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  1. #1
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    dirtjumper bmx seat tube

    Hi,
    I'm working on a 26" wheeled bmx dirtjumper project and trying to scope out a set of steel tubes. The seat tube is stumping me. The external butted seat tubes are all made for longer lengths. I need about 300-320mm length. And I need a beefy tube at the chainstay/top tube cluster for strength. I could do an external sleeve. But that seems overly complicated for this application. The seat will be super low and almost never be touched I imagine. So the ask is that it has an ID that works for some standard sized seatpost.

  2. #2
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    Buy a straight guage steel tube with enough wall thickness to ream to whatever seat post size you want? Or use a shim?

  3. #3
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    Use a topper for a straight gauge chromoly tube like paragon and some others offer. If you have access to a lathe they are fairly simple to make. I would suggest avoiding an internally relieved topper for you application. I have had seat stays crack because the topper deformed. Rider was not the lightest and never seemed to rotate 360ís all the way around.


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  4. #4
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    I have just purchased some 28.6 at 1.6 wall and plan to use a standard bmx post (25.4). Hoping to try turning down the od to create an external butt at the length I want for a dj.

  5. #5
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    Thanks for the ideas. I think I have a plan. I'll order some 1.25" OD .032" wall 4130 for my seat tube. And I see that there are these inserts at paragon that should nicely slide into that tube to get my seatpost clamping area:

    CLMS2068: Steel Seat Collar, 1-1/4" x 27.2 mm ID, 91 mm Long, 30.8 mm Step, Relieved ID

    Without a lathe, this seems like a pretty straightforward way to built my seat tube.

    I have the main triangle built from 1-1/4", ie 31.8 butted tubes. So that should match up nice for the joints too.That's what I was after.

  6. #6
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    I've used a sleeve a couple times, and have a frame where the builder used a collar. Both solutions seem pretty bombproof. The collar looks super clean but i'm pretty sure a sleeve is stronger and easier to implement. It's probably moot in this case since dj/bmx bikes don't lever the seat tube that much.

    [i linked the actual parts i'm familiar with]
    "Things that are complex are not useful, Things that are useful are simple."
    Mikhail Kalashnikov

  7. #7
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    Quote Originally Posted by qwkswede View Post
    Thanks for the ideas. I think I have a plan. I'll order some 1.25" OD .032" wall 4130 for my seat tube. And I see that there are these inserts at paragon that should nicely slide into that tube to get my seatpost clamping area:

    CLMS2068: Steel Seat Collar, 1-1/4" x 27.2 mm ID, 91 mm Long, 30.8 mm Step, Relieved ID

    Without a lathe, this seems like a pretty straightforward way to built my seat tube.

    I have the main triangle built from 1-1/4", ie 31.8 butted tubes. So that should match up nice for the joints too.That's what I was after.
    If you do 360ís or anything that places a heavy side load on the seat stays avoid the relieved ID version.

    Take a look at laird frame on Instagram. He has documented what he uses for seat tubes in his custom bmx frames.


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