Bottom Bracket Manual Torque

Discussion in 'Bicycle Mechanics and Repairs' started by RhythMick, 9 Aug 2012.

  1. RhythMick

    RhythMick Über Member

    Location:
    Barnsley
    Yesterday I fitted my first BB and Cranks (wife's bike) and want to make sure of torque before I send her out on it. (Must check insurance lol)

    Searched through the forums and web and read lots of interesting info on torque. I have a very old and huge torque wrench but I don't trust it as it hasn't been used for 30 years and it only goes up to 20 Nm anyway. Torque wrenches for bikes seem massively expensive. There's a Draper on Amazon that covers 10 - 60 Nm for about £25.

    The general opinion seems to be to not bother with one unless you have carbon. I don't.

    The torque on the BB (Shimano M590) range starts I think at 35. In foot pounds that's about 29. if I have it right in my head that means a foot long spanner with 29 lbs of weight on the end. Still hard to visualize.

    I'm a heavy bloke at 213lbs. I figure put a big wrench in my hand and there's more danger of stripping the threads and ruining the frame than leaving loose. I've done it so that I didn't have to strain or use "gorilla force" as Sheldon puts it, but it definitely feels good and tight.

    any guidance ? because of the different threading will it self tighten anyway?
     
  2. ianrauk

    ianrauk Tattooed Beat Messiah

    Location:
    Atop a Ti
    Just good and tight will be fine. It's just one of those things you know when it's tight. You won't strip the threads unless you have inserted the cups/BB incorrectly.
    Just remember to grease the threads first.
     
  3. Pauluk

    Pauluk Senior Member

    Location:
    Leicester
    10Nm minimum wouldn't be any use for some of the more delicate stuff like stems and brakes if you are a bit ham-fisted.

    If you are concerned about your wife's BB and cassette lock ring then go for it, for £25 seems like an ok purchase to put your mind at ease.

    Personally, I wouldn't bother because I would need two to cover the ranges required, but I'm not a big bloke and I think I have a good enough feel when it comes to tightening fittings on a cycle (and I don't have carbon anything).
     
  4. OP
    OP
    RhythMick

    RhythMick Über Member

    Location:
    Barnsley
    Thanks guys. I'm not ham fisted just aware of my strength and careful. Threads were greased.

    The two bolts on the left crank needed to be tightened alternatively up to 12Nm. I set the old torque wrench to 12 but felt it was getting too tight before it clicked so I stopped.

    I'm good. Thanks.
     
  5. Rob3rt

    Rob3rt Man or Moose!

    Location:
    Manchester
    From what I gather, the reason wrenches made for bikes are expensive is that it is harder to detect with reasonable resolution and accuracy changes in torque at lower values. You will find the cost of any low nM value torque wrench to be high vs a high torque wrench, regardless of if it is marketed to bike users or not. You need a torque wrench that goes down to about 4 or 6 Nm to be much use on a bikes smaller components.

    Also they need periodic calibration, so if something critical needs a certain torque and you only have a 40 year old wrench that has been rotting in the bottom of a toolbox, it might be a safer bet to buy a new one.
     
  6. OP
    OP
    RhythMick

    RhythMick Über Member

    Location:
    Barnsley
    Makes sense thanks
     
  7. slowmotion

    slowmotion Quite dreadful

    Location:
    lost somewhere
    Use a large adjustable wrench ( they usually have a handy hole in the end of the handle) and put the hook of a spring balance in the hole. Measure the lever arm, do the calculation, and pull on the spring balance to apply the correct force.

    If the lever arm is about 250mm, apply 14 kg to get 35Nm

    Job done.
     
  8. OP
    OP
    RhythMick

    RhythMick Über Member

    Location:
    Barnsley
    cool ta
     
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