Facing your bottom bracket, or not?

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  • Facing your bottom bracket, or not?
  • Well, I don’t know what to say. What I do know though, is that if I screw a cup into a shell and the face isn’t perpendicular so I have a gap on one side and not the other. Then I keep tightening ’til that gap goes away, then something has either moved or deformed. Whether that has an impact on the smooth running or longevity of the BB I guess depends on the BB or where/how it’s moved/deformed. But I know it happens.

    Just for balance, and to illustrate I have no axe to grind, I very rarely face the shell ends on bikes we build.

    I face bb shells as a matter of course when bullding bikes.
    It’s amazing to see that mid way through facing a frame, you can sometimes see paint and material removed from one part of the face but not the other.
    I also chase threads through to make sure they are aligned with one another.
    For me it’s simply correct practice to make sure that the bb is starting life off in the best circumstances possible.

    Premier Icon mick_r
    Subscriber

    To the OP – just fit a square taper crank and UN55 bb and forget about alignment 🙂

    Slightly off topic so apologies to the OP (talking head tubes not bb shells).

    toys19’s headset analysis is very nice but unfortunately does not describe the actual mechanism of most headtube misalignment. It shows a misaligned tube where the end faces are not perpendicular to the bore. Pretty much all mass produced quality frames start with a head tube that has been faced / bored in a lathe prior to assembly (so begins life perpendicular and parallel).

    Welding or brazing the frame concentrates a lot of heat on the rear face of the head tube, and if you get the heat wrong it pulls into a visible banana shape. So the end faces and bore are still locally perpendicular, but the top and bottom faces are no longer parallel (more than 1mm out if you really go crazy with the torch).

    Similarly, if the head tube has short overhangs above top tube / below down tube (so ends are near the heat), then it can also flare / ovalise.

    Provided these misalignments aren’t too big, then they can be corrected by facing, without which it can be impossible to fit cups / headset will have tight spots. Sometimes the headset problems are actually caused by the out of round / localised excessive interference causing deformation of the (realitevly thin) bearing race.

    toys19 also suggests that it needs 1400 kg to close the misalignment gap on his headset analysis model. This is probably correct – and the thread on a relatively stout headset press can generate forces far in excess of that figure. In the day job I’ve run bolt torque vs clamp load tests on M24 fine pitch bolts and generated forces well in excess of 100 kN (actually a lot more force than that but haven’t got the data at home to quote). So briefly back on topic – maybe a fully tightened bb thread generates a lot more axial force than you think.

    Similarly, any animal fitting a headset by the hammer and wood block method can attest that it is very easy to fit a cup out of parallel with the bore, and it is only when it comes up against the faces that it squares up.

    toys19
    Member

    mick_r cheers for the response there are a couple of points I would like to raise.

    it is very easy to fit a cup out of parallel with the bore, and it is only when it comes up against the faces that it squares up.

    I am not sure I agree with this, and have never seen it. In my experience you can start it off out of parallel, but very soon, long before the cups touch the face, the thing straightens out. If it does start out of parallel you tend to get marks on the cup parallel faces too. There are many pics that will attest to the fact that even after pressing your headset in there can be a gap on one side due to a non square face that no amount of torquing with the press will resolve (because they tend to have parallel press tool faces so they cannot force the cup out of parallel) . (can I find a pic like this now? When I need it? No, but I have seen them, honest)

    I agree about the possibility of weld distortion, but I will admit I have never seen it, if I bought a frame with this I would send it back.
    But to try and remedy it with any of the market available hand tools is a ridiculous notion, there is so much backlash in the contact between the reamer/facer and the shaft that it rotates on that it will just conform to whichever direction it decides to take, plus how do you set your datum? The cone it uses at the other end will never ensure centralisation. To do this you need a lath, mill or boring equipment. Any aftermarket facer/reamer just makes it look pretty.

    bencooper
    Member

    Facing BBs – my feeling is that it’s more about removing paint bulges and the like. The BB shell does distort fractionally when welded or brazed (though very little if made properly), the issue is more a powder or paint drip causing a bearing shell to distort slightly.

    Head tubes are a different issue – the cups are thinner and more easily deformed, and there’s no thread to control the insertion and alignment.

    The hand tools might not get it absolutely perfect, but they get it close enough to solve the problem.

    natrix
    Member

    My LBS told me that if I didn’t face my BB I’d die of cat aids 😳

    Personally as yes I am quite the homo, I prefer the rear mounted bracket. Gives me more support in the general anus area.

    Peace
    #mountainbikesaregay

    Premier Icon scotroutes
    Subscriber

    The wrote:

    Personally as yes I am quite the homo, I prefer the rear mounted bracket. Gives me more support in the general anus area.
    Peace
    #mountainbikesaregay

    😕

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