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Story with Bowman?

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Comments

  • Registered Users Posts: 6,997 ✭✭✭ cletus


    I'd agree broadly about the welding, but these welds were not good. I've done plenty of ugly but structurally sound welds. In this frame there was oxidisation holes (pinholes or blowouts), there were many points where there was no penetration on the weld, so that the bead was sitting on top of the joint, so very prone to cracking or shearing, and there were a number of places where it looked like they hadn't welded at all.

    I understand your dislike for Hambini, but in this case he's not wrong



  • Registered Users Posts: 676 ✭✭✭ gn3dr


    The dimple on the weld behind the seat tube was described as a blowout but it looked to me like the classic instant stop in an aluminium weld which always leads to a crater in the weld. It then becomes a stress riser / failure point. It's the classic "do not do this" from aluminium welding 101. You are supposed to back off the current slowly at the end of the weld.



  • Registered Users Posts: 6,997 ✭✭✭ cletus


    You're very probably right. I've never chanced welding aluminium 😁



  • Registered Users Posts: 1,364 ✭✭✭ hesker


    So you reckon most modern BBs have give in them. Interested where you got that from as I haven’t heard that before.



  • Registered Users Posts: 6,997 ✭✭✭ cletus


    It's not so much that they have 'give' in them, more that they can operate properly with a relatively large tolerance.


    Tolerances, in machining terms, are tiny. 10'000ths of an inch is a measurement commonly used in machining. It equates to about 0.025mm. Building things to these levels of tolerance increase the cost.

    Having components that can operate even when the tolerances are (relatively) big is important in mass production



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  • Registered Users Posts: 1,364 ✭✭✭ hesker


    What characteristics do they have that allow them to operate properly in this situation.



  • Registered Users Posts: 6,997 ✭✭✭ cletus


    I couldn't say for sure, I'm not an engineer, but one of the ways tolerances are allowed for would be in thread formation, whether the 'peak' of the thread has been flattened or not.

    Another way might be shell wall thickness, so that ovality in the bottom bracket hole doesn't bind the bearing.


    I'm sure there are other things, but I'm only a PE teacher.

    There are a variety of levels of tolerance accepted in all areas of manufacturing. If a hole is specified at 10mm diameter, there is a range of sizes that might actually suit, depending on the requirements of the piece.

    If I drill a hole in order to tap it, there are a couple of different sizes of hole that will suit, depending on the amount of thread engagement I might need, for any nominal hole size.



  • Registered Users Posts: 1,364 ✭✭✭ hesker


    I think I understand what tolerances are well enough. I was just probing a line of reasoning proposed by some in this thread that to me seems really out of whack with reality.



  • Registered Users Posts: 6,997 ✭✭✭ cletus


    Well, if you understand tolerances, what was the line of questioning? That plug and play parts are built with those tolerances in mind? I'm not sure I understand



  • Moderators, Sports Moderators Posts: 22,769 Mod ✭✭✭✭ CramCycle


    Apologies, tolerance would be more appropriate, I was using more layman's terms. In work we would have other terms but not used in laymans terms. Basically, there is an acceptable margin of not being in line that the BB (that you buy) can tolerate, alot of modern BBs, can tolerate these micrometers, they tolerate it through the threading on the bike and the BB not being completely flush (damn near impossible), or the pressfit taking the almost unnoticeable compression somewhere. So imperceptable that once faced off, no person on their own could tell. Some companies when QCing, allow wider criteria than they should but realistically, the bigger companies have assessed that if they apply certain criteria with a suitable level of sub sampling, that even though there will be outliers that may fail, the majority of frames are more than acceptable. There are some smaller companies that may be more stringent, which may potentially increase cost, or less stringent (if they bother at all), which will reduce cost.

    To make it more simple, wheel manufacturing might be a simpler one to visualise. I know there was a video on here on how to make money quick rebadging cheap chineses wheels. Well imagine that the cost difference between HUNT or similar and the direct from factory, isn't solely the rebrand, but it is the local team QCing on this end, they may need to send X% back to the factory or scrap, depending on their business model but the mark up they charge means they can tolerate those returns and also tolerate the lower than you would get from direct sales returns, as they QC'd them locally, possibly every set of wheels.



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