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Seatpost - Seat Tube Clearance - How much is too much?

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Seatpost - Seat Tube Clearance - How much is too much?

Old 11-28-22, 07:52 PM
  #76  
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Originally Posted by maddog34 View Post
Or.. head to a hardware store for a proper Shim material, cut and bend as needed, as i suggested a few dozen posts ago... sadly, people became too enthralled with protecting an overpriced seat post, and a titanium frame.... which was bought "new" then sat around for a long time for an undisclosed reason.

General message... have fun... go ream out Seat tube sleeves that are already oversized, as was suggested earlier.... i've got a Fox Float shock to clean thief paint off of... same color as my old Rally Car, actually, a lovely shade of blue.. i really miss that car.......later.
I am headed to the hardware store tomorrow but may end up at MSC or McMaste-Carr.
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Old 11-28-22, 08:13 PM
  #77  
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Originally Posted by Steve_sr View Post
I am headed to the hardware store tomorrow but may end up at MSC or McMaste-Carr.
If you've decided to accept 2nd rate, at least fix it right.

Measure carefully and find stainless steel shim stock of 1/2 the difference and 4x5 or 6" or so. Trim it to 3.75" and use a hammer to form it over a improvised mandrel ie. 1" water pipe to form a cylinder.

Cover the outside with something tacky like gasket compound and position it in the frame with the slot lined up. Allow some time for the adhesive to set, then fit your greased post.

Be careful to measure carefully and use the right thickness shim stock. The goal is to get as close to 31.6 as possible without going under. You also want the tube to be about 5-6" or so to ensure good support and prevent rocking.

BTW - you'll also want to seal the exposed ends to prevent water from wicking in and possibly causing corrosion.

Last edited by FBinNY; 11-28-22 at 08:19 PM.
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Old 11-30-22, 10:18 AM
  #78  
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Originally Posted by FBinNY View Post
If you've decided to accept 2nd rate, at least fix it right.
I don't really have a choice. The builder doesn't consider it a problem and even if they did they have demonstrated that they either don't have the correct tooling and/or expertise to do this properly.

Originally Posted by FBinNY View Post
Measure carefully and find stainless steel shim stock of 1/2 the difference and 4x5 or 6" or so. Trim it to 3.75" and use a hammer to form it over a improvised mandrel ie. 1" water pipe to form a cylinder.

Cover the outside with something tacky like gasket compound and position it in the frame with the slot lined up. Allow some time for the adhesive to set, then fit your greased post.

Be careful to measure carefully and use the right thickness shim stock. The goal is to get as close to 31.6 as possible without going under. You also want the tube to be about 5-6" or so to ensure good support and prevent rocking.
I was thinking about using brass shim stock. It is more malleable and still corrosion resistant. Before I order the shim stock I was thinking about trying several layers of Coke can to make sure that the hole is not tapered.

Originally Posted by FBinNY View Post
BTW - you'll also want to seal the exposed ends to prevent water from wicking in and possibly causing corrosion.
I think that the gap in the shim is going to be insignificant to the keyhole in the seat tube. On my other Ti frame I cover this with a small piece of electrical tape.
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Old 11-30-22, 01:45 PM
  #79  
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Here is an interesting data point:

https://www.ticycles.com/news/servic...-fat-beat-yoke

Moots apparently had a MTB design that used the Ti chain stays as part of the suspension. Apparently they weren't concerned about titanium fatigue or perhaps they use a special Ti allow that was more fatigue resistant?
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Old 11-30-22, 03:07 PM
  #80  
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Originally Posted by Steve_sr View Post
Here is an interesting data point:

https://www.ticycles.com/news/servic...-fat-beat-yoke

Moots apparently had a MTB design that used the Ti chain stays as part of the suspension. Apparently they weren't concerned about titanium fatigue or perhaps they use a special Ti allow that was more fatigue resistant?
Metal fatigue is a complicated process. It occurs after repeated stress up to near the V yield limit. Note that metals vave various allowable distortion within limits, and repeated stresses well within the working range are OK, and when I consider about risk of fatigue, it's because of concern about uncontrolled stresses near the limit.

A cantilevered seat tube extension is not strong enough on its own, and relies on a rigidly constrained seat post to take a share of the load. This is why bikes of this design require that posts are inserted to 2-3 inches below the top tube.

