Bicycle Mechanics - Front Derailluer Help

OK, this is kind of an odd one

I cannot shift into the small chainring on my mountain bike. The front derailluer is XT and haven't had any problems with it in the past. When I try to shift into the small ring, the derailluer doesn't move over enough. I thought it was an issue with the limit screws, but when I had it in the stand I noticed that the derailluer itself is hitting the seat tube.

Basically, if I'm in the middle chainring and I shift into the smaller one, the derailluer moves, but is stopped by the seat tube before it can move over enough to shift the chain.

Any help is appreciated.

Thanks

First off, have you ever been able to shift into the lowest chain ring? Other than that, the derailluer must be able to shift it over. Make sure it is positioned about .5mm higher than the largest chain ring and that it is also angled correctly.

This doesn't make sense. If the derailleur is resting on the seat tube and it used to work alright before than the only logical possibility is that the small ring moved inward for some reason. Not sure how this can happen without changing BB spindle and or crankset.

Thanks for the replies. I agree that it doesn't make sense.

My original thought was that the angle was off but it appears to be straight. I've had this setup for 5 years and shifting into the small chain ring hasn't been a problem before. This weekend was the first time I tried shifting to the small ring this season and that is when I discovered the problem. The derailluer can clear the big chain ring but is then stopped by the seatube before it can fully move the chain (the chain ends up rubbing against it).

It may be excess friction in the shift cable or in the shifter. Check the routing of the cable looking for any interference or kinks. Try cleaning the shifter with WD40.
You may need new cables and cable housings.

Al

pictures please.

Thanks for the replies. I agree that it doesn't make sense.

My original thought was that the angle was off but it appears to be straight. I've had this setup for 5 years and shifting into the small chain ring hasn't been a problem before. This weekend was the first time I tried shifting to the small ring this season and that is when I discovered the problem. The derailluer can clear the big chain ring but is then stopped by the seatube before it can fully move the chain (the chain ends up rubbing against it).

It sounds like your front derailer has moved. Are the derailer plates nearly parallel with the chainring teeth? If not, you've probably twisted around the seat tube a little. Alternatively, did you have a bad shift or crash? Dropped something on the bike? Perhaps the derailer has been bent. Overall, if I'm understanding correctly, your front derailer shouldn't even come close to hitting the seat tube, so it sounds like an alignment problem.

I'm as baffled as everyone else--mainly because the description doesn't seem to make sense. I'd go with Al1943's suggestion to check the cables. Also, try grabbing hold of the exposed cable on the downtube and giving it a few good tugs to make sure the derailleur is moving freely. Also, it sounds like the FD may have slipped so it's not angled correctly (explaining why it's hitting the seat tube), but you mentioned that the angle is okay. At this point I think pics are in order...

Possibilities:

1) cable adjustment
2) bent dérailleur
3) incorrect chainline

One of my Trek 4300's came stock with the front der. resting on the seat tube. The cause was somehow they put the wrong BB in the bike.

and that it is also angled correctly.


It sounds like your front derailer has moved. Are the derailer plates nearly parallel with the chainring teeth? If not, you've probably twisted around the seat tube a little.


Also, it sounds like the FD may have slipped so it's not angled correctly (explaining why it's hitting the seat tube)

Shall I +1 or is this enough?

Which part of the derailleur hits the seat tube? I didn't pick up on this before.

First off, thanks to everyone for all of the replies. It is great that to have this many people helping out. Now, I just hope it doesn't turn out to be something stupid.

I took a few pictures that (not the best, sorry). One shows the derailluer in the middle chain ring position and another shows it in the small chain ring.

The derailluer doesn't look like it slipped (the middle and big rings shift fine) and in any case it doesn't look like that would cause it to hit the frame.

Anyway, please keeping the ideas coming. It is probably something simple but I can't figure it out.

Thanks

In my opinion the derailleur could go down a bit. I know it worked before, but perhaps the pivots have developed play. Also check the inside of the outer cage for wear. A chain can rub its way right through the cage eventually, and if it has worn it at all it could compromise shifting. You can always cheat and bend the rear part in a bit. Usually does not affect other gears as the lