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Educate Me On Seat Tube Shims In A Ti Frame

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Educate Me On Seat Tube Shims In A Ti Frame

Old 11-24-22, 07:16 PM
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Steve_sr
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Educate Me On Seat Tube Shims In A Ti Frame

Hello,

Happy Thanksgiving everyone! I hope that this post is in the right place.

I have a 2005 titanium Litespeed with about 93K miles on it. It has a Ti seat tube with a shim that was honed/reamed to a snug fit. No issues with the seatpost slipping or any frame cracks in all those miles.

I recently received a new Ti frame and was surprised that the seatpost just dropped right into the frame! It also has a Ti seattube with what looks like an aluminum shim insert construction.

I took some caliper measurements and the seattube opening measures 32.13mm for a 31.6mm nominal seatpost. This is a diameter difference of .020" (0.5mm).
I have been in contact with the builder who claims that .020" is "within tolerance". Their original response was that the insert was reamed to the correct size before the slot was cut and that the top of the seattube may have expanded after the slot was cut. I don't know that I buy this argument as the seattube O.D. is round within a couple of thousandths and the diameter is the same within .005/.010" above and below the top tube. I don't have a bore gauge but based on my O.D measurements I would assume that the seattube bore is as sloppy all the way to the end of the insert shim.

While the builder doesn't consider this a problem I would like a second opinion. Hopefully there are some builders here with Ti experience. My main concern would be frame cracking of the seatpost due to metal fatigue as the post wobbles back and forth in the loose bore. Is this a valid concern or not? About the only thing that I could guarantee is that if there was no flex there would be no frame cracking!

I am also curious on what should be the proper process for installing a seatpost shim in a Ti frame. I am assuming that the insert shim is aluminum. Is it press fit or glued? If it is decided that this shim should be replaced and properly sized what special skills and tools are required to do it properly? Could a builder that only does steel frames do it or should I find someone with Ti experience (much harder)?

Thanks,
Steve
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Old 11-24-22, 07:45 PM
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Subscribing to this thread.

I do not have an answer to your questions.

However, I plan to shim an aluminum dropper post into a titanium mountain bike frame soon. For my application I will use a liberal amount of copper antisieze paste grease. I haven't had a seatpost or bottom bracket cups get stuck using that stuff with titanium before

I would like to hear what others may say about your questions.

Strictly by intuition, I advance that the more seat post is exposed, the greater the flexion and potential for damage.

A 6'5" friend of mine who had a crack form in the seatstay/top tube/seat tube junction on his titanium frame. There was no extraneous play or errant fitment however.

Last edited by Sorcerer; 11-24-22 at 07:49 PM.
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Old 11-27-22, 04:29 PM
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A poorly fitting seatpost is the worst, they will always slip or spin on me. Never had luck with carbon friction paste either.

32.4mm posts exist for some brand of frame I've never seen, I'd be tempted to ream the current sleeve to accept one of those.

https://www.ebay.com/itm/14482869652...Bk9SR6KyqrOXYQ
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Old 11-29-22, 07:56 PM
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Did I say something wrong? I was hoping to get some input from some of the builders here. I am just trying to solve a problem. I am not looking for trade secrets.

Thanks,
Steve
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Old 11-29-22, 08:50 PM
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Steve- I think you will get similar replies as you did in the Bike Mechanics thread you started (70+ replies to date).

One of the questions i had, in that other thread, is if the seat post fit is an actual issue when riding. Does the post slip when in use? Andy
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Old 11-30-22, 09:56 AM
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Originally Posted by Andrew R Stewart View Post
Steve- I think you will get similar replies as you did in the Bike Mechanics thread you started (70+ replies to date).

One of the questions i had, in that other thread, is if the seat post fit is an actual issue when riding. Does the post slip when in use? Andy
I don't know if this is an issue as the bike hasn't been ridden yet. They sent a chain that was cut too short and I am waiting on the replacement.

I was hoping that someone in this forum could provide some more process details as to what skills and tools would be required to perform a replacement and also where these shims come from and whether they are press fit or glued? Would a competent machine shop be able to perform the replacement or does it need to be a frame builder with special tools?