Also note that the constant umrelieved stress of the distortion involved in clamping to an illfitted post acts a a dead load (imagine parking cement trucks on a bridge) on top of which the normal riding loads would be added.

I'm not saying this frame is doomed to premature failure. What I'm saying is that the OP address the issues and implements a fix consistent with basic design principles.

He has a number of viable approaches, and IMO should decide on one of integrity consistent with what he spent serious dough to buy.
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Old 11-30-22, 05:51 PM
  #81  
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I agree with the idea of some stainless shim. But while it would be a great idea to form it around a mandrel of some manner that is a bit smaller than the seat post I doubt it'll need a hammer. We're only talking about a few thousands of springy material. Just hand form it around a roughly 1" round item and it'll take enough of a curl to do just fine.

What you can also do is something I've done in the past. I put the shim around the seat tube and put the two into place. But I leave about 0.04 to 0.05 inch (1 to 1.4mm) sticking up past the edge of the seat tube. Then with a scriber or a chisel shaped too. I catch the little edge and form it otwards. Then with a hardwood stick or similar and a hammer I tap it down to form a narrow flange that prevents the shim diving deeper into the seat tube. It's a bit fussy to do this of course. But it's not all that hard once you get the trick and work in the stretch it needs in small amounts with a soft non marring chisel shaped tool.
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Old 11-30-22, 05:55 PM
  #82  
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Originally Posted by Steve_sr View Post
Here is an interesting data point:

https://www.ticycles.com/news/servic...-fat-beat-yoke

Moots apparently had a MTB design that used the Ti chain stays as part of the suspension. Apparently they weren't concerned about titanium fatigue or perhaps they use a special Ti allow that was more fatigue resistant?
There's almost as many different alloys of titanium as there are of aluminum. I've even seen titanium alloy springs in some applications. So I suspect FB hit the nail on the head when he suggested that Moots picked an alloy for those stays that will stand the test of time. The industry has, I hope , learned a thing or two about the selection of Ti alloys for use in cycle frames since the early mistakes that still make us doubt.
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Old 12-05-22, 07:05 PM
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I did some testing today with some half inch wide .003" pieces of an aluminum Coke can. As I suspected the bore is tapered. Smaller at the bottom than the top. Bottom looks to be about .014" while top is .020" dia. gap.

So it looks like I will need to find some .007" shim stock. The other question is what material should I use. I was thinking about either stainless or brass. As suggested I will be stretching (rounding) over the top to keep the shim from sliding down the bore. Is stainless malleable enough for this without cracking or should I use brass instead?

The local hardware store was zero help. I have found some shim material online but it is much larger quantity than I really need at significant cost. Only need a 4" x 4" piece. Any suggestions on other places to look for shim stock?
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Old 12-05-22, 08:19 PM
  #84  
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I'm still irritated that your frame builder screwed you with a defective product.

But if you insist on accepting mediocrity, we all deserve to know who this shoddy, substandard huckster is so that we can avoid him in the future.

To answer your question: Any well stocked hobby store that deals in model trains, dioramas, figurine casting, RC cars/planes, rock polishers, & the like should have a display rack with brass sheets & rod stock of various dimensions. Look near the balsa wood building supply area.
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Old 12-05-22, 11:32 PM
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Originally Posted by FBinNY View Post
Metal fatigue is a complicated process. It occurs after repeated stress up to near the V yield limit. Note that metals vave various allowable distortion within limits, and repeated stresses well within the working range are OK, and when I consider about risk of fatigue, it's because of concern about uncontrolled stresses near the limit.

A cantilevered seat tube extension is not strong enough on its own, and relies on a rigidly constrained seat post to take a share of the load. This is why bikes of this design require that posts are inserted to 2-3 inches below the top tube.

Also note that the constant umrelieved stress of the distortion involved in clamping to an illfitted post acts a a dead load (imagine parking cement trucks on a bridge) on top of which the normal riding loads would be added.

I'm not saying this frame is doomed to premature failure. What I'm saying is that the OP address the issues and implements a fix consistent with basic design principles.

