how is a frame's BB shell determined if in spec?

Discussion in 'Cycling Equipment' started by jasong, Mar 23, 2007.

  1. jasong

    jasong New Member

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    I've recently bought a new frame. I can't install 1 new bottom bracket in it, because the shell is distorted. Another one I can get in there, but needing to use more and more force as the BB's ends approach the edges of the shell. When getting that one further in, I can already feel the increase resistance on the spindle, presumable from the frame's shell causing the cartridge to distort and further bind the spingle against the ball bearings inside.

    The manufacturer is implying that this can't be a warranty item. Of course, this means the frame must be replaced, which is very bad for him.

    How is a BB objectively determined to be in spec?

    I can see the disparity as I screw in the drive side of the BB, easily being able to see the gap difference from side to side.
     
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  2. matagi

    matagi Well-Known Member

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    Stupid question ..... are you sure the BB you are trying to install is the correct thread?
     
  3. jasong

    jasong New Member

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    Yeah; I didn't mention another part of my test.

    I installed both BBs with no problems (hand tightened all the way) into 2 other blank, new frames. I've never had a BB install problem in 30+ times. Each side of the cartridge BBs install fine, its that there's an inherent tolerance that the shells assume as they get closer to being fully threaded in and need to start gently touching each other. If the BB shell is out, then you simply can't thread each side in, or you must thread them with a lot of force.

    I could get the BBs started by hand into the suspect frame. I really think the shell was either poorly tapped (poor alignment between the opposing sides) or that the shell was tapped and then welded, and was distored during the welding.

    Since I have an older frame by the same mfctr, I tried removing its BB soon after. That BB had even worse problems.

    I know it's a problem in their assembly or QC; but the mfctr is implying that there is no way that a shell can be out of spec. I disagree, but don't have a way others might quantify this.
     
  4. daveryanwyoming

    daveryanwyoming Well-Known Member

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    If you're sure this manufacturer doesn't make both english and italian threaded BBs then the most obvious answer is to have the BB chased and faced. Assuming the frame has enough value to keep then chasing out the threads is a way of checking with a known calibrated tool. You'll also get nice clean threads and parallel BB faces out of the deal which is a good idea anyway. It only takes a few minutes and any decent bike shop should be able to do it for you.
     
  5. jasong

    jasong New Member

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    Italian isn't an possibility.

    If the shell is distorted, I don't think chasing will fix this, as it's no longer cylindrical. Or if the threads were tapped off axis, same thing. Chasing will destory the threads and bring the threads themselves out of spec.
     
  6. matagi

    matagi Well-Known Member

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    Have you got some Vernier calipers? Get them out and measure the shell.

    If the shell isn't cylindrical, you shouldn't be able to get the correct sized BB even started, let alone part way in.

    Have you checked the underside of the shell to see if it has an obvious "ding" or something which might indicate it had been dropped or subjected to some external force?
     
  7. jasong

    jasong New Member

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    Measuring a 3d surface from the outside and ensuring that it's cylindrical isn't so easy. The cylindricity can be "violated" in many ways, the shell may be cylindrical, but the threading inside can be off. Imagine a V formed inside where the axes intersect somewhere below the axis formed by the outer shell.

    It's obvious the shell tappings aren't on the same axis- but what is the spec for this? How far is acceptable? With the close tolerances of the left sleeve/right cartridge, there is very little room for error (< 1 mm).
     
  8. matagi

    matagi Well-Known Member

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    I meant taking an inside measurement then going 90 degrees and checking the measurement again - that should give you an idea of whether or not the shell is cylindrical. You could take a couple of measurements at 45 degrees as well for the sake of completeness.

    If you are so certain of the problem, why are you even bothering to discuss it here? Go back to whoever sold you the frame and take it up directly with them. You don't need to know the specs for the BB shell if the thing is so obviously wrong.
     
  9. jasong

    jasong New Member

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    Because I don't have formal knowledge of what defines a BB shell/tapping to be in spec. Not looking for "laymans" opinions.. Fodder for a warranty claim or refund on the credit card.

    PS - thanks for you suggestion, but you can still have a skewed tapping that measures perfectly this way in every degree and is still off center. I think the tolerance of measuring this is way too tight, as the projection through 68mm results in only needing a very small difference in dimension to project a 1-2mm difference on the opposite side. This is outside the scope of operator measuring, as we're talking about a very small fraction of a mm (visualize the triangle).
     
  10. Retro Grouch

    Retro Grouch New Member

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    Check the little screw that holds the cable guide. Could that be causing your bottom bracket to bind on the inside?
     
  11. jasong

    jasong New Member

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    No routing under BB on this frame
     
  12. alfeng

    alfeng Well-Known Member

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    NOT all BB shells are tapped as far into the shell as others ...

    NOT all threads on a BB cup are cut "equally" ...

    I had some cups on a "Track" BB which I could only run fully into ONE (road) frame but not into the frame I wanted to install the BB.

    It took me FOREVER to figure out that the threads in on the cups were probably cut so precisely (?) by a CNC "machine" that the OD wasn't seemingly reduced AND the tap that cut the shell were probably worn so that the ID was smaller by just enough to create a situation similar to what you are describing (in part) ... so, the HEIGHT of the threads on the cups needed to be reduced OR the shell's threads "chased" (as suggested) or tapped further into the shell (older, steel cups often do NOT have as much "depth" so shells were/are often not tapped as far!).

    While you may be encountering a similar problem, I can't account for the skewed spindle that you describe. Does the spindle START out skewed or does it skew after the cartridge is half-way in?







    BTW. Here's the quick way to verify that the BB shell is "round" & the faces are parallel:
    Put a piece of typing paper (or, equivalent) inside the shell, uncurl it with the edge of the paper aligned to EITHER face. Look at the OTHER edge of the paper that is NOT in the shell. If the shell & face are square, the OTHER edge will be perfectly aligned, too! If not, the shell needs to be faced.






     
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