Bent Shimano middle ring

Cornish Trekker
Cornish Trekker Posts: 79
edited May 2013 in MTB workshop & tech
After hours of searching and checking and relubing and re-torquing everything on my bike to find the cause of an annoying creaking i finally found it to be a loose chainring bolt. so i thought it would be a good time to strip down and rebuild (retorque) all back together nicely. WRONG.

According to the shimano tech sheet that came with my crankset (Deore M950) the required torque setting for the blts that hold the middle ring to the outer one are 14nm. upon gradually applying this force to the 4 bolts in opposite (gradual) increments the force f 14nm has bent the steel ring so it is ready for the bin.

Could someone please tell me how you do this properly (without relying on the advice of shimano's 14nm which is clearly far too much for the soft steel ring).

what annoys me more is that on the FSA sets the chainring is nicely spaced between the other rings and can not be bent, you'd shear the blts before you bent the ring. but the shimano has nothing to bear onto , it sort of just floats on the bolts.

Any advice greatly appreciated, thanks people.

Comments

  • nicklouse
    nicklouse Posts: 50,673
    you can not have positioned it correctly.

    pics of what you have now?
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • Cornish Trekker
    Cornish Trekker Posts: 79
    i've disassembled it now and ordered a new one.

    it was deffo positioned correctly, with the tab aligned with the crank, etc.
    it seems that 14nm is enough to deform the ring to the area immediately around the bolt (it pulls it closer to the outer chainring and bends the ring), this is especially true of the bolt hole which has no ramps behind it to support it.

    i have i guess i will have to just hand tighten with an allen key unless my £75 torque wrench is faulty.

    any other ideas?
  • Chunkers1980
    Chunkers1980 Posts: 8,035
    Yep, don't rely on torque, feel what is correct.

    What cranks are they? I think you've got the models mixed as that number is for some old xtrs.
  • nicklouse
    nicklouse Posts: 50,673
    sounds like you have the incorrect fitting rigs.

    or did not mod them to fit.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • Cornish Trekker
    Cornish Trekker Posts: 79
    sorry my typo Deore M590 not 950. it's a set so has to be right.
  • nicklouse
    nicklouse Posts: 50,673
    Cornish Trekker wrote:
    sorry my typo Deore M590 not 950. it's a set so has to be right.
    it sort of just floats on the bolts.
    wrong ring.

    shimano have a habit of changing the diameter of the shoulders that suport the rings. you will find that that ring has the right spacing for the out side position?

    floating on the bolts is not an issue just dont use alloy ones. buying by name is no good you need to buy by part number if you want things to fit correctly.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • Cornish Trekker
    Cornish Trekker Posts: 79
    damn, well i've just ordered an exact replacement. may scrap that idea then if the new one won't fit either.

    do you mean don't buy an alloy ring? the one i've just screwed was steel.

    and if not shimano 36t which flavour would you recommend, FSA?

    cheers
  • The Rookie
    The Rookie Posts: 27,812
    nicklouse wrote:
    floating on the bolts is not an issue just dont use alloy ones.
    No mention of alloy rings......
    Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.
  • nicklouse
    nicklouse Posts: 50,673
    The Beginner wrote:
    nicklouse wrote:
    floating on the bolts is not an issue just dont use alloy ones.
    No mention of alloy rings......
    bolts.
    "Do not follow where the path may lead, Go instead where there is no path, and Leave a Trail."
    Parktools :?:SheldonBrown
  • The Rookie
    The Rookie Posts: 27,812
    Indeed.....
    Currently riding a Whyte T130C, X0 drivetrain, Magura Trail brakes converted to mixed wheel size (homebuilt wheels) with 140mm Fox 34 Rhythm and RP23 suspension. 12.2Kg.
  • Cornish Trekker
    Cornish Trekker Posts: 79
    people i am the biggest twat alive! whilst regarding myself as mechanically proficient i have dropped a cataclysmic blunder!!!! i can laugh at it now.

    on my fsa sets the outer ring and middle ring were both attached inside the crank arm.; not so the shimano which of course sandwiches the outer and middle ring having the crank rotor in the middle. so there was me bolting the 2 rings together inside the rotor, no wonder there was nothing for the middle ring to bear against and stop it buckling.

    £10.99 lighter and with much more experience i can now safely torque up the bolts to a steady 14nm as specified by mr shimano with no problem.

    thanks for your help and enjoy the laugh at my expense, i deserve it for being a nob!

    ps. if it still creaks when i test ride it i am chucking the whole lot over the nearest cliff (which is very close by!)

Categories