He has a number of viable approaches, and IMO should decide on one of integrity consistent with what he spent serious dough to buy.
.... bought a used Independent Fabrication steel frame for like 25 bucks 4 or 5 years ago, with exactly this sort of failure beginning in the seat tube. Two hairline stress cracks running down in a gentle V, one from each lower corner of the seat tube expansion slot, maybe 1/2". I thought, worst case, I could use the fork for some other frame. It was one of those very light bikes that IF was famous for, but someone had run it with a seat post one size too small was what I surmised. Certainly the clamping ears and slot were distorted.

Before I could invest too much time and ingenuity in trying to salvage it, I discovered it also had drillium dropouts, and one of them was busted, too. The fork works nicely on another frame, though.
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Old 12-05-22, 11:40 PM
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Originally Posted by Steve_sr View Post
Hello,

This is with a new titanium frame with (I am assuming) an aluminum insert that was supposed to be honed to match the seatpost. Well, the builder (I think) got a bit overzealous with the hone. I have another titanium bike, a Litespeed, where the seatpost is tight enough to be held by friction which is sort of what I was expecting for this new frame.

Your thoughts?

Thanks,
Steve
...looking for shim stock would not be my response, in the case of a new, custom frame. I would get it fixed by the guy who built it. I was considering brazing in a reinforcing insert sleeve in my IF frame's seat tube, before I discovered the additional issue of the dropouts being drilled, with one already busted. But that was because it only cost me 25 bucks, and the frame had been abused.
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Old 12-06-22, 02:23 PM
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Originally Posted by Steve_sr View Post
I did some testing today with some half inch wide .003" pieces of an aluminum Coke can. As I suspected the bore is tapered. Smaller at the bottom than the top. Bottom looks to be about .014" while top is .020" dia. gap.

So it looks like I will need to find some .007" shim stock. The other question is what material should I use. I was thinking about either stainless or brass. As suggested I will be stretching (rounding) over the top to keep the shim from sliding down the bore. Is stainless malleable enough for this without cracking or should I use brass instead?

The local hardware store was zero help. I have found some shim material online but it is much larger quantity than I really need at significant cost. Only need a 4" x 4" piece. Any suggestions on other places to look for shim stock?
My not so local hobby shop sells both brass and aluminium sheets in various thicknesses. Those are about 4" by 8" or so. I'd check a hobby shop as suggested elsewhere.

Cheers
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Old 12-06-22, 02:58 PM
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Originally Posted by 3alarmer View Post
...looking for shim stock would not be my response, in the case of a new, custom frame. I would get it fixed by the guy who built it.
They have already proven that they couldn't do it right the first time. What makes you think that they could do it correctly the second time? Isn't that one of the definitions of insanity?
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Old 12-06-22, 03:05 PM
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Originally Posted by Miele Man View Post
My not so local hobby shop sells both brass and aluminium sheets in various thicknesses. Those are about 4" by 8" or so. I'd check a hobby shop as suggested elsewhere.

Cheers
I have checked the (few) local hobby stores and they sell K & S Precision Metals products and K & S doesn't offer anything in .007" thickness.

I did however have some luck finding some Maudlin .007" 302/304 stainless shim stock online at Zoro via Ebay.
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Old 12-09-22, 01:24 PM
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Originally Posted by Steve_sr View Post
I did some testing today with some half inch wide .003" pieces of an aluminum Coke can. As I suspected the bore is tapered. Smaller at the bottom than the top. Bottom looks to be about .014" while top is .020" dia. gap.

So it looks like I will need to find some .007" shim stock. The other question is what material should I use. I was thinking about either stainless or brass. As suggested I will be stretching (rounding) over the top to keep the shim from sliding down the bore. Is stainless malleable enough for this without cracking or should I use brass instead?

The local hardware store was zero help. I have found some shim material online but it is much larger quantity than I really need at significant cost. Only need a 4" x 4" piece. Any suggestions on other places to look for shim stock?
Originally Posted by 3alarmer View Post
...looking for shim stock would not be my response, in the case of a new, custom frame. I would get it fixed by the guy who built it. I was considering brazing in a reinforcing insert sleeve in my IF frame's seat tube, before I discovered the additional issue of the dropouts being drilled, with one already busted. But that was because it only cost me 25 bucks, and the frame had been abused.
Originally Posted by Steve_sr View Post
They have already proven that they couldn't do it right the first time. What makes you think that they could do it correctly the second time? Isn't that one of the definitions of insanity?
Originally Posted by Steve_sr View Post
I have checked the (few) local hobby stores and they sell K & S Precision Metals products and K & S doesn't offer anything in .007" thickness.

