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Thread: BB posts

  1. #1
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    BB posts

    Just after a bit of info. re. the design of these, specifically the best method for clamping of BB shell ie is it simply a case of clamping either side of the shell to a square surface, or are machined cones necessary?

    Thought about getting one from Joe Bringheli but he has a min 4 week waiting list + the shipping to the uk hikes the price up a bit.

    My other option is getting something machined locally, but am unsure as to what's the best method - hence the thread.

    Thanks as always.

  2. #2
    tuz
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    A lot of builders use the Bringheli (including me) so it's a good one. Not a big fan of the design however.

    Some have one machined. I actually just made one! What you want is that both sides of the post are as close to parallel as possible, as well as sufficiently hard. For the top cap I made a brass cone. A shop with a big-ish lathe should be able to do it.
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    Quote Originally Posted by tuz View Post
    A lot of builders use the Bringheli (including me) so it's a good one. Not a big fan of the design however.

    Some have one machined. I actually just made one! What you want is that both sides of the post are as close to parallel as possible, as well as sufficiently hard. For the top cap I made a brass cone. A shop with a big-ish lathe should be able to do it.


    So does the Bringheli post have cones or is it just two parallel surfaces?

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    tuz
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    No cones on the Bringheli, flat parallel surfaces. Check flickr.
    homebuilt commuter, mixte, road and track (+ Ryffranck road)
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    To make it work, you need the ability to machine the surfaces in relationship to the threads, and those threads still need to be in a concentric relationship to each other. Which given the fact the whole issues arises in the first place due to heat distortion, is a... testament to builders everywhere.

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    Quote Originally Posted by tuz View Post
    No cones on the Bringheli, flat parallel surfaces. Check flickr.

    Ah! The ever bountiful flickr - just what I was after, thanks.

    Also came across a couple of pics of a post following the Paterek design (which looks in essence the same as the Bringheli post), as well as the Paterek design drawings themselves.

    Would I be ideally looking for the section that fits inside the BB shell itself to be quite tight fitting, (what would be a good diameter for an English BB?) or is something that loosely centres the shell in the clamp adequate if the clamping forces are focussed on the side faces of the shell (the reason I ask is that the photo and the drawing don't seem to match - the drawing has a spindly little .708" middle section whereas the photo looks much 'beefier')
    Last edited by c_booth; 03-22-11 at 09:57 AM. Reason: removing a daft question

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    tuz
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    I made my post flat without a raised section that fits in the shell's ID. Making a raised section tight fitting is not that useful; I've come across an older BB shell that didn't fit, but that was on another fixture. And even having it there in the first place might interfere with the chainstays just after you braze or tack them (I let them stick in a little), but at that point you can get away with only using a true wheel to track alignment.
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    Quote Originally Posted by tuz View Post
    I made my post flat without a raised section that fits in the shell's ID. Making a raised section tight fitting is not that useful; I've come across an older BB shell that didn't fit, but that was on another fixture. And even having it there in the first place might interfere with the chainstays just after you braze or tack them (I let them stick in a little), but at that point you can get away with only using a true wheel to track alignment.

    Ah yes, this was what I thought. I guess one can always use a washer just to centre the shell, and this isn't likely to interfere with the stays.

    Something else that I was wondering - if you're going to such lengths to ensure the faces of the clamp are as square as possible, is it therefore necessary to face the BB shell before you start? This is something I've not really heard mention anywhere, but I was under the impression that as well as being a mm or so too wide, BB shell faces are more than likely not square in their 'raw' state, and can't be 'trusted' as a guide?

    Think I'll prob head off down the machine shop tomorrow and put an order down so yeah, thanks to all for the insight, as ever.

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    The last couple of BB shells I've seen have had machined faces when I got them raw. I'd bet they are pretty good.
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    tuz
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    You can check that the face is good by checking it with a machinist's square. I personally wouldn't trust them to be perfect: any run-out across the BB face is magnified by about 15 at the end of a 57 cm tube (0.2 mm --> 3 mm). Which makes alignment a bit iffy to interpret generally, even more so post-weld.

    Here is a pic of my BB post:

    PICT0045.jpg
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    Quote Originally Posted by tuz View Post
    You can check that the face is good by checking it with a machinist's square. I personally wouldn't trust them to be perfect: any run-out across the BB face is magnified by about 15 at the end of a 57 cm tube (0.2 mm --> 3 mm). Which makes alignment a bit iffy to interpret generally, even more so post-weld.

    Here is a pic of my BB post:

    PICT0045.jpg

    Hey thanks for the pic tuz.

    Am I right in deducing that the presence of the two nuts in your pic suggests the stainless base and brass cone are not threaded? If so, was this just a tooling issue or a deliberate design choice? Also I'm curious as to the purpose of the washer screwed to the top of the brass cone, and the bolt head protruding off-centre from the stainless base?

    (not wishing to sound like an inquisition )

  12. #12
    tuz
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    My post is made to fasten to a T-slotted table. Hence the T-bolt. The post and cap are not threaded by design. I couldn't think of another way for the post, and the cap would have required handles which would have made it bulky and heavy. The washer is screwed simply to have one less part to worry about when taking the BB in & out. The off-centre bolt is such that the post won't rotate when the assembly is tightened, it fits in the T-slot.
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