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Old 08-19-14, 09:02 PM   #1
bjtesch
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accidentally telescoping seat tube

Well it's not supposed to telescope. I got a new used frame (steel), swapped my components from my old frame, rode it a little bit and a few blocks from home the seatpost telescopes. (Slides as far down as it would go.) When I got home I tightend the bolt as much as I thought I could get away with. I always read that you should put a very thin coating of grease on the seatpost so it won't rust and get frozen in position so when I built the bike that is what I did. I went out Sunday afternoon for a longer ride and a few miles from home I miss a pedal and the impact telescopes the seat post again. Tonight I pulled the seat post out, cleaned out the inside of the seat tube and the outside of the stem with rags and acetone. I put it back together and again tightened it as much as I thought I could get away with. Tomorrow I'll check it out again. This time I marked the position on the post with a marker so if I have to make emergency adjustments on the road I can get it back to where it was.
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Old 08-19-14, 09:07 PM   #2
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slipping happens .. .. maybe the seat tube is a bit oversize , post a Bit under ..
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Old 08-19-14, 09:09 PM   #3
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Wrong size seat post (too small).
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Old 08-19-14, 09:09 PM   #4
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If all is right, the post should clamp to where it can't budge under any load less than what would bend the saddle rails. (grease or no grease)

First thing to check is whether the pinch bolt ears touch in back. Then check if the tops of the slot touch each other. Thy should be just slightly closer than the width of the slot at the bottom.

If either of these checks are positive, you won't be able to clamp effectively, either because the post is undersized, or in the case of the ears touching because they've cracked or collapsed from prior over tightening.

The other possibility is that you're running out of thread, or coming to a defective thread on the pinch bolt, so you think it's tighter than it actually is.
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Old 08-19-14, 10:14 PM   #5
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The new frame may have a larger seat tube diameter than your old one. There are many sizes, and some of the steps between each are very small. If you swapped over the entire seatpost + seat combo to the new bike, that could explain the problem.

If the new frame came with a seatpost, you could look and see if the diameter is imprinted on it. Look for the designated circle with a slash through it, along with a number in millimeters. Or use calipers on each seatpost and see if/how they differ.

Your seat can be detached from the current post by loosening the clamping mechanism and clamping it onto a new post, if it's appropriate.
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Old 08-19-14, 10:37 PM   #6
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If the seatpost is the right size and still won't clamp just rub some dirt on the greasy s/p and you will be gtg.




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Old 08-19-14, 10:42 PM   #7
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Also noticed you didn't include any measurements or frame model. Likely you have the wrong s/p.


ie: the seatpost from my Raleigh Special (27.0) almost but not quite works in my Kona Explosif (27.2).




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Old 08-19-14, 10:48 PM   #8
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Seatpost too small for new frame, most likely.
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Old 08-20-14, 11:32 PM   #9
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I thought I remembered the seat post being 27.2, but I bought it over 20 years ago. I measured it tonight and it does measure 27.2. The frame is a Schwinn Paramount, from 1987. I don't recall that larger seat posts were used back then. As far as I can tell the Paramount is supposed to use a 27.2.
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Old 08-20-14, 11:38 PM   #10
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Quote:
Originally Posted by bjtesch View Post
I thought I remembered the seat post being 27.2, but I bought it over 20 years ago. I measured it tonight and it does measure 27.2. The frame is a Schwinn Paramount, from 1987. I don't recall that larger seat posts were used back then. As far as I can tell the Paramount is supposed to use a 27.2.
There's always the off chance that it calls for a 27.4 post.

But if you read my earlier post, I explain how the frame itself tells you if the post is the right size by the tapering of the seat clamp slot. No taper, the post is right, taper of almost 1mm means the post is undersized.
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Old 08-20-14, 11:47 PM   #11
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Originally Posted by FBinNY View Post
There's always the off chance that it calls for a 27.4 post.

But if you read my earlier post, I explain how the frame itself tells you if the post is the right size by the tapering of the seat clamp slot. No taper, the post is right, taper of almost 1mm means the post is undersized.

I looked tonight when I measured the seatpost and I didn't notice any tapering of the slot, or bottoming out of anything. I did some other internet searching and found Paramounts with 27.2 seat posts but I also found hints that Waterford bikes used 27.4 seat posts. I found a good number of 27.4 seat posts on ebay so I may see what I can find locally since I need the bike for an event this weekend. I read about shimming the seat post but that seems like it would be though to do. A 0.2mm difference in diameter would require a 0.003"-0.004" shim wrapped around the post, and that is a pretty thin shim.
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Old 08-21-14, 12:11 AM   #12
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A 0.2mm difference in diameter would require a 0.003"-0.004" shim wrapped around the post, and that is a pretty thin shim.
Wait is my ability to do math suspect? wouldn't the shim need to be .1 and not .003-4? How'd you come up with that?

I've heard of cutting a strip of a beer/soda can as a shim, that seems pretty thin
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Old 08-21-14, 01:13 AM   #13
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Wait is my ability to do math suspect? wouldn't the shim need to be .1 and not .003-4? How'd you come up with that?
0.003" refers to 0.003 inches, that's how.
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Old 08-22-14, 11:20 AM   #14
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I sliced up one of my wife's diet coke cans and it measures 0.004" thick.

I pulled out the seat post and with the pinch bolt loose the post slides smoothly up and down but with it in its ride position you can detect a very small amount of rocking side to side.

I wrapped the shim around the post and it would not go into the tube. I cut a sliver of shim about an inch wide and tried it on one side, it wouldn't go. I tried lots of ways such as putting half of the shim in the tube and then putting the post in. I managed to get a piece of shim about 3/8" to 1/2" wide in on one side, about 2.5" total depth. I tightened the bolt as much as I dared, we'll see how it goes.

When I could detect that small amount of rocking I thought maybe it did need the 0.2mm larger diameter. Depending on how the shim works, I'll try to find a 27.4 post on the net sometime after this weekend.
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Old 08-22-14, 12:01 PM   #15
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If shimming isn't working, you can be riding this weekend with this quick fix.

Visit an auto mechanic, and swap a beer for a teaspoon of medium to coarse lapping compound (grit in grease). Determine the desired height of the post and coat the clamping area of the post with the lapping compound (top 2 inches inside the frame). When clamped the grit will bite into both the post and frame, providing a mechanical interlock between the two, and greatly improve gripping strength.

This is not a substitute for using the right diameter post, but it may also be necessary if you do.

In any case, until or unless you solve the problem another way, this will have you riding now.
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Old 08-23-14, 08:11 AM   #16
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Originally Posted by bjtesch View Post
I thought I remembered the seat post being 27.2, but I bought it over 20 years ago. I measured it tonight and it does measure 27.2. The frame is a Schwinn Paramount, from 1987. I don't recall that larger seat posts were used back then. As far as I can tell the Paramount is supposed to use a 27.2.
Measured how?
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