Last edited by Steve_sr; 11-30-22 at 10:01 AM.
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Old 11-30-22, 11:32 AM
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I have serviced a number of Ti bike that had a shim insert in the seat tube but as these were not a problem never spent much time exploring in detail. However I have read of both a press fit and a bonded in fit being used by various builders. There might be some excess bonding agent (epoxy?) at the lower end of the shim that could be felt with a hooked probe. One could consider opening up (reaming?) the OEM shim and adding another custom made one inside the OEM one. Still a lot of work for a situation that we don't know is an actual riding issue (like seat post slippage). I have zero concerns about the long term reliability of this frame, WRT the seat tube and clamping area, as long as the post is not rocking about after the clamp is tightened there should be no increased wear on the frame. The shim will keep the post from contacting the actual tube ID. Now I suppose there could be some shim degradation if the post were to not stay tight but as the Al shim is softer and more prone to contact wear it would likely be the shim that would show any issues before the frame.

As to removing the OEM shim I would avoid that if possible as the removal process might cause some issues if done poorly. Shimming a seat tube to fit a post is not the most common method to size a frame to a post and I suspect most all steel frame guys don't have much experience with this. Andy
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Old 11-30-22, 01:37 PM
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One place I worked, our Ti frames had an alu shim that was machined to be a press fit, and it was glued with a green loctite (I forget which number but a strong one). Loctite can be loosened with heat, but the shim would still be ungodly difficult to remove due to the press fit, and not having much of a lower edge to hammer on (say, with a slide-hammer). Luckily we never had to remove one, they were 100% reliable during the few years I worked there. If I really had to remove one I'd melt it out, since the melting point of alu is not hot enough to hurt a Ti frame. Unless you're one of the idiots who paints Ti. Did I mention I hate painted Ti frames?

At that shop, the Ti was lightly reamed before the shim went in, just to remove the worst of the artifacts from welding, then the shim ID was reamed precisely, after the Loctite set, but before slitting. Shim ID started slightly small before reaming, so there was some extra meat to take up any imperfections in the Ti and still end up with a nice-fitting bore for the seatpost. Slitting was done last, with the slit then being lightly deburred. Posts always fit very well unless the post was too undersized... which is depressingly common. (Measure your post!)

At the place I worked at before that, I welded on a thicker piece of Ti at the top of the seat tube and reamed that to fit the post directly, no alu shim. I thought of that as the Gold Standard, and thought of the alu shim as a bodge, until I made some frames with the shim and realized it is an excellent way to go, from any practical perspective. And reaming Ti is really hard, so I was glad to never have to do that again! In fact the very word "shim" says bodge to me, so instead of shim I will henceforth call it an "interface", which sounds so much more purposeful.

Mark B
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Old 11-30-22, 01:48 PM
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Originally Posted by Andrew R Stewart View Post
I have serviced a number of Ti bike that had a shim insert in the seat tube but as these were not a problem never spent much time exploring in detail. However I have read of both a press fit and a bonded in fit being used by various builders. There might be some excess bonding agent (epoxy?) at the lower end of the shim that could be felt with a hooked probe. One could consider opening up (reaming?) the OEM shim and adding another custom made one inside the OEM one. Still a lot of work for a situation that we don't know is an actual riding issue (like seat post slippage). I have zero concerns about the long term reliability of this frame, WRT the seat tube and clamping area, as long as the post is not rocking about after the clamp is tightened there should be no increased wear on the frame. The shim will keep the post from contacting the actual tube ID. Now I suppose there could be some shim degradation if the post were to not stay tight but as the Al shim is softer and more prone to contact wear it would likely be the shim that would show any issues before the frame.

