Bicycle Mechanics - chainline problem

I'm currently having problems getting my chainline sorted out. I am currently using the stock BB with STX cranks. I've had to replace the cranks because I bent the arm of the original ones. now, for some reason the chainline isn't quite right, It always sounds like it's trying to shft to the next gear being either higher or lower. can anyone help.

Sight down from above the crank. Where is the inner face of the inner chainring, relative to the edge of the bottom-bracket shell? Usually the inner face of the small chainring is approximately flush with the edge of the bottom-bracket shell, or a few millimeters outboard of it.

either tighten the crank onto the bb more, or re-set your front derailleur.

Maybe, it's not a chainline issue......?? Derailleur(s) may just need a little tweeking, instead??............assuming you replaced your crank with another of the same??

On a triple crankset, the chainline is correct when the middle chainring runs in line with the middle of the gear cluster. You can get a rough idea of how your chainline is by using a yardstick, putting it on the top of the teeth of the middle chainring then seeing where it falls on the rear cassette. This is only a rough guide though and if you have a carpenters square or a machinists straight edge, these would be more accurate.

If the chainline is off, you need to measure how much and purchase a BB that will move the crankset in the direction you need to go. DO NOT TIGHTEN THE CRANKSET ON THE BB AXLE IN AN ATTEMPT TO MOVE THE CRANK IN TOWARDS THE FRAME! This will deform and possible ruin your crank.

Crank BB's come with length designations, eg. 107, 113, 102. The difference between a 107 and a 113 for example is 3mm on each side of the bb axle. So, if you need to move the bb in 3 mm to align the crankset middle chainring with the middle of the cassette, and you have a 113mm bb, you need to get a 107.

I have seen a new, off the shelf bicycle that had a 113mm bb require a 102 mm bb to correct the chainline. Without going into the reasons for this, chainline can make a huge diff in how your drive train runs and operates, if that is in fact your problem.

Originally posted by don d.
DO NOT TIGHTEN THE CRANKSET ON THE BB AXLE IN AN ATTEMPT TO MOVE THE CRANK IN TOWARDS THE FRAME! This will deform and possible ruin your crank.

yeah but first of all you have to make sure the crank is installed correctly, torqued to the correct figures. if not then it will fall off during riding.

Please note that not all cranks seat equally deeply onto a given spindle. When I put what SHOULD have been the correct-length Shimano UN-72 cartridge BB in my son's Specialized HardRock, his chainline was dreadful, because the cranks did not fully seat.

With most frames, one can obtain a good chainline by selecting a short enough spindle to place the innermost chainring within a couple of mm of the chainstay.

I used a rubber mallet to initally "seat" the cranks, was this a bad idea? I then used a torque wrench to finish it off. Looks good except for the fact that my crank bolts are stressed. Will be picking some new ones up tomorrow. Fixed the problem, I had adjusted the deraileurs out so far that I had nothing left in leeway. Thanks for all the help guys, I also realize that going to a 9 speed crankset on an 8 speed setup was not the smartest move I've ever made.