Which diameter seat post? (or, to shim or not to shim) (or Alu vs Ti post)- Mtbr.com
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  1. #1
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    Which diameter seat post? (or, to shim or not to shim) (or Alu vs Ti post)

    I'm considering an Eriksen post for my Hifi 29r. My Hifi takes a 31.6mm post. Eriksen comes in 27.2 or 30.9. Either way I'll have to shim it. I'm leaning towards 27.2 to make it the most compatible with other bikes should I want to move it in the future. Any reason I should go with the 30.9?
    Last edited by hellomcfly; 12-26-2010 at 07:16 AM.

  2. #2
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    I know you don't want to hear that shims are a bad idea period. Without proper fit you run risk of cracking the seat tube. It puts stress in area where it was not designed for and shouldn't be. Also in my opinion you wouldn't get the benefit of a Ti post on a full suspension. If you want to save weight I'd go with carbon.

  3. #3
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    more bikes are 30.9

  4. #4
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    Quote Originally Posted by bandersen7
    Also in my opinion you wouldn't get the benefit of a Ti post on a full suspension.
    This was my first thought after reading the OP's post.

  5. #5
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    Quote Originally Posted by bandersen7
    I know you don't want to hear that shims are a bad idea period. Without proper fit you run risk of cracking the seat tube. It puts stress in area where it was not designed for and shouldn't be.
    I've done a bunch of searching before asking this question. I found plenty of cases of people running shims with no problems. The worst issue seems to be that the shim will be difficult to remove after some period of time. If you have links to examples of people breaking seat tubes with shims, I'd be interested in reading about it.

    Quote Originally Posted by bandersen7
    Also in my opinion you wouldn't get the benefit of a Ti post on a full suspension. If you want to save weight I'd go with carbon.
    My main reasons for going with Ti are it's longevity, and it's failure mode properties (bends instead of snaps). Seems I'd get these benefits on a full suspension.


    Quote Originally Posted by Endothermic Cavewalker
    more bikes are 30.9
    Uh, yeah. But even more bikes are 30.9 and smaller.

  6. #6
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    Quote Originally Posted by hellomcfly
    Any reason I should go with the 30.9?
    Not really. Especially considering you are buying a $200 post. It would nice to be able to use it on any bike.

  7. #7
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    I cracked a steel frame... with a shim... and they wouldn't warranty.... in opinion longevity of a seat post is not worth the shortened life of my frame.

  8. #8
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    Quote Originally Posted by bandersen7
    I cracked a steel frame... with a shim... and they wouldn't warranty.... in opinion longevity of a seat post is not worth the shortened life of my frame.
    Why didn't you swap the post before service?

  9. #9
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    Quote Originally Posted by hellomcfly
    Why didn't you swap the post before service?

    Took the bike in for something else.... they noticed two cracks where the seat stays meet the seat tube...

  10. #10
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    Use a post that fits properly.

  11. #11
    craigsj
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    You might want to look for 30.9 -> 31.6 shims before deciding.

  12. #12
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    Quote Originally Posted by bandersen7
    Took the bike in for something else.... they noticed two cracks where the seat stays meet the seat tube...
    That sucks, maybe I'll reconsider. Anyone think it not a bad idea to shim?

    Quote Originally Posted by craigsj
    You might want to look for 30.9 -> 31.6 shims before deciding.
    Eriksen will supply the appropriate shim.

  13. #13
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    I ran a 27.2 Eriksen post on a Niner with a 31.6 hole and never had an issue, this was on an Air9. With maintenance, proper install and proper post length I don't see an issue.

  14. #14
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    I wanted an Erikson Sewatpost when I built my Ti hard tail, but the seat tube was 31.6. I decided to just go with the linskey post, because I didn't want to shim.

  15. #15
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    [QUOTE=hellomcfly]That sucks, maybe I'll reconsider. Anyone think it not a bad idea to shim?



    Eriksen will supply the appropriate shim.[/QUOTE

    I wanted to share my misfortune with you so I didn't happen to you. If Eriksen supplies a full length shim I wouldn't see a problem with it... just wouldn't recommend... but it would be terrible to find out the wrong way...

