Shimming/lathing dropper post for odd sized tube.

Discussion in 'Parts 'n Stuff' started by moorey, Apr 3, 2012.

  1. moorey

    moorey Isn't flammable.

    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 no engineer, but .45mm......hmmmmmm)
    Last edited: Apr 4, 2012
  2. moorey

    moorey Isn't flammable.

    :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.....? ;)
  3. ajay

    ajay ^Once punched Jeff Kennett. Don't pick an e-fight

    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!
  4. Dales Cannon

    Dales Cannon Likes Bikes and Dirt

    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.
  5. moorey

    moorey Isn't flammable.

    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)
  6. Sethius

    Sethius Crashed out somewhere

  7. moorey

    moorey Isn't flammable.

    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 :pirate:
  8. John U

    John U MTB Precision

    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. Nerf Herder

    Nerf Herder Wheel size expert

    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

    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

  10. Dales Cannon

    Dales Cannon Likes Bikes and Dirt

    So are you suggesting the generous use of silicone?
  11. moorey

    moorey Isn't flammable.

  12. ajay

    ajay ^Once punched Jeff Kennett. Don't pick an e-fight

    Looks good!
  13. crank1979

    crank1979 Likes Bikes and Dirt

    This is a problem. Most seat post shims are very short in length. It is possible to find 120mm long shims though that should do the job. Minimum insert on most seat posts is 100mm.

    I wouldn't be turning the seat post down. Putting a step around the circumference of the post doesn't sound like the smartest idea I've ever heard.
  14. Shredden

    Shredden Eats Squid

    1. Machine it down
    2. Cover the sides in vaseline
    3. When it snaps, enjoy being forcefully probed.
  15. moorey

    moorey Isn't flammable.

    Ok. Got it. Will do thanks :)
  16. Dales Cannon

    Dales Cannon Likes Bikes and Dirt

    Just make sure there is no notch or sharp edge to the transition between the machined part of the seatpost and the existing diameter. Don't advocate this approach but if you have to make sure you don't leave a crack initiator where you least want it. Nice radius would be good.
  17. moorey

    moorey Isn't flammable.

    Yeah, it's a seamless transition. We'll see how it goes. Risk considered and no tears if it doesn't pan out. (well, there MAY be tears)
  18. Dales Cannon

    Dales Cannon Likes Bikes and Dirt

    Is that tears as in 'water falling from eyes' or tears as in 'God that won't fit in there!'
  19. moorey

    moorey Isn't flammable.

    A little from column A, a little from column B ;)
  20. Isildur

    Isildur Likes Bikes and Dirt

    Definitely subscribing to this one! For the laughs already, it's worth it!

    That said, the post pictured above does look good, and I'll be interested to see how it all pans out!

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