Crank arms off center

check the chainline first: the middle chainring on the drive side should line up with the middle cog of the freewheel or cassette in the back, if you have only 2 chainrings then the gap between them should line up with the middle cog... you can eyeball this or have a shop look at it, they may be able to do it with the bike on the floor...

if the right crank is too far in (lines up with the bigger cogs in the back) then it may be possible to flip the bottom bracket spindle or cartridge and improve everything... bring the left arm in and pushing the right arm out where it should be... if the chainline is correct then you can consider getting a different bottom bracket to move your left crank arm in...

if the difference is less than 4mm i wouldnt bother with it... for example if my left crank arm was 7mm from the frame and the right one was only 4mm i would leave it like that, even though the symmetry of my bike world would be off... 😉

Very interesting, the other day I was checking out my chain line on an 06 105 triple crank because as I ride it appeared to me that 50/16 [5th smallest] was more crossed chained than 30/17 [5th largest] When I knelt & looked along the chain in various gears the CL appeared as I would have thought i.e. middle front lined up with middle of rear. Conclusion... optical allusion due to seeing more chain on the 50 than on the 30 when looking from above, but then again maybe I am cross eyed.

Sorry, back to your question, whilst doing the above I noticed that the left crank arm was about 4mm closer to the chain stay [opposite to yours] than the right. The outside of the arms at BB level was also the same discrepancy from the BB outboard shell.

My bike is as bought new.

8200rpm said:

Just installed a new square taper BB (Shimano UN73, 68x110).

I think the BB shell on my frame is 68, but the spindle seems long on the cup side (left). So my left crank arm is further from its chainstay than my right crank arm.

What gives? Any thoughts? Help!😕

Several years ago most BBs were asymmetric (longer on the chainring/drive side), whereas almost all now are made symmetrically (Campagnolo record triple BB remaining one example of asymmetric).

If you replaced an asymmetric with one that is symmetric (which the UN73 is), you will have misalignment of the crank arms with the left crank arm farther out. It may be a small enough difference to be functionally insignificant. If it is a problem you can use spacers under the fixed cup (chainring side) to move the whole assembly rightward. You will need to check the chainline first (as previously suggested) to see if moving the chainring side rightward will mess up the chainline too much.

The suggestion to switch the cartridge BB around is not possible with your BB since the right and left cups are threaded differently. It would also be of no benefit because the BB is symmetric to start with.

See sheldon brown's discussion of BBs and chainline:

http://sheldonbrown.com/bbsize.html
http://sheldonbrown.com/bbsize.html#chainline

BTW, the frame is a brand new 2005 aluminum MTB frame, and the BB shell measures exactly 68mm. The cranks are the square taper Shimano LX (FC M570). My BB (UN73) is definitely 68 x 110; I even uninstalled the thing to double check and read the dimensions off the BB!!!

I checked the chainline and the middle ring does line up with the middle cog in the cassette (9 spd).

The lateral distance from the downtube to the driveside crank arm (right) is 45mm whereas the distance from the downtube to the left crank arm is 52mm. The left crank is 7mm out further than the right.

Could this be a spindle length issue? The left side spindle seems to stick out further than the right. With the cranks installed, the left crank has a 3mm gap to the cup whereas the drive side crank is nearly flush with the left side cup.

I'm going to rule out crank arm seating issue since there is virtually no more additional square taper for the left crank arm to go into. It's in there as far as it's designed to go.

It's almost as if the BB is designed for a shell slightly larger than 68mm; but as I said, the BB is 68 x 110mm.

😕

sounds like you have equipment that is designed to be run together...

is there any feature of the frame design that would account for some of this difference?

in the old days when i was faced with this problem i would simply install a spindle that was 7mm shorter on the non-drive side and see how it looked... today the options are not as straight forward... would this be possible in your situation? do you have 7mm of space remaining between the bb shell cup and the point where the crank arm sits right now?

frame alignment should be checked in your situation although it doesnt sound like it could account for your entire discrepancy... you can check one aspect of this yourself by running a string from the outside of the rear dropout on the drive side (held in place by QR skewer if need be) around the headtube (without any obstructions ie: head badge, cablestops, cables etc.) and back to the outside of the dropout on the non-drive side... then measure the distance from the string to the seat tube on both sides of the bike, should be the same distance...

let us know how this comes out...

all's'miles

curby