    Happy Holidays
    And good luck with your decision

  16. #16
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    Quote Originally Posted by [email protected]
    I wanted an Erikson Sewatpost when I built my Ti hard tail, but the seat tube was 31.6. I decided to just go with the linskey post, because I didn't want to shim.
    I was considering the Lynskey post but looking at the pics on their site, the clamp hardware looks kinda kludgey. Does it look any better in real life?

  17. #17
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    Quote Originally Posted by hellomcfly
    I was considering the Lynskey post but looking at the pics on their site, the clamp hardware looks kinda kludgey. Does it look any better in real life?
    No, it doesn't compare to the Sweatpost, but it works, and I still prefer it to a shim.

  18. #18
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    Quote Originally Posted by hellomcfly
    That sucks, maybe I'll reconsider. Anyone think it not a bad idea to shim?



    Eriksen will supply the appropriate shim.
    Any shim used should be long enough to provide full support. Many are too short. The length required is the same as the minimum seat post insertion for the frame/post. Longer is better. Many frames use internally butted set tubes which support the post only to the end of the butt. If you use a shim, it should also reach to, or past the butt end.
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  19. #19
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    Quote Originally Posted by hellomcfly
    That sucks, maybe I'll reconsider. Anyone think it not a bad idea to shim?
    dont ask me why but i can tell you from experience its a bad idea
    Quote Originally Posted by craftworks750
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  20. #20
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    Quote Originally Posted by hellomcfly
    My main reasons for going with Ti are it's longevity, and it's failure mode properties (bends instead of snaps). Seems I'd get these benefits on a full suspension.
    Do you go through seat posts at an alarming rate? As a 250lb rider who has never even bent a post, despite my 'more exposed seatpost than most' needs, I'm baffled by a $200+ seat post. If you say you 'want' it rather than 'need' it, I understand that more!

  21. #21
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    Quote Originally Posted by TooTallUK
    Do you go through seat posts at an alarming rate? As a 250lb rider who has never even bent a post, despite my 'more exposed seatpost than most' needs, I'm baffled by a $200+ seat post. If you say you 'want' it rather than 'need' it, I understand that more!
    My understanding is that if you're riding an AL or carbon post you never will experience a bent post, but you might experience a snapped one.

    I guess I'm paranoid because I'm on my 3rd aluminum frame in 3 years, and both frame failures were clean breaks that happened with no warning, and no bending. A similar handlebar or seat post failure is something I'd really like to avoid. $200 is actually less than the cost of 2 high end al or carbon posts, and I'd expect the ti post to last a lifetime.

  22. #22
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    Quote Originally Posted by GTR-33
    Use a post that fits properly.

    This is about ALL you need, right here.
    ( but I will pontificate anyway)

    SHIMS are a useful do-dad when setting up a $150 bike for your mom to take of the paved paths. If a mountain bike is actually going to be ridden off road a SHIM is a horrible idea... the same way you wouldn't run a seat post with it's MINIMUM INSERTION mark an inch above collar, having a post LEVERING around inside your fame is an expensive, avoidable problem waiting to happen.
    As for knowing people who've used shims with no problems........ you likely also know people who have driven drunk or without a seatbelt or even ran with sissors ( maybe even on the pool deck) but thats not the way to understand if something is a good idea

  23. #23
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    31.6 Thomson seatpost and Foooget 'bout it......I'm just saying....

  24. #24
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    Quote Originally Posted by bandersen7
    I wanted to share my misfortune with you so I didn't happen to you. If Eriksen supplies a full length shim I wouldn't see a problem with it... just wouldn't recommend... but it would be terrible to find out the wrong way...
    Most seattubes are butted, some with the butting starting fairly close to the seatpost clamp. Any seatpost length, or shim length, beyond the point where the butting effectively removes any contact between the frame and the seatpost is wasted.

    FWIW, on practically every bike I've owned over the last 25 years, once I get my saddle height set, I mark (or tape) the post, pull it out, and cut any excess off, effectively putting my seatpost at the new, improved 'max height' limit. I've never had a problem with either a frame or a seatpost.

