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Can I shim a seatpost/seat tube?

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Can I shim a seatpost/seat tube?

Old 08-24-12, 06:04 PM
  #1  
sunburst
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Can I shim a seatpost/seat tube?

can't believe I did this. bought and installed a 26.0mm seatpost and it spun around when I tightened it up. looked at the old one and it is 26.2. I measured before purchase, but obviously not carefully enough.

can't return the new one scratched up. can I shim it? soda can or whatever? I've heard of successfully shimming handlebars, and even seatposts. In fact my first fancy/expensive mtn bike suspension seatpost came with a plastic shim/collar to fit my bike, but I've only got 0.2mm to work with here.

this is for an old English bike from the 70's. I only need to set this once and leave it for eternity.
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Old 08-24-12, 06:25 PM
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I'm surprised it didn't work anyway - 0.2mm difference works fine for handlebar clamps.
OK, a 0.1mm shim is like the thickness of tin foil - I'm not sure how you'll keep it from ripping though..
A real seatpost shim (for matching larger disparities) will have a lip so it doesn't fall down the seat tube.
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Old 08-24-12, 07:58 PM
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I'd give it a try. First thing I can think of would be a can of spam, throw the stuff in the garbage and keep the can. The metal tends to be thin. Get the snips out and cut and try a few pieces. Maybe dip one in oil to help it last longer, then see if it could fit in. With oil it would have to be good and tight.

At .2 mm, the piece to do the job might not need to go all the way around.

If it doesn't fit well it would give you an idea of the next possible thing to attack in the pantry.

(I enjoy this kind of stuff. )
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Old 08-24-12, 10:14 PM
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You might try one of the weaker loctite varieties
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Old 08-25-12, 01:03 AM
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a digital caliper will best you measure the thickness of the material required
in order to be that 0.1mm thickness that you say you need.

Last edited by fietsbob; 08-25-12 at 01:07 AM.
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Old 08-25-12, 08:05 PM
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Originally Posted by DiabloScott View Post
OK, a 0.1mm shim is like the thickness of tin foil - I'm not sure how you'll keep it from ripping though..
.
Originally Posted by fietsbob View Post
a digital caliper will best you measure the thickness of the material required
in order to be that 0.1mm thickness that you say you need.
First thing I tried was a beer can. It measured 0.2 so I knew that wouldn't do. I figured I was screwed because that was pretty flimsy aluminum. couldn't imagine anything thinner except foil, and how would I keep foil in place while inserting the post?

well, as luck would have it (for once in my favor), right behind my bike tool box is my electronics tool box, and I noticed an unopened roll of Scotch foil tape that I purchased to shield one of my guitar cavities. It had been sitting there for years waiting for me to open up my guitar. I measured it and it was ~0.1mm. I wrapped it around the post and the post measured to 26.2mm exactly.

I installed the post and it's holding. will post again with a long term report (after 1st serious ride).

https://www.amazon.com/3M-Scotch-2-In.../dp/B000NG622Q

Last edited by sunburst; 08-25-12 at 08:12 PM.
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Old 08-25-12, 08:20 PM
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Good to read you got the job done. I'm surprised you needed a shim I have used a lot of seat posts that where 0.2m too small on bikes without using a shim. You can get really nice commercial reatail seat post handle bar shim off of Ebay or on the net but the smallest I have seen is about .4m
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Old 08-27-12, 03:11 PM
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well, happy to say it held up during my one hour test ride. and the new seatpost, which I bought for its offset, placed my B68 in the right spot. It only gave me about 5mm or so more setback than the old Laprade but it was enough so I'm not sitting on that rear support rail of the Brooks (not a good place!).
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Old 08-27-12, 04:41 PM
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That's good to know it's possible. I find the fractional difference in seatpost size very annoying, and have been caught out on more than one occasion
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Old 08-27-12, 04:55 PM
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I needed to put a longer seat post on my 12-year old daughter's bike this weekend. The OEM part had the separate saddle clamp thingy common on kids' bikes and it was labeled 25.1mm - actually the 25 was kind of a big font and the 1 was kind of a superscript so I'm not really sure. My caliper said 25.0 which is more common. Couldn't find either one in a longer version at any of my favorite shops but the last place I went had a 25.4 in a long version with an integrated clamp so I risked it... daughter's bike is aluminum with a collar seatpost clamp. Went in just fine with a 0.3 or 0.4 mm larger diameter than the original post - might not've worked on a steel frame with a lug clamp though.
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Old 08-28-12, 07:01 PM
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Originally Posted by DiabloScott View Post
Went in just fine with a 0.3 or 0.4 mm larger diameter than the original post - might not've worked on a steel frame with a lug clamp though.
with my luck, 0.3-4 would not have worked. at least not on my Peugeots, which I have the most experience with. I've filed stems a number of times trying to remove 0.2mm. but now that I say that, I guess the headset/headtube has less (or no!) wiggle-room. and yeah, I know what you're talking about with that saddle clamp. most of my Peugeots are old enough to need one.

this 26.2 post was for my 1st English restoration.
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Old 07-15-22, 06:24 PM
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Ancient, mummified, dust encrusted thread, but neverthless I'll give the foil tape a try! I have the exact same 0.2mm difference situation right now. Unless of course I can locate a small amount of some stainless shim stock first. I've used some paper thin brass stock on cantilever brake bosses before to take out slop with success.

