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Oversized seat tube on titanium frame

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Oversized seat tube on titanium frame

Old 06-20-21, 12:11 PM
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Shinkers
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Oversized seat tube on titanium frame

Hello,

I've got an inexpensive titanium frame that is supposed to fit a 31.6 seatpost but really feels too big for that. When the post is inserted you can feel some movement and it will slide all of the way into the tube.

When I contacted the dealer about it, they said it should be fine as long as I'm using a good seat clamp. I'm using a Salsa lip-lock and haven't really had a problem with it. I did end up getting a carbon post I had laying around painted with an extra coat of paint and that made the post tighter in the frame.

I'm needing to switch to a 0 offset post though and I'm wondering if there's a better solution here. I was considering switching to a 27.2 post and using a 27.2-31.8 shim. I suppose another option would be to get another carbon post and paint it again as well, but that feels kind of like a band-aid.

I did try using a pop can shim with another post and it was too tight to get in the frame.

Here are some pictures of the clamping area with the painted carbon post.

I guess what has me wondering here is I think I can see some space between the seat tube and the post at the bottom of the slot but it's hard to say. I've heard of people cracking seat tubes by over clamping them and I'm hoping I haven't done anything by riding this the way that it is currently.






Edit: the residue below the clamp is from electrical tape btw.
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Old 06-20-21, 02:59 PM
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Originally Posted by Shinkers View Post
Hello,

I've got an inexpensive titanium frame that is supposed to fit a 31.6 seatpost but really feels too big for that. When the post is inserted you can feel some movement and it will slide all of the way into the tube.


Edit: the residue below the clamp is from electrical tape btw.
I'll bet a dollar this is the root of your problem.
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Old 06-20-21, 03:09 PM
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Originally Posted by cxwrench View Post
I'll bet a dollar this is the root of your problem.
Absolutely. For what I paid I expected some issues. Just wondering what the best way to deal with this would be.
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Old 06-20-21, 03:26 PM
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If you check on E-Bay, you'll find several 31.8 seatposts including a couple of zero-offset posts.

Measure your frame if you can.
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Old 06-20-21, 03:38 PM
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The best path is to test fit a 31.8 post before buying anything. Also posts are well known in general for A- being slightly undersized and 2- like seat tube IDs the post ODs drift from the labeled size.

There is what's known as shin stock. Sheets of various metal (brass is very common) in varying thicknesses ( .0015", .002", .003"...). I have used it to shim bars and posts before. Cut to fit. Andy
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Old 06-20-21, 08:05 PM
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As Andrew suggests, you could already have an undersized post.

31.8 may not be a standard post size. If you do choose to buy one off of E-Bay, you might ask the seller to double check the size before shipping.

It is possible to sand down an oversized post, but you will lose any factory finish that is on it.
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Old 06-20-21, 08:32 PM
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A slice of an aluminum Coke can may be a sufficient shim.
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Old 06-20-21, 08:46 PM
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Is there any potential for a crack to develop from having had a 31.6 post in there for the first 100 or so miles on the frame (this is an MTB)? I'm assuming that until a crack develops everything should be fine and as long as I get the appropriate post size in there now I'm okay, correct?
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Old 06-20-21, 10:04 PM
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Originally Posted by Shinkers View Post
Is there any potential for a crack to develop from having had a 31.6 post in there for the first 100 or so miles on the frame (this is an MTB)? I'm assuming that until a crack develops everything should be fine and as long as I get the appropriate post size in there now I'm okay, correct?
My opinion is that is a fair bet (your 2nd sentence).
Never had a Ti frame, but from what I've seen of other frame materials, it would take more than a .1 or .2 mismatch for a short amount of time like that to cause an issue with the top of your seattube. When frames/clamp ears get mangled there it's typically due to someone using a post with a really different diameter vs. the frame, and/or clamping the hell out of it without regard for proper torque range. Installing a post that works down the whole tube should rectify things.
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Old 06-20-21, 10:17 PM
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I think Titanium is a fairly springy metal, so you should be fine. But it does experience fatigue, and crack and break.

I'd encourage you to get the right post as soon as possible rather than thinking it may not damage the frame.

There are certain areas on bikes that people post damage photos, inevitably due to poorly fit parts.
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Old 06-20-21, 10:26 PM
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I am used to vintage steel bikes where the seat post is often tight enough that one has to twist and wiggle the seat/seatpost to get them out, sometimes leaving zig-zag scratches if there is a burr.

Many modern bikes have the posts loose enough that they can essentially just drop in. Perhaps a long enough post that they aren't putting a pressure point right at the clamp.

MTB riders often have a quick release allowing rapid adjustment of saddle height.

It is hard for us to tell for sure how much undersized you are.

Are your photos above with the clamp tight or loose? It really doesn't look too bad.

Remember that circumference = πD. So, if you increase the post diameter by 0.2mm, then that will change the clamp by about 0.63m.

That may not be bad, but your post may be pretty tight.
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