  25. #25
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    Quote Originally Posted by Jeromelo
    31.6 Thomson seatpost and Foooget 'bout it......I'm just saying....
    Those look like nice posts, but I'm still leaning towards titanium. Maybe i'm over simplifying it, but my understanding is that aluminum has a finite fatigue life, ti doesn't. Aluminum WILL break eventually, it's just a matter of when. Also, aluminum won't bend when it fails, it will snap. Titanium will bend, reducing the chance of a catastrophic crash if failure does occur. If this is incorrect I'm open to be corrected!

    By the way, I've been convinced to avoid a shim. It sucks that there's such a limited selection of 31.6 ti posts.

  26. #26
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    Quote Originally Posted by hellomcfly
    Those look like nice posts, but I'm still leaning towards titanium. Maybe i'm over simplifying it, but my understanding is that aluminum has a finite fatigue life, ti doesn't. Aluminum WILL break eventually, it's just a matter of when. Also, aluminum won't bend when it fails, it will snap. Titanium will bend, reducing the chance of a catastrophic crash if failure does occur. If this is incorrect I'm open to be corrected!

    By the way, I've been convinced to avoid a shim. It sucks that there's such a limited selection of 31.6 ti posts.

    Some hate on here against shims. You'll find plenty of posts (with a search) on MTBR to the contrary. My Cane Creek shim goes way beyond the safe point on my current frame. I don't even think about it. As long as the shim goes as far into the frame as the seatpost should the forces exerted on the frame are no different.

    Getting the shim out? As long as it's greased when you put it in you should be fine (and pulled out and re-greased occasionally like you would do with a regular post). It is as likely to get stuck as a seatpost. There is less to grab on to, but the shim should be slotted so you can grab it with some channel locks and compress it slightly as you pull it out.

    That said, I would buy a 31.6 Thomson and never look back... especially on a full suspension bike. I'll give you odds that the Thomson post will outlast your frame.
    Last edited by teamdicky; 12-26-2010 at 07:41 AM.
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  27. #27
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    Quote Originally Posted by teamdicky

    That said, I would buy a 31.6 Thomson and never look back... especially on a full suspension bike. I'll give you odds that the Thomson post will outlast your frame.
    Like I said above, aluminum will break eventually. If not on my current frame, then on a frame after that. A ti post will last a lifetime, I'd move it to future frames without worrying about fatigue life. And the failure properties of ti (bend vs snap) are much more desirable. If you can refute this I'm open minded.

    By the way, so far I've found 2 options for 31.6 ti posts, a Lynskey and a PMP. Comments on these would be welcome.

  28. #28
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    Quote Originally Posted by hellomcfly
    Like I said above, aluminum will break eventually. If not on my current frame, then on a frame after that. A ti post will last a lifetime, I'd move it to future frames without worrying about fatigue life. And the failure properties of ti (bend vs snap) are much more desirable. If you can refute this I'm open minded.

    By the way, so far I've found 2 options for 31.6 ti posts, a Lynskey and a PMP. Comments on these would be welcome.

    How 'bout that? Didn't know about any ti 31.6 posts.

    There's some aluminum/carbon in the clamp mech on the Lynskey..

    Shaft: Aerospace quality 3al-2.5v titanium

    Tension rod/upper saddle rail clamps, internal and external sleeves: 6061 anodized aluminum

    Lower saddle rail cradle: Carbon fiber composite

    Tension bolts: Aerospace quality 6al-4v titanium


    I'm not saying that it's gonna break. Just saying it's not all ti.

    Same goes for the PMP with it's bonded aluminum head.

    I would do some searches on Thomson before you discount them.

    I'll admit I run a ti post on my hardtail (with a shim).

    Can't refute the "the failure properties of ti (bend vs snap)" thing. Not in my realm...
    Last edited by teamdicky; 12-26-2010 at 08:32 AM.
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  29. #29
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    Quote Originally Posted by hellomcfly
    Like I said above, aluminum will break eventually. If not on my current frame, then on a frame after that. A ti post will last a lifetime, I'd move it to future frames without worrying about fatigue life. And the failure properties of ti (bend vs snap) are much more desirable. If you can refute this I'm open minded.

