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Thread: Shimming/lathing dropper post for odd sized tube.

  1. #1
    Isn't flammable. moorey's Avatar
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    Default Shimming/lathing dropper post for odd sized tube.

    I think i know the answer to this, but here goes anyway. I want to put a dropper post on my bottlerocket. Seat tube is 30.0, which is a silly fuck off size, but shit happens.
    Thinking the best option is to shim out a 27.2. Something like a KS Supernatural 272.
    Happy with KS gear. Running a i950, but they only do larger diameter in that model.
    OR....incase my research missed it, does anyong do a dropper in 30.0? Black Mamba Rapid Adjust is out there possibly, but not convinced about that one.
    OR OR....a mate suggested lathing down a 30.9 (only 0.45mm shaved off each side), but on a bike I want to use for FR, AM and a bit of DH, its not an area I think I want to weaken, even if the seat would only really be sat on when pedalling. (tempting notion, i have to admit.....im no engineer, but .45mm......hmmmmmm)
    Last edited by moorey; 04-04-2012 at 11:36 AM.
    Quote Originally Posted by JTmofo View Post
    Think I'll just buy an Orange!

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    Isn't flammable. moorey's Avatar
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    :insert chirping crickets:

    I'm going to try out the WRONG (but more interesting) option, of lathing down an old spare 30.9 to 30.0, and ride it around for a while and see how it goes. If its, say 300mm long, I'll lathe down about 200mm of it, leaving top 100mm of unlathed post sitting out of the seat tube, factoring in that if I get a dropper post with 4-5" raise, that will get me up to a pedallable height.
    Meh...whats the WORST that could happen.....? ;)
    Quote Originally Posted by JTmofo View Post
    Think I'll just buy an Orange!

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    ^Once punched Jeff Kennett. Don't pick an e-fight with this guy ajay's Avatar
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    It's hard to know without testing if it will fail, .9mm off the O.D doesn't sound like heaps but it could be 20% of the wall thickness... Also, I'm not sure what coating is on the post ( straight annodising?) and how interacts with the frame material. When you turn it down, taking half a mm doesn't allow for a lot of cutting, so you'd want to use some wet and dry to get a decent finish... Also, aluminium is rank shit to machine... Give it a crack though!

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    Senior Member Dales Cannon's Avatar
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    Looks to me as though your only option is to go 27.2 and shim it to suit. I wouldn't be skimming the diameter, especially since you are a big bloke with big balls (alledgedly). If you can track down the shim I have a 27.2 KS seatpost I don't use anymore (don't know the number but can check) that you can try out before you buy something. Just cost you postage.
    Itoko no roba wa ichirinshi ga dekimasen

  5. #5
    Isn't flammable. moorey's Avatar
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    Quote Originally Posted by Dales Cannon View Post
    Looks to me as though your only option is to go 27.2 and shim it to suit. I wouldn't be skimming the diameter, especially since you are a big bloke with big balls (alledgedly). If you can track down the shim I have a 27.2 KS seatpost I don't use anymore (don't know the number but can check) that you can try out before you buy something. Just cost you postage.
    Appreciate that mate. I'll keep that in mind as second option. i've just dropped an old 30.9 into a mate to lathe down. Its a VERY thin walled XC post.(2mm thick). If THAT can handle it, i'm going to jump in the deep end and try it on a dropper post. The skimmed section will sit entirely inside the frame on the one i'm testing, and eventually (if this works), on the dropper.
    Aside from the gonads of an infant, i'm actually a little fella, of little more than 70kg, and on the bike its going on (hopefully), the only time I will be on the seat is riding it up the hill to bomb down again with the seat dropped, and my arse (and tiny testicles), not touching the seat.
    I'll keep you 'Posted' (pardon the pun)
    Quote Originally Posted by JTmofo View Post
    Think I'll just buy an Orange!

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    BSS geelong Sethius's Avatar
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    Isn't flammable. moorey's Avatar
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    Quote Originally Posted by Sethius View Post
    Cheers mate, but they are all shimmed to get to 30.0. If shimming, I'll stay with my tried and tested KS's
    I'm going to play with the lathing down idea, and when out of hospital, evaluate the results
    Quote Originally Posted by JTmofo View Post
    Think I'll just buy an Orange!

  8. #8
    MTB Precision John U's Avatar
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    Back in the day when we were importing gravity dropper and they were the only option around they only came in 27.2 and were shimmed out for all other sizes. Don't think this caused many issues for people. Should work ok for you. I have heaps of shims left over so I could probably dig out the correct size if you plan on going the 27.2 route.

  9. #9
    Senior Member Nerf Herder's Avatar
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    I recall when I first started looking into dropper posts reading an article that recommended 'not' to shim dropper posts. I was searching last night through my shizzle but couldn't find it sorry.

    I searched through the KS site and the only real mention of it is in the user manual


    Please be sure your new KS seatpost is designed to fit in your frame.


    All Supernatural, Supernatural Remote, Supernatural Beam, Dropzone and Dropzone Remote models are designed to fit either 30.9mm or 31.6mm seat tube inner diameters. Supernatural 272 and Supernatural 272 Remote is only compatible with 27.2mm seat tube inner diameters. Improper fit may cause slippage, faulty performance, injury and may result in void of warranty.


    Make sure your frame is clean and free of debris, grease and the inside of your seat tube is smooth and free of any object that may score the seatpost.
    Apply grease to all seatpost bolt threads. Apply a friction or anti-seize compound to the inside of the seat tube and inserted surface of the seatpost. Do not use grease.
    Now it doesn't specifically say 'Do not use a shim' ... but the bit underlined above is pretty specific by saying 'only' compatible with 27.2mm seat tube inner diameter.

    The main difference between a dropper seat post and a standard post (in terms of clamping) is the thickness of the main seat post mast. I think this is why manufacturers (not just KS) don't like using shims ... which is related to uneven pressure on the mast. Maybe if you had a shim that was the correct length to cover the entire insertion depth of the post, so that there was even surface contact between post and seat tube then this might be OK. but I'm guessing.

    Sorry I don't have a definitive answer,

    In the end I say ... beware of the Shim no matter how tempting he may look ... unless of course you take responsibility for your own actions ... then enjoy

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    Senior Member Dales Cannon's Avatar
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    So are you suggesting the generous use of silicone?
    Itoko no roba wa ichirinshi ga dekimasen

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