Chainrings seem bent
I rode my daughter's bike into the City today: a 7-speed basic Trek aluminium hybrid, with freewheel and square tapered bottom bracket.
I looked down to see the chain line moved about a centimetre with every revolution, so I assumed the large chainring had a bend in it. But it did the same on the second or middle chainring, too.
I have now taken the chainrings off and they seem OK. If she has bent them, she has managed to bend them all – which is an undertaking.
I am also suspicious about the bottom bracket, which is some sort of sealed unit and probably very cheap.
Could a bearing have gone inside this, causing the wobble in the chainline? It turns, but feels loose then stiff.
Apart from the chainrings or the BB, I don't know what can be causing this. I also don't know how you check the chainrings, except by eye; or lay them on a flat surface and see that they are level and regular.
The bike is not well cared for and the chain is shot. I have this idea that it is new, but it must be eight years old by now.
I looked down to see the chain line moved about a centimetre with every revolution, so I assumed the large chainring had a bend in it. But it did the same on the second or middle chainring, too.
I have now taken the chainrings off and they seem OK. If she has bent them, she has managed to bend them all – which is an undertaking.
I am also suspicious about the bottom bracket, which is some sort of sealed unit and probably very cheap.
Could a bearing have gone inside this, causing the wobble in the chainline? It turns, but feels loose then stiff.
Apart from the chainrings or the BB, I don't know what can be causing this. I also don't know how you check the chainrings, except by eye; or lay them on a flat surface and see that they are level and regular.
The bike is not well cared for and the chain is shot. I have this idea that it is new, but it must be eight years old by now.
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Comments
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Most likely the spider or crank is bent, replace.0
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My bets the bottom bracket,
I had the same thing with the chain line moving, changed the BB and all is good again. One of the symptoms was with everything done up properly, is there any lateral to chain line 'wobble' in the cranks?
My BB was an FSA mega Exo, when removed, the drive side bearing was about to collapse, as above, refitted new BB, everything was as new.FCN 90 -
A 7 speed citybike, 8 years old, will have a square taper axle.
The chance that the BB is worn out so badly that the rings wobble 10 mm is very small to zero.
Most likely the bike has fallen hard on the driveside and bent the crank/spider.0 -
Cranks/spiders are very strong. I have subjected mine to some big hits and never bent one. I would think it's more likely that something is not properly screwed down and has seized in place, e.g. one of the BB cups has wound itself out of the frame maybe. But without pictures I am guessing really; in your shoes I would put an hour or two aside to properly strip it down.- - - - - - - - - -
On Strava.{/url}0 -
This might sound a bit basic but have you checked the chain ring bolts? Might be loose or might even be one or two missing. I had exactly this problem and I only noticed it initially by looking down at the chain rings as I cycled. Worth a check, could become a really serious problem if not sorted.0
