ryanmm

This started happening a couple weeks ago, when shifting from the middle ring to the big ring the chain sometimes slips off and wraps around the crank arm. Doesn't happen to/from any other shift combo on the front derailleur, and doesn't happen every time...maybe 20% of the time I do that shift. The bike is an early 90s Giant Iguana mtb, I think the front derailleur is original to the bike, the rear one is a few months old. The chain is lubed and isn't squeaking or binding. No issues shifting the rear derailleur. When this happens it's easy enough to put the chain back on and get going again but would certainly like to stop this from happening.

Is there anything simple that I can adjust to try & fix this? Could improper shifting cause this?

cny-bikeman

First Adjust height and rotation according to Park Tool https://www.parktool.com/blog/repair-...ur-adjustments then adjust outer limit screw. If still a problem something else must have changed to cause the problem. Check for play in the front derailleur, bending of the cage, bent chainwheel or damaged teeth, etc. You will have to investigate and eliminate possibilities one by one, as any symptom can have several causes.

Homebrew01

To add the good advice above ......The High limit screw may have an 'H' next to it, and it is often the screw further out (away) from the frame. It controls how far out the front derailleur moves. It probably needs 1/4 turn clockwise (In) to "limit" the derailleur's movement. I usually make adjustments in 1/4 or 1/2 turns, nothing more. Makes it easier to undo if you go a bit too far.

The Low "L" limit screw controls how far the front derailleur can move at the small chainring.

ryanmm

Thanks I checked the height & rotation as best I could, it looked aligned. I then turned the H limit screw a bit and will do a ride later today & see if it works without problems.

ryanmm

Got back awhile ago, wouldn't shift from middle to big ring at first. So I turned the screw back a little, same thing. Then a bit more and it shifted normally. Did that shift more than usual just to test, around 20 times, and the chain never came off. Think it's resolved.

Thanks guys.

dsbrantjr

The proper setting for that stop is loose enough to allow crisp upshifts to the big ring and no looser. It sounds like you have it pretty much correct. Note that the exact setting is fairly easy to overshoot; I make adjustments in 1/8 turns or so when I am getting close.

musicgalaxyman

Three adjustments to the front derailleur can cause this.

The height the derailleur rides above the largest chainring/ The angle of the derailleur/ and the likely culprit...the screw settings controlling movement of the derailleur arms.r

If you have a friend with mechanical know how to help, that would be best, or take to a shoppe.

It's a common problem.

musicgalaxyman

See you have it fixed...great.

Now's a good time to go over the entire bike, looking for ways to tweak just right.