I resurrect thee, oh thread

Last edited by prairiepedaler; 07-16-22 at 07:02 AM.
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Old 07-15-22, 10:22 PM
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Originally Posted by Closed Office View Post
I'd give it a try. First thing I can think of would be a can of spam, throw the stuff in the garbage and keep the can. The metal tends to be thin. Get the snips out and cut and try a few pieces. Maybe dip one in oil to help it last longer, then see if it could fit in. With oil it would have to be good and tight.

At .2 mm, the piece to do the job might not need to go all the way around.

If it doesn't fit well it would give you an idea of the next possible thing to attack in the pantry.

(I enjoy this kind of stuff. )
A travesty! Fried spam at 1am, what could be better?

I use pop can shims on one of my bikes with assembly paste on both sides of it. Works great.
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Old 07-16-22, 07:48 AM
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You guys are the cheapest people I know. Shims? Really?? Pry open your wallets and buy the correct size seatpost. Sell the wrong one on eBay. Sheesh.
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Old 07-16-22, 05:09 PM
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Originally Posted by smd4 View Post
You guys are the cheapest people I know. Shims? Really?? Pry open your wallets and buy the correct size seatpost. Sell the wrong one on eBay. Sheesh.
Surely not the cheapest! Frugality and lack of funds yes, but cheap no. In my case, I wanted a suspension post which is tough to find economically in a 26.0 if at all. A 25.8 showed itself up at the local coop in decent condition and the old Kuwahara Sage it will go into won't mind.
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Old 07-17-22, 06:46 AM
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Originally Posted by prairiepedaler View Post
i resurrect thee, o thread
fify!
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Old 07-17-22, 08:52 AM
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Originally Posted by sweeks View Post
fify!
O... I forgot.
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Old 07-20-22, 06:07 PM
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I just bought an expensive seat-post based on the wrong diameter posted on the frame's website (I didn't yet have the frame to measure myself). It turns out they had the wrong size information, so now I get to choose between a shim and re-buying a seat-post. Since the now slightly used one is 30.6 mm, it is unlikely I am going to be able to sell that thing to anyone.
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Old 07-21-22, 08:06 AM
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Using a shim to get a proper fit between a seat tube and seat post isn’t nearly as “cheap” or blasphemous as some might like to think. I’ve come across more than one bike with a shim that I’m pretty sure was OEM! The fact that shims are commercially available generally indicates widespread use.
Anyone that might be worried about getting disparaging looks from their “bike snob” friends, should make sure that their “home-made” shim is placed just below the top of the seat tube AND isn’t visible through the compression slot on the back of the seat tube. That way, only the individual and mechanic will know for sure .
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Old 07-21-22, 08:18 AM
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In my case, it is a new Ti frame and new carbon seat-post. I had to go out of my way to find the (wrongly-stated) size, and of course it wiggles around in the seat tube. The shim gives me about 100mm x 2(pi)r of interface, and a properly-sized post would give me about 300mm x 2(pi)r of contact interface, so it will be much more stable (the length is probably most important, since the rider will be torquing about the clamp site). The shim basically gives you the minimum allowed post insertion length. Perhaps the only advantage of having short legs in this sport is being able to have a long seat post inserted into the seat tube.

The other annoyance is instead of having a carbon/titanium interface, I now will have a carbon/aluminum/titanium interface. Let the galvonic festivities begin!
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Old 07-21-22, 09:02 AM
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Great job solving. For the information of those who might blanch at the $25 price for a roll, smaller rolls can be procured for $3 or so.
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Old 07-21-22, 09:25 AM
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I posted a picture (can't find it) I'd taken of a local bike in the wild whose owner had shimMacGyvered up his post with a handkerchief. It looked kind of like a two wheeled Molotov. I'm not ready to go that far.
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Old 07-21-22, 04:03 PM
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This whole “0.2mm” difference requiring an exact “0.1mm shim”….not if you only go partially around, right? We’d always cut a thin strip (less than a cm wide) of a pop can (~0.2mm) and just slide that in front—that would shim fine (or we’d add another piece) with the bonus advantage of having most of the circumference contact remain between the seat tube and seat post at the point of clamping.
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Old 07-21-22, 07:29 PM
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Yes, you can, and it works great. You can see the short portion of a steel can rim poking out from above the rear bolt (below in this picture), which keeps it in place instead of falling down the seat tube. My aluminum-can shim has no rim, but shows on the side a little so I know where it is. Both are cut to about 2/3 of the seatpost diameter.


Half-wrap-around seat stays, a la Woodrup

I also use commercial shims, for example to use an expensive spring seatpost that I bought for one bike, on another.

No need to feel bad about it, either. If you ride with snobs, just keep your middle finger ready!
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Old 07-22-22, 02:02 AM
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I have an old bike with 26.4 mm seat tube. Trying to find a decent seat post for it was a nightmare. I eventually found one from U.S.E.; the seat post itself was, I think (this was many years ago now), 25mm, and they sold shims in all sorts of sizes to match any tube. Seems an eminently more sensible solution than trying to provide a post in every single size.

My suggestion for strong, thin shims would be to buy a cheap set of feeler gauges and sacrifice the appropriate size.
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