Replaced crank

[rokee]

Hi I replaced my crank that was 170m to a 175m do i need to replace the derailleur also? the reason im asking is the derailleur would not go to the high gear. if you have any ideas pls let me know thanks

[nameused]

You shouldn't need to replace the derailleur. I just did this swap on my bike and I dropped down to a smaller large gear in the front. I just had to lower the derailleur on the frame and then readjust the high and low limit screws since the new crankset had a little bit more spacing between the gears than the old one.

[rokee]

i tried everything the derailleur wouldn't go all the way out so the chain can go on the larger gear. I released the cable to see if that helped but it did not

[trevgbb]

changing the crank arm length would not have any effect on the gears unless, you also changed the tooth count on the chainset?
or, the chain line has been affected?

[rokee]

i kept the teeth count the same 42-32-22 was the count

[rokee]

well I just checked it out and my teeth count did change went from 42-34-24 to 42-32-22 do you think that's the problem?

[RobAR]

rokee - changing the arm length or the tooth-count that you have now, will not result in the problem you described.

What WILL cause this problem is installing a crankset (other model or brand) that requires a shorter bottom-bracket spindle.

If you did, perchance, replace like with like, just a diff arm length - I would guess that you have a square spindle. The aluminum arms will, over time, distort/ enlarge. This is why we need to check the tightness of the crank arm bolt during a maintenance session.
As they are repeatedly tightened, they move inboard and we compensate by adjusting the Hi limit screw on the drlr. When you install a new crankset of the same, the drlr Hi limit screw must be backed out until the shift will complete.

Make sense?

Rob