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Splined bottom bracket

Discussion in 'Bicycle Mechanics and Repairs' started by Pete, 7 Aug 2007.

  1. Pete

    Pete Guest

    This is my first encounter with one of these, surprisingly: had plenty of experience with tapers (as well as the old cotterpins - yonks ago! ;) ). Anyway, I know splines are commonplace these days: my son's left crank loosened and fell off today on the ride. He managed to re-attach it, and I had a look this evening and tightened it a bit more. My question is: how tight? There are two allen bolts holding the thing on. I'm forever scared of stripping threads. Any other tips?
     
  2. Steve Austin

    Steve Austin The Marmalade Kid

    Location:
    Mlehworld
    thats a hollowtechII chainset by the sound of things. The two bolts need to be tight, but have a bit of a reputation for stripping.
    I tighten them tight then a smidge more.

    I think this is the only bolt on a bike that 'might' need a torque spanner so you know its done up tight enough. tight and a smidge more works for me
     
  3. derosa

    derosa New Member

    Location:
    Off the back
    Use a torque wrench. You won't belive how tight those bolts have to be to keep the cranks secure.
     
  4. alecstilleyedye

    alecstilleyedye nothing in moderation Staff Member

    if it's a shimano bb you can get the torque settings from their website.
     
  5. asterix

    asterix Comrade Member

    Location:
    Limoges or York

    Mine kept loosening.. ..until the LBS put a very long allen key onto 'em. No way I could get them as tight with the normal sort. The LBS is a Shimano agent so they ought to know.
     
  6. Pete

    Pete Guest

    Hmmm... looks to me like a weakness in modern bike design ... Cranks take a lot of stress and if the fixing is inherently dodgy?

    OK, OK, so I grew up with tapers, they were around when I was a lad, know them well, had plenty of trouble, true, but no problems tightening the bolt onto the BB spindle. You're tightening a heavy steel bolt into a steel thread so you'd be very unlucky to strip. Yes, taper cranks do break occasionally, but seldom work loose.

    No, with tapers the trouble comes with the outer extractor thread, cut straight into the alloy. Often I've wished for some sort of 'recovery' extractor, to use once the worst has happened...

    Back to splines: well I cannot understand why they were ever invented. I would have thought the split in the crank would weaken the whole thing, make it more likely to break... No doubt this will all be explained to me...;)