How much can you shim a seatpost

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  • How much can you shim a seatpost
  • I have a rather nice 27.2 Ti seatpost. Is there any problem in shimming it to 31.6 for a new Ti frame?

    Or should i sell it and buy the correct size?

    thanks

    greyspoke
    Member

    I’ve shimmed the same size gap with no probs.  72Kg and old, so not a heavy rider though.

    Premier Icon nickdavies
    Subscriber

    Shimming it width wise is fine, it’s the length of shim that’s the issue. Most shims are 100mm, if you’ve got plenty of post down below that and/or the shim isn’t well below the welds the you could have problems.

    ok, i understand that….i think a new post would be better…

    Premier Icon scotroutes
    Subscriber

    I have a 27.2 Ti post in a 31.6 Ti frame with a shim. 27.2 flexes more giving added comfort.

    johnners
    Member

    I used a 27.2 post in a 30.9 tube for years before I eventually bought a dropper – USE thermoplastic shim with about 90mm in the seat tube, I weigh about 90kg. It did clear the lower weld on the top tube though, and there’s a brace.

    I’d be guided by what your frame maker says, they may have a specified minimum insertion. Ooh er etc. It’d be a shame to have to retire a nice post if you didn’t have to, more of a shame if you damage your frame though!

    Premier Icon martymac
    Subscriber

    I shimmed my surly from (i think) 31.6 down to 27.2 with a USE thermoplastic shim, it has 100mm insertion, which is the same as the minimum insertion for the thomson post.

    surly didn’t recommend shimming, but wouldn’t give me an answer on what the minimum insertion was, they just said to follow the post manufacturer guidelines.

    bike has been fine so far, i prefer a 27.2 post, as it fits all my other bikes too.

    Premier Icon momo
    Subscriber

    My genesis tarn came with a shim to run 27.2 posts, as said above far comfier than 31.6!

    hols2
    Member

    As above, check the minimum insertion for the frame (not the seatpost). In my experience, 100 mm is fine, as long as the shim extends down to the lower weld on the toptube. My old Giant NRS came from the factory with a 30.9 seattube shimmed down to use a 27.2 post. I’ve run 27.2 Gravity Dropper posts on all my bikes for well over a decade, they’ve mostly had seattubes 30.9 mm or bigger.

    thanks for the input guys. its a shame there arent longer shims to benefit from the longer seatpost in the downtube.

    I will speak with the frame maker.

    Premier Icon Yak
    Subscriber

    I have also shimmed 31.6 down to 27.2. But to get a long enough and well supported insertion below the top tube weld, I used 2 USE shims bonded together. I had to cut the collar off the lower one. Worked fine.

    Put shim on post and run insulation around bottom of post to space it out to 31.6 it will minimise any flex

    im liking the two shims bonded together with a collar cut off

    Premier Icon martymac
    Subscriber

    Yes im liking that idea too

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