I did however have some luck finding some Maudlin .007" 302/304 stainless shim stock online at Zoro via Ebay.
...just as a clarification, the idea of shimming a seat post to fit the tube is not unheard of. I do not know how you are measuring this (it's a difficult thing to measure accurately, down inside the seat tube). But presuming you are correct, shimming as a repair sets you up for a lifetime of problems with this frame. I don't see how you're going to shim it at various spots with different supporting thicknesses, up and down the length of the post, or conversely, inside the seat tube.

If I had given up on the builder to make it right (which it seems you have), personally, I would either hire someone local to replace and re-ream the aluminum insert to standard, or at the very least, ream it out myself to the oversized measurement you have at the top, so it is at least a cylindrical fit for the length of the seat post. Then I would probably just find an oversized seat post to fit that new seat tube dimension as a nice sliding fit, or barring that, find one that is just a little oversized and carefully reduce the outside diameter of that new post by abrasion turning.

Maybe I am missing something in your overall plan here, which is certainly possible. But adjustable reamers are available cheap enough that even I have a few of them, in various ranges. Reaming a seat tube (or in this case your aluminum insert in yours), is not beyond the abilities of the average home mechanic, if you can't find someone local. You do have to be very careful in setting the diameter, and you need to test ream something else, and measure the result, before you do your frame. But once set, the adjustable ones seem to work well enough for occassional (or eve one time) use.

I only add this as a postscript because I think you might be setting yourself up for bigger problems down the road. Obviously, I'm not standing in the same room with your frame. But the whole idea here is to get a measure of support for the entire length of the post in the frame, as the ideal solution, with the clamp at the top serving to keep the position fixed. This is a difficult thing to achieve with shims. Good luck, however you decide to proceed. Maybe you are way ahead of me.
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Old 12-10-22, 03:34 PM
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Originally Posted by 3alarmer View Post
...just as a clarification, the idea of shimming a seat post to fit the tube is not unheard of. I do not know how you are measuring this (it's a difficult thing to measure accurately, down inside the seat tube). But presuming you are correct, shimming as a repair sets you up for a lifetime of problems with this frame. I don't see how you're going to shim it at various spots with different supporting thicknesses, up and down the length of the post, or conversely, inside the seat tube.

If I had given up on the builder to make it right (which it seems you have), personally, I would either hire someone local to replace and re-ream the aluminum insert to standard, or at the very least, ream it out myself to the oversized measurement you have at the top, so it is at least a cylindrical fit for the length of the seat post. Then I would probably just find an oversized seat post to fit that new seat tube dimension as a nice sliding fit, or barring that, find one that is just a little oversized and carefully reduce the outside diameter of that new post by abrasion turning.

Maybe I am missing something in your overall plan here, which is certainly possible. But adjustable reamers are available cheap enough that even I have a few of them, in various ranges. Reaming a seat tube (or in this case your aluminum insert in yours), is not beyond the abilities of the average home mechanic, if you can't find someone local. You do have to be very careful in setting the diameter, and you need to test ream something else, and measure the result, before you do your frame. But once set, the adjustable ones seem to work well enough for occassional (or eve one time) use.

I only add this as a postscript because I think you might be setting yourself up for bigger problems down the road. Obviously, I'm not standing in the same room with your frame. But the whole idea here is to get a measure of support for the entire length of the post in the frame, as the ideal solution, with the clamp at the top serving to keep the position fixed. This is a difficult thing to achieve with shims. Good luck, however you decide to proceed. Maybe you are way ahead of me.
There are no local frame builders in this area. I located the one who closed up shop during the pandemic and he moved back to the left coast somewhere.

Someone can correct this if my thinking is backwards but my objective here is to suitably constrain the seat post so that it does not rock in the bore and eventually fatigue the frame near the clamp. So the added shim will take up the slack at the bottom of the existing shim while the clamp will take up the slack at the top of the frame and prevent it from moving. I am not sure that support (or contact to the shim/seat tube) between these two points is relevant.

