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  1. #1
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    Not shure where to ask this, but...

    Do you think is possible to machine down a 30.9 mm adjustable seatpost down to 30 mm?

    I just saw a 30.9 mm Joplin adjustable seatpost in ebay, and wondered if it would be possible to machine it down to 30 mm, so they will fit mi old Kona Coiler.

    How thick the seatpost walls are? I think removing 0.45mm all around wouldn't compromise any strenght...

    Give me a reason not to try it :-)

  2. #2
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    Quote Originally Posted by JackStephen View Post
    Give me a reason not to try it :-)
    If it fails then you are out the money. Why not 27.2 with a shim?
    Just keep pedaling, don't stop pedaling.

  3. #3
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    because there is no 27.2 on ebay. also 30.9 is really cheap (149.99 and free shipping)

  4. #4
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    Removal of OD material and reducing wall thicnkess = less strength.

    Less material to absorb stresses means the materials yield strength will be achieved will less stress. It's a law of physics you cannot avoid.

    Good luck.

  5. #5
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    Not worth the risk for minimal money savings in my opinion. Wait until a 27.2 comes available.
    Santa Cruz Blur TRc

  6. #6
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    Would you pay 150 bucks for having a sharp, pointy piece of metal pointed at your nuts and/or rear end???

  7. #7
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    Quote Originally Posted by JackStephen View Post

    How thick the seatpost walls are? I think removing 0.45mm all around wouldn't compromise any strenght...

    Give me a reason not to try it :-)
    Uh this sounds like a painful plan. Bike parts are already pushing the edge of being just strong enough in efforts to save weight taking away half the material will effectively give you a useless pile of aluminum sitting trailside

  8. #8
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    This is hilarious.
    If you go for it, make sure you post up pictures of the damage when it breaks...
    6'5" 230lbs
    My Build: Vitalmtb - Bike Check

  9. #9
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    overall bad idea and further more joplins are just bad posts.

  10. #10
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    I live in Mexico, here a machinist would charge about 20 bucks to do it, The thing is, I don't know how thick the seatpost walls are, but I think the internals in the 30.9 and 31.6 seatposts are the same. The point is, maybe only the outer tube of the 30.9 has an already reduced diameter to fit this measure, It wouldn't hurt to remove a little more (0.45 m is like nothing) and see what happens, haha.

    Or, I can buy the seatpost, measure the outer tube OD and ID, and see if it's feasible. If not, I can sell it

  11. #11
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    The Joplins are definitely a shaky product, but if you have to have it; you should consider taking the material out of the seat tube on the bike instead. The wall thickness on the Kona seat tubes is pretty thick and I know of a few people that have done this mod with no ill effects.
    "If you can get both wheels sliding with no brakes, that's when you really know that you're cookin." Nathan Rennie

  12. #12
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    Quote Originally Posted by JackStephen View Post
    I live in Mexico, here a machinist would charge about 20 bucks to do it, The thing is, I don't know how thick the seatpost walls are, but I think the internals in the 30.9 and 31.6 seatposts are the same. The point is, maybe only the outer tube of the 30.9 has an already reduced diameter to fit this measure, It wouldn't hurt to remove a little more (0.45 m is like nothing) and see what happens, haha.

    Or, I can buy the seatpost, measure the outer tube OD and ID, and see if it's feasible. If not, I can sell it
    I say go with the seattube on your frame. Take it to a bike shop and they might be able to ream it out with a minimal amount of fuss, or possibly your machinist can do it as well. Be wary of overheating the frame and consider on many bikes there is only a couple of inches at the top of the seat tube that is the size for the seatpost and below it is larger ID.
    Try this: HTFU

  13. #13
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    I really wouldn't want to machine it down. As others have said, they wouldn't have added extra material if they didn't need to surely?

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    The walls on any adjustable seatpost are thin enough that you can bind up the post by tightening the seat collar even a little too much. Not only would that issue be amplified by machining the post down, I bet it would be susceptible to getting crushed and deformed when you attempt to tighten down the seat collar. I think your chance of success is less than 1%.

  15. #15
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    No. You will break it and pay twice.

    I use Coiler with AMP post. No more AMP, but gravity dropper should work fine.

    Avoid Joplin.

