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Discussion Starter · #1 ·
Hello all, not posted much as google and searching the forums helps.. but im drawing a blank here ... Or maybe im missing something obvious ..

Fitted a bb and crank I picked up off ebay, to my trek bruiser frame ..

The drive side crank is pretty far over and the chainring is so far over that the chain comes straight off in 1 and 2 cogs on rear, and sketchy feel on all others ..

The chainline is straight when I'm set on the 8th rear cog, so anything inside of 8th gear is didgy, up at 1st and 2nd it comes straight off with no pedaling pressure

Bottom bracket is an FSA square taper 68x113 ...I think its 113, it says 113 on the axle anyways..

Running single ring upfront .. pictured below with 40 tooth, but going down to 32 once payday arrives

Will switching to a 103mm bb bring the crank arms nice and close and sort my chainline out?

One more thing, the bb I have is not symettrical? The drive side sticks out more which is maybe whats pushing my chain ring too far out

Would a symettrical 113m bb sort it? Or should I go down to 103mm

My lbs is a large retail bike shop and the guy their doesn't have much of a clue if im.honest so id rather ask here for help ...

Help please :)

Chainline


Drive side , chainring is attached outside of spider in pic, since moved inside spider, but still only brings chainline to 6th or 7th gear, still comes straight off in 1,2 and 3 gears


Non drive
 

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I'm guessing the reason it is offset is because you're using a 68mm bb in a 73mm shell. Turn your bike upside down and measure across the bb shell with a ruler to check it.

Is the chain ring on the inside or outside of the crank? If it's mounted where you usually see the big ring on a 3-ring crank, simply moving it to inside may do the trick.

This is all guessing.... If it's a 73 shell, measure from the center of the seat post to the chain ring. It may be ~52.5mm now. Using a 73x113 may move it inward 2.5mm. Using a 73x108 will move it an additional 2.5mm, resulting in a 47.5mm chainline. Do you have room between the frame and ring to move it in 5mm - I can't tell from the pic?
 

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Discussion Starter · #3 ·
Thanks for the reply .. turns out its a 73mm shell, oops ... I clearly missed the obvious ... New frame so didnt have bb to refer to for sizing

There is room to move the crank arm in 5mm, just about ... Moving chainring in and changing bb would bring it in by probably 10mm which would hopefully sort it ...

Thanks :)
 

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To second rlouder, try moving the chainring to the middle position.

My wife's bike has a similar chainline, though the chain doesn't come off in the lower gears. I've ordered a new BB because I can't move the chainring to the middle position...
 
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