. I am pretty sure it was the right
sized post for the frame - it was secure when I bought the bike, but after
removal I couldn't get it tightened up enough. I eventually gave up and
used an aluminium post.

2) More recently, someone asked me to do up a bike for him. I noticed
the new steel post he had used was loose, but again I could not get it tight enough. I measured the post and frame, and both seemed to match, but
even with the clamp done up very tight the post slipped when I put any significant weight on it. In this case, I replaced the original post with another
old steel one I had in my bits box, which I was able to get tight enough to ride.

Any idea why these very frustrating problems occur with steel posts? I know there are inaccuracies with the measurement of frames and posts by manufacturers, but that doesn't explain the first case, where the post seemed to fit at first, then didn't later on... Is it something to do with the
use of chrome on the posts? Or the flexible nature of steel compared to aluminium?

Sorry for the long rant, this has been quite annoying...

Johnny


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 Post subject:
PostPosted: Mon Oct 01, 2012 1:25 pm 
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Joined: Wed Apr 25, 2012 2:08 pm
Posts: 2185
Location: Shrewsbury
Bit of an odd one, I can't say I've really had that problem.

Are you sure where the slot is at the top of the seat post tube, that the two sides are not completely pulled together? If they are then there would be no way of getting it to clamp the post any tighter.

Alternatively, it might be worth another seat post bolt in case the one you are using is stripped and not tightening as much as you think :?


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 Post subject:
PostPosted: Mon Oct 01, 2012 1:38 pm 
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Joined: Tue Dec 21, 2010 6:31 pm
Posts: 1111
Thanks for the reply.

The slot at the top of the post did get pulled together so the clamps were touching in the second case, which is why I tried the second post that worked a bit better. Is it possible/advisable to file the slot a bit to create more room to close the clamp?

I also tried various different bolts to make sure I wasn't using the wrong
type, but this didn't help much.

I suppose it's good to know I am not making some obvious mistake...

Cheers,

Johnny


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 Post subject:
PostPosted: Mon Oct 01, 2012 3:43 pm 
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Joined: Wed Apr 25, 2012 2:08 pm
Posts: 2185
Location: Shrewsbury
If its the right diameter seat post the slot at the top of the seat tube shouldn't be touching but I wouldn't file it away, I would guess you have the wrong size seat post. Try and straighten out the slot and measure the tube diameter.

You could try making a shim from an old coke or beer can. Cut a strip, they'll cut with scissors, make it about 2" deep and long enough to wrap once around your seat post. Try fitting that and tightening again to see if its any better :)


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 Post subject:
PostPosted: Tue Oct 02, 2012 8:54 am 
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Joined: Mon Mar 26, 2012 5:21 pm
Posts: 589
I had a similar situation to your first case: it turned out that the seat post that the bike came with was too small, but the passage of time had helped to secure it. If you assume that the parts a bike comes with are the correct ones, you are placing your confidence in the work of persons unknown.

I'd warrant that if you checked the post, you will find gouges in it where the clamp was over-tightened (because it was too small). When it was replaced, it would have been in a slightly different position, hence the movement.

I don't think it would be a good idea to file the slot: not because this would weaken the frame particularly, but because the slot is a good indicator of a correct fit. As Robbie wrote: if the slot is closed-up then the post is too small.

The rule of thumb that I work to, is that a post should be a "gravity fit" when unclamped; ie the post should stay in place without the bolt being tightened. Then you are assured it will be secure when clamped.


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 Post subject:
PostPosted: Tue Oct 02, 2012 10:31 am 
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Joined: Tue Dec 21, 2010 6:31 pm
Posts: 1111
Thanks again for the replies.

Yes, filing is a bad idea, I won't do that...

I may try the coke can shim idea on the bike I've just done up. I did
measure the posts and the frame, but it's possible I made a mistake.
The guy who owns the bike needs a very long seatpost, so it might
be a struggle for him to find a suitable one in the correct length.

I'm intrigued by what process happens that makes a post that's too small
fit after a while. Rust or corrosion? Heat? Using grease to install?

I've also learned not to trust other peoples' component installation. I had
one bike that seemed like an exam test for bad installation. Everything was done wrong, from the brake blocks being put in backwards to the ergopower levers being put on the wrong sides so the thumb levers stuck out like little Mickey Mouse ears. Cute, but not very practical...

Johnny


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 Post subject:
PostPosted: Tue Oct 02, 2012 7:06 pm 
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Joined: Mon Mar 26, 2012 5:21 pm
Posts: 589
Johnsqual wrote:
...I'm intrigued by what process happens that makes a post that's too small fit after a while. Rust or corrosion? Heat? Using grease to install?...

My guess would be that the rider's weight and road vibration would gradually work the post lower, forcing the top of the seat tube into the post.

Throw in accumulated road crud, the assembly grease etc etc.

Once you take it out that "breaks the seal" and you're back to square one.


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