As to removing the OEM shim I would avoid that if possible as the removal process might cause some issues if done poorly. Shimming a seat tube to fit a post is not the most common method to size a frame to a post and I suspect most all steel frame guys don't have much experience with this. Andy
Thanks for the input. I'll probably just go with an additional internal shim until I can find someone with the proper tools and expertise to to a correct replacement.
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Old 11-30-22, 02:01 PM
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Originally Posted by bulgie View Post
One place I worked, our Ti frames had an alu shim that was machined to be a press fit, and it was glued with a green loctite (I forget which number but a strong one). Loctite can be loosened with heat, but the shim would still be ungodly difficult to remove due to the press fit, and not having much of a lower edge to hammer on (say, with a slide-hammer). Luckily we never had to remove one, they were 100% reliable during the few years I worked there. If I really had to remove one I'd melt it out, since the melting point of alu is not hot enough to hurt a Ti frame. Unless you're one of the idiots who paints Ti. Did I mention I hate painted Ti frames?

At that shop, the Ti was lightly reamed before the shim went in, just to remove the worst of the artifacts from welding, then the shim ID was reamed precisely, after the Loctite set, but before slitting. Shim ID started slightly small before reaming, so there was some extra meat to take up any imperfections in the Ti and still end up with a nice-fitting bore for the seatpost. Slitting was done last, with the slit then being lightly deburred. Posts always fit very well unless the post was too undersized... which is depressingly common. (Measure your post!)

At the place I worked at before that, I welded on a thicker piece of Ti at the top of the seat tube and reamed that to fit the post directly, no alu shim. I thought of that as the Gold Standard, and thought of the alu shim as a bodge, until I made some frames with the shim and realized it is an excellent way to go, from any practical perspective. And reaming Ti is really hard, so I was glad to never have to do that again! In fact the very word "shim" says bodge to me, so instead of shim I will henceforth call it an "interface", which sounds so much more purposeful.

Mark B
Thanks for the input. i was wondering how the press fit was done. Are these shims pre-made to a given titanium seat tube I.D. size or do you have to have a lathe (and know how to us it) to turn each shim individually? Are there manufacturers that make these shims stock? I looked at some frame builders supply websites and didn't see anything like these shims.

Yes, I think that it is a great way to do an interface.. provided that it is executed properly. Seat tube out of round, weld distortion at the top tube or seat stays, etc.
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Old 11-30-22, 08:39 PM
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Originally Posted by Steve_sr View Post
i was wondering how the press fit was done. Are these shims pre-made to a given titanium seat tube I.D. size or do you have to have a lathe (and know how to us it) to turn each shim individually?
We made our own, no idea if this is a part you can buy from a catalog. To be a press fit, they need to be made to fairly tight tolerance. Each diameter and wall thickness of seat tube requires a separate shim OD, so if anyone sells stock shims, they'd probably only stock the few most common sizes. My guess is, you need someone with a lathe.
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Old 12-01-22, 05:45 AM
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I've always been confused about how much clearance you should have for a seatpost. I don't think there is an industry standard. Some seatposts are undersized, and sometimes people want to use them on frames that have an oversized seat tube ID.
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Old 12-01-22, 09:10 AM
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Originally Posted by unterhausen View Post
I've always been confused about how much clearance you should have for a seatpost. I don't think there is an industry standard. Some seatposts are undersized, and sometimes people want to use them on frames that have an oversized seat tube ID.
This is my current dilemma. The seat tube is 0.5mm diameter larger than the nominal post. (31.6mm post in a 32.1mm hole) There are some (probably most) in the industry that try to make these as tight as possible. Is there a valid reason for that due to a history of frame failures or just paranoia? The only given is that if it is tight enough there will be NO flex and therefore NO possibility of fatigue failure.

I can also see a potential issue with retention and slippage as a large clearance will reduce the contact area between the post and the tube. Basically the only contact is at the top of the seat tube and not the whole width of the clamp.
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Old 12-01-22, 01:37 PM
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Until relatively recently in bicycle history timeline, nobody chose what seat tube bore to use. They got a tube and used it (hopefully after checking that a shop seatpost fit okay). Even today, I think most people either do that or buy a pre-made sleeve that they weld in. Generally at that point there is enough distortion it needs to be touched up with a reamer a little. You want a sliding fit, so not too tight. Everyone hates it when the seatpost takes effort to get in the frame. It seems a bit surprising that someone would ream larger than a sliding fit.
But when I look up sliding fit, it seems tighter than what is used by the bike industry.
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