The bottom of the bore was measured by how many .003" x 0.5" Coke can shims that I could fit down the bore with the seat post. Crude but effective measurement.
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Old 12-10-22, 03:50 PM
  #92  
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Originally Posted by Steve_sr View Post
Someone can correct this if my thinking is backwards
.....
Since you ask -------
Yes. It's backwards. From the beginning you were given solid advice on achieving a first class solution, either by going to a pro, or DIY.

Yet, you prefer to make excuses over doing it right.

So, last suggestion.

Any proper solution starts by establishing a truly cylindrical bore. Then shimming full depth, or at least 5-6 inches, to match the seatpost.

Any skilled shop mechanic can do this, or you can DIY at low cost, including buying the required tool. I, and others have given you the "how to", but it's up to you to care enough to do it right.

I'm out.
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Old 12-10-22, 04:45 PM
  #93  
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Originally Posted by Steve_sr View Post
There are no local frame builders in this area. I located the one who closed up shop during the pandemic and he moved back to the left coast somewhere.

Someone can correct this if my thinking is backwards but my objective here is to suitably constrain the seat post so that it does not rock in the bore and eventually fatigue the frame near the clamp. So the added shim will take up the slack at the bottom of the existing shim while the clamp will take up the slack at the top of the frame and prevent it from moving. I am not sure that support (or contact to the shim/seat tube) between these two points is relevant.

The bottom of the bore was measured by how many .003" x 0.5" Coke can shims that I could fit down the bore with the seat post. Crude but effective measurement.
I would question that this is an effective measure.

I would get ID calipers https://www.amazon.com/iGaging-Insid...711357&sr=8-11

or a bore tool

https://www.amazon.com/Beslands-Tele...s%2C164&sr=8-2

the idea that the bore is tapered is really odd to me.

Did you ever ride the bike with the loose seatpost and maybe increase the size at the top? or maybe the builder did?

here is a video about moots and it talks about seat post insert at around minute 14
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Old 12-10-22, 05:15 PM
  #94  
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Originally Posted by Steve_sr View Post
There are no local frame builders in this area. I located the one who closed up shop during the pandemic and he moved back to the left coast somewhere.
...once more, you don't really need a frame builder. Any machinist, in any machine shop, will instantly understand the required operation. But since you remain unconvinced, here is a cheap adjustable reamer with handle (handle costs extra), in use in my back yard.



In your case, the oversized measurement at the top of your seat tube means that the standard measurement for a 31.6 post ship has sailed. That horse has left the barn. But it ought to be possible to re-bore the whole thing down about 6 or 8 inches, to something approaching a cylindrical bore at some new, oversized dimension, and fit a new, oversized post to that. This depends on how much thickness of aluminum liner you have left to work with. Obviously, reaming all the way through it into the thin titanium tube walls is a bad idea. But presumably it was a single thickness for it's entire length originally. So presumably if the builder managed to ream it to a certain inner diameter at the top, there's enough aluminum to do that.

Originally Posted by Steve_sr View Post
Someone can correct this if my thinking is backwards but my objective here is to suitably constrain the seat post so that it does not rock in the bore and eventually fatigue the frame near the clamp. So the added shim will take up the slack at the bottom of the existing shim while the clamp will take up the slack at the top of the frame and prevent it from moving. I am not sure that support (or contact to the shim/seat tube) between these two points is relevant.
...this is a misunderstanding of the forces in play. They have been explained above, earlier in the thread. What will happen, should you approach it in this manner, is that you will gradually see one or two stress cracks that appear and begin to propagate downward, from the bottom of the slot, cut in the seat tube. It's not the rocking, it's the dead load of your body weight, pushing down on that seat post. Shims down lower in the seat tube do nothing to relieve that. You're not just trying to establish two points of support.

Originally Posted by Steve_sr View Post
The bottom of the bore was measured by how many .003" x 0.5" Coke can shims that I could fit down the bore with the seat post. Crude but effective measurement.
...it's certainly crude, but I would not trust it as effective. It's very, very difficult to ream something that is off in the way you describe with either a standard, or an adjustable reamer. They really are cylindrical in nature. Thus the results they yield are, most times, cylindrical. nyway, once more, good luck with it. I'm sure it's been a less than pleasant experience.
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Old 12-10-22, 06:51 PM
  #95  
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Originally Posted by 3alarmer View Post
...once more, you don't really need a frame builder. Any machinist, in any machine shop, will instantly understand the required operation......
It's not even that complicated, and there's no need to search for a new OS seatpost.

