to thick seat post. Can a lathe help?

ChILd_ReBoRn
ChILd_ReBoRn Posts: 92
edited February 2016 in MTB workshop & tech
It's a bontrager SSR post, that's got Walls, thick like a money safe. It's 31,6mm, and I need 30,8mm. So the machined part would be 0,8mm. that should work. I'll have a shiny post. Any Cons to the idea?

Comments

  • ste_t
    ste_t Posts: 1,599
    Being impaled through the arse?

    Would definitely be a con for me, but each to their own
  • WhyteSky
    WhyteSky Posts: 70
    I'd honestly just buy a new post. It's as thick as it is for a reason. 0.8mm is a lot.

    ste_t is absolutely right. Impalement is a distinct possibility and a real downside to your plan.
  • rockmonkeysc
    rockmonkeysc Posts: 14,774
    That's a LOT of material to loose, especially considering the thickness is unlikely to be uniform along the whole length.
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  • Angus Young
    Angus Young Posts: 3,063
    Sounds like a brilliant idea. Let us know how you get on...
    All the gear, no idea and loving the smell of jealousy in the morning.
    Kona Process 134 viewtopic.php?f=10017&t=12994607
  • WhyteSky
    WhyteSky Posts: 70
    Angus Young wrote:
    Sounds like a brilliant idea. Let us know how you get on...

    :D
  • rockmonkeysc
    rockmonkeysc Posts: 14,774
    Angus Young wrote:
    Sounds like a brilliant idea. Let us know how you get on...

    Or video it and share the x ray of the broken seatpost inserted the full 350mm.
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  • ChILd_ReBoRn
    ChILd_ReBoRn Posts: 92
    you guys are either all material experts, or just very paranoid. The aluminum is 3mm thick. I will take 0,35mm from both sides. That's less than a milimeter. The post will still be 2,3 mm thick walled. I don't do DH... so the post will hold .. no worries there. If anything happens, it will bend at the clamp.. not snap and impale me in any kind of homosexual way you all hope.

    Try to bend a 2,5mm thick aluminum tube. I dare you. You'll bend it.. not...

    I'll buy a new one. Not because I'm afraid of injury, but because I'm not sure my work lathe can be that precise. It's 30+ years old...
  • ste_t
    ste_t Posts: 1,599
    ChILd_ReBoRn wrote:
    Try to bend a 2,5mm thick aluminum tube. I dare you. You'll bend it.. not...

    You're absolutely right. Nobody has ever bent a seatpost, particularly one that has been blindly bodged. :roll:
  • ChILd_ReBoRn
    ChILd_ReBoRn Posts: 92
    well, nobody around me did...

    I know there are different aluminium types out there, but i saw a syncross post thin as a paper....only it was
    way to expensive..
  • rockmonkeysc
    rockmonkeysc Posts: 14,774
    ChILd_ReBoRn wrote:
    you guys are either all material experts, or just very paranoid. The aluminum is 3mm thick. I will take 0,35mm from both sides. That's less than a milimeter. The post will still be 2,3 mm thick walled. I don't do DH... so the post will hold .. no worries there. If anything happens, it will bend at the clamp.. not snap and impale me in any kind of homosexual way you all hope.

    Try to bend a 2,5mm thick aluminum tube. I dare you. You'll bend it.. not...

    I'll buy a new one. Not because I'm afraid of injury, but because I'm not sure my work lathe can be that precise. It's 30+ years old...

    Not strictly a material expert but a mechanical design engineer.
    You will loose a very significant percentage of stiffness in the seat post by taking 0.35mm (try a moment of inertia calculation) from it's thickness and don't forget it's thicker at the ends than in the middle.
    I have also seen loads of bent seatpost. When you hit a bump seated you're seat post can easily see loads of three to four times your weight, combine that with the length of the seat post out of your frame and that's some big bending loads. To simulate that you would need to be swinging on a big lever, you won't generate those loads by hand.
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  • The Rookie
    The Rookie Posts: 27,812
    For the price of a seatpost, it's one of the daftest suggestions I've heard.

    I've seen a few bent seat posts, and a snapped one (fortunately no injury).
    Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.
  • ChILd_ReBoRn
    ChILd_ReBoRn Posts: 92
    It's not the price.. it's the availability. There is only one 30,9 option in my city that's not 40$+. rest will have to be ordered... Ah well... the weather is crappy anyways, so no point in forcing the bike build before the weekend
  • The Rookie
    The Rookie Posts: 27,812
    Try a shim then with a 27.2 post, or buy online.

    Cheap posts of cheap material are thick to compensate for that, an expensive post will be of a better material and designed to be OK with a thinner wall.
    Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.
  • FishFish
    FishFish Posts: 2,152
    If you are cycling properly you do not need a seat, and therefore a seatpost.
    ...take your pickelf on your holibobs.... :D

    jeez :roll:
  • Thewaylander
    Thewaylander Posts: 8,594
    I really want him to do it.... I have bent a fair few posts in my younger years.

    do it do it do it do it do it :D
  • ChILd_ReBoRn
    ChILd_ReBoRn Posts: 92
    naah..ordered the original GIANT post
  • rockmonkeysc
    rockmonkeysc Posts: 14,774
    ChILd_ReBoRn wrote:
    It's not the price.. it's the availability. There is only one 30,9 option in my city that's not 40$+. rest will have to be ordered... Ah well... the weather is crappy anyways, so no point in forcing the bike build before the weekend

    Get one mail order with next day delivery. It's a very common size. All Giant bikes use that size and so do lots of others.
    Transition Patrol - viewtopic.php?f=10017&t=13070235

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