Bent square tapered Bottom Bracket

bice
bice Posts: 772
edited February 2017 in Workshop
I am working on my daughter's bike, a unspecial hybrid used off road and on and not treated very carefully.

The cheap chainrings were wobbling untrue and the chain was skipping.

I did some bending with an adjustable spanner, not effectively, so I bought another cheap set of triple chainrings.

It is much better, but the chainrings still don't turn true. On the big ring the chain is about 5mm from the front mech, and then slightly rubs it.

I don't think it is possible to tighten up a set of chainrings crookedly, so something is wrong.

The bottom bracket is a cartridge type. It turns, but can it be bent? And if it is "bent", what does that actually mean in this context?

I doubt the solid spindle can be bent, so what is actually happening here? Any thoughts appreciated.

Comments

  • dj58
    dj58 Posts: 2,217
    Not bent, more likely the bearings are worn, causing the axle to run out of true. Fit a new BB and performance should be restored.
  • arthur_scrimshaw
    arthur_scrimshaw Posts: 2,596
    With square taper the cranks often won't run true just by fitting them and tightening the bolt. Try the crank in a different position (rotate in 90 degree steps) and see if you can find a position where they run true. If not use a soft faced mallet to nudge the crank to run true before tightening the bolt. I've done this on quite a few occasions and it works.
  • keezx
    keezx Posts: 1,322
    It's highly unlikely that a square taper axle (steel) is bent.
    Chainrings and/or spider are bent.
  • SME
    SME Posts: 348
    The above comments are good advice.

    Just to add to them... I once had a crank that wouldn't run straight. It turns out it had come loose and been rubbing on the BB tapered axle end, kind of burring it or rounding it off. No amount of tightening the crank bolt would straighten it so, as above, a new bottom bracket was required.
  • The Rookie
    The Rookie Posts: 27,812
    Erm, no way was a BB needed for that, it would be new cranks. I've yet to see a soft alloy crank wear a hardened steel BB axle!
    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.
  • bice
    bice Posts: 772
    Thanks all. Very useful advice here. I didn't realise you could put cranks on a square tapered BB slightly crookedly.

    I will try a different position and (gently) tap with wooded mallet to coax into place.
  • SME
    SME Posts: 348
    The Rookie wrote:
    Erm, no way was a BB needed for that, it would be new cranks. I've yet to see a soft alloy crank wear a hardened steel BB axle!

    OK Yep, you're right... I should've said cranks (that's what I get for having a long day and staying up too long... Sorry)!

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