ANY decent mechanic in ANY decent bike shop can do this correctly in under 1 hour.

Refer to my post #77 for a quick outline. If I knew where you were, I'd give you a name. However, you can visit your local shop, show them the outline, and they can reach out to me here with any questions
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Old 12-11-22, 02:35 PM
  #96  
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re-reading this whole thread
1) still think if it came this way from the builder it is the builders issue
2) something is very off
3) lot's of assumptions are being made without accurate measurements
4) the idea that the seat tube is tapered is curious if accurae

IMHO nothing should be done until detail ID measurement are made the full seatpost insertion length, using real measurement tools this will tell a better picture of what is reality is

once that is known then a solution can be determined

1) go back to builder with facts and see what response is

2) get a machinist to build a seatpost shim to fit size and go for smaller post size like 27.2 using the shim (something like the problem solvers) https://www.universalcycles.com/shop...ils.php?id=914
2.1) ream to a consistent size if needed but from pics does not appear to be a log of material in the current insert

3) try to shim with sheet material

I personally think number 2 would be best assuming as stated builder is doing nothing
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Old 12-11-22, 03:20 PM
  #97  
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Originally Posted by squirtdad View Post
re-reading this whole thread
1) still think if it came this way from the builder it is the builders issue
2) something is very off
3) lot's of assumptions are being made without accurate measurements
4) the idea that the seat tube is tapered is curious if accurae

IMHO nothing should be done until detail ID measurement are made the full seatpost insertion length, using real measurement tools this will tell a better picture of what is reality is

once that is known then a solution can be determined

1) go back to builder with facts and see what response is

2) get a machinist to build a seatpost shim to fit size and go for smaller post size like 27.2 using the shim (something like the problem solvers).....
WHY spend serious dough for a machinist for a kludge solution?

If going to a machinist, then ream out the original liner down to the titanium, and fabricate a new liner to the original design specs.

However the approach I outlined a few posts above is almost as good, and can be done in a bike shop, or DIY, at low cost.
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Old 12-14-22, 11:01 AM
  #98  
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Thanks for the continued discussion. However, I am sort of stuck lacking available resources and expertise.

1. Finding an oversize seat post is pretty much a non-starter as they aren't available in the size that would be needed to compensate for the oversize bore.

2. The existing aluminum shim is only 4" long so is a long way from supporting the length of the post. I might consider replacing the existing shim and starting over... if I could find one. I suspect the builder knows of a source for these but I don't. Having a machinist make one from scratch would likely run into hundreds of dollars.

3. I could buy and figure out how to use a reamer but it is currently pointless because I don't have a replacement shim.

4. All of the local bike shops are pretty much parts changers on big name mass market bikes. They don't deal with customs or frame specific issues. They just replace frames as needed. They don't have the expertise and proper tooling to replace a seat post shim.

5. As for the tapered bore I can pretty much guarantee that whomever reamed the shim didn't know what they were doing and likely added too much side force on the handle when reaming. The builder said that they don't cut the slot in the shim until after reaming and that this could cause the bore to open up slightly when the slot is cut. I am afraid that I don't buy that story. Assuming that the aluminum shim is press fit into the tube I would expect the bore to close up rather than open up. It also tends to show that the builder used a straight flute reamer on the shim. BTW, The bike has not yet been ridden so that couldn't have caused any of the excess play.

6. As for finding a local machinist my part of the country is not known for manufacturing. What little manufacturing that was here has mostly disappeared over the past 3 decades.

7. In case someone might know of resources that I don't I live in central NC near Raleigh.

Thanks,
Steve
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Old 12-14-22, 06:49 PM
  #99  
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Originally Posted by Steve_sr View Post
7. In case someone might know of resources that I don't I live in central NC near Raleigh.
Might be worth contacting these guys and seeing if they'll take a look at it. They seem to be about a 45 min drive from downtown Raleigh, on the west side of Chapel Hill.

https://kishbike.com/
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Old 12-14-22, 11:07 PM
  #100  
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I still can't see why the frame builder won't make you happy!

Oh, and you might get some good ideas from the Framebuilders forum I'd give them a try.
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