  16. #16
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    Quote Originally Posted by cornfish View Post
    The walls on any adjustable seatpost are thin enough that you can bind up the post by tightening the seat collar even a little too much. Not only would that issue be amplified by machining the post down, I bet it would be susceptible to getting crushed and deformed when you attempt to tighten down the seat collar. I think your chance of success is less than 1%.
    That's what I wanted to read. I thought the outer tube walls were thick enough, given all the hydraulic stuff packed inside.

    After all, it was a bad idea, thanks everyone for your input.

    I hate Kona for using such an odd seatpost tube haha.

  17. #17
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    Quote Originally Posted by gratefulbiker View Post
    The Joplins are definitely a shaky product, but if you have to have it; you should consider taking the material out of the seat tube on the bike instead. The wall thickness on the Kona seat tubes is pretty thick and I know of a few people that have done this mod with no ill effects.
    Ask the machinist what it would cost to make you a custom seat post.

    6061 is the aluminum you want

    heat treating it will be good too.

  18. #18
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    Quote Originally Posted by KAZU View Post
    Ask the machinist what it would cost to make you a custom seat post.

    6061 is the aluminum you want

    heat treating it will be good too.
    Anytime I am asked to make one custom part I let people know that if there is a viable production part/product out there it would be cheaper for them to purchase it than to pay me for my time to produce just one.

  19. #19
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    maybe

    I will play devils advocate to everyone else.
    Turning it down .9mm is only taking off .45 mm per side, how thick is the tubing? I dont think its going to be half the thickness of the tube. if it is that would make it 2mm thick...
    I think you could scratch a tube more than .45 mm scraping on a rock and make that the weak spot in the tube. Would you toss the seat post in that situation?

    Maybe you should ask what you weigh, how hard you ride and how sensitive the seat post structure would be in that situation. What are common seat post tubing thicknesses anyway?
    Last edited by trythis; 09-22-2011 at 04:15 PM. Reason: enhancement

  20. #20
    Axe
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    Quote Originally Posted by trythis View Post
    I will play devils advocate to everyone else.
    Turning it down .9mm is only taking off .45 mm per side, how thick is the tubing? I dont think its going to be half the thickness of the tube. if it is that would make it 2mm thick...
    I think you could scratch a tube more than .45 mm scraping on a rock and make that the weak spot in the tube. Would you toss the seat post in that situation?
    .45mm is a lot. It could very well be half the thickness.

    Yes, I would toss a seatpost with a half mm gash on it - though I have never got anything even close to that.

  21. #21
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    This is just retarded. man up, wait for a 27.2 post to come up.

  22. #22
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    Quote Originally Posted by Axe View Post
    .45mm is a lot. It could very well be half the thickness.
    .
    Your saying they make 1mm thick seatpost tubes?

    I just mic'd a seatpost, only one handy, and it was roughly 3.37 mm thick. I wouldnt worry about taking it down 0.5mm of this one, but I suppose that if you riding a 1mm post that half its thickness would be alarming. I would be scared to ride on a 1mm thick post. That's only twice the thickness of my thumbnail!

  23. #23
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    Quote Originally Posted by trythis View Post
    Your saying they make 1mm thick seatpost tubes?

    I just mic'd a seatpost, only one handy, and it was roughly 3.37 mm thick. I wouldnt worry about taking it down 0.5mm of this one, but I suppose that if you riding a 1mm post that half its thickness would be alarming. I would be scared to ride on a 1mm thick post. That's only twice the thickness of my thumbnail!
    Here is my Thomson Masterpiece. At least near the end it is less then 1mm thick. No, I am not afraid to ride it.
    Attached Thumbnails Attached Thumbnails Not shure where to ask this, but...-img-20110922-00031.jpg  


  24. #24
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    No poop! I am really surprised, but would like to know the thickness 2 inches up from there. I get the idea of shaving it to save the .023 grams of aluminum near the base of the tube, but a couple inches up where the real stress would be, can it really hold up at 1mm thick? Just askin, not trying to be as arsvoid. <-sensor dodge

  25. #25
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    Quote Originally Posted by Luigiugueto View Post
    Would you pay 150 bucks for having a sharp, pointy piece of metal pointed at your nuts and/or rear end???
    Ha, funniest thing I've read on here in a while.

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