    By the way, so far I've found 2 options for 31.6 ti posts, a Lynskey and a PMP. Comments on these would be welcome.
    I have seen several bent Ti posts, and at least one broken one just among local riding group. Ti is not indestrustable.
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  30. #30
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    Quote Originally Posted by shiggy
    I have seen several bent Ti posts, and at least one broken one just among local riding group. Ti is not indestrustable.

    MTBR blasphemy and hearsay.

    Go pick on some carbon fiber your own size.
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  31. #31
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    Quote Originally Posted by teamdicky
    There's some aluminum/carbon in the clamp mech on the Lynskey..
    ...
    Good point. I'm probably being over-sensitive because of my recent frame break, and should just stick with a decent alu post.

  32. #32
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    Quote Originally Posted by hellomcfly
    Good point. I'm probably being over-sensitive because of my recent frame break, and should just stick with a decent alu post.
    Yeah, now you got it. Ti seatposts are nice to smooth out the ride of a hardtail or to get all matchy on a ti frame, but don't buy one because you think you will break your alu post.

    I have ti posts on my ti frames and have shimmed one of the same ti posts to fit a prior steel frame.

    I have broken a different ti post that was the proper size for the steel frame it was fitted to. FWIW - I have never broken an alu post.

    For your FS bike, get the proper size Thomson and forget about it. They are great posts. Since you are saving all that money, get a matching Thomson stem too.

    Have fun!

    LP

  33. #33
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    Quote Originally Posted by hellomcfly
    Like I said above, aluminum will break eventually. If not on my current frame, then on a frame after that. A ti post will last a lifetime, I'd move it to future frames without worrying about fatigue life. And the failure properties of ti (bend vs snap) are much more desirable. If you can refute this I'm open minded.
    I think those points can be refuted but it's not important. What Shiggy said spoke to that, they can still break and their fatigue properties aren't literally infinite. There can be workmanship issues and materials problems with smaller parts as teamdicky said. I don't think this is the core issue though.

    The question is why you believe that any post could truly be a lifetime purchase or why you would want it to be. Why assume that any bike in the future will require a length, offset, and diameter that can be met by today's purchase? Why assume that your fit won't change over your lifetime; it most certainly will. Why assume that trends in frame design will never impact the seatpost? They do now.

    If you think that a post purchased today will be your choice 20 years from now then ask yourself how suitable a 30.9 post with minimal offset would have been 20 years ago. You think all the evolution that will ever be just got through taking place?

    I think you should buy a post to match a frame and not worry about reusing it on your next one. If you can, consider it a bonus. I switched last year to a saddle type that requires more setback than I have used in the past. That combined with more vertical STAs has caused me to switch to setback posts when I couldn't have imagined doing so before. Good thing I'm not married to a 20 year old post because I bought titanium. I couldn't be even if I wanted to; all my bikes are less than 5 years old due to theft. A lifetime post is unrealistic.

    I own a Lynskey Ti 31.6 post. The clamp is fairly typical of a lot of lightweight posts. It is fussy and offers no setback but works fine and seems well made. I got it to go into a Ti frame---a pure vanity purchase as Ti posts really are. Nothing wrong with that.

  34. #34
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    Quote Originally Posted by hellomcfly
    I'm on my 3rd aluminum frame in 3 years, and both frame failures were clean breaks that happened with no warning, and no bending.
    Did those frames snap at the welds or mid tube? I'm on my 2nd and 3rd alu frames in 15 years, none of which have ever broken. Either you have remarkable bad luck, poor frame choice or you're one brutal cyclist. A ti seat post won't prevent you doing the same to another alu frame!

  35. #35
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    Quote Originally Posted by TooTallUK
    A ti seat post won't prevent you doing the same to another alu frame!
    Thanks for pointing that out. I thought that a ti post would magically protect the whole bike!

    I've already been convinced that a ti post isn't what I need. My original reasoning was that a ti post would be less likely to snap, and if it did fail, it would bend and I wouldn't end up with a sharp piece of metal in my arse and/or have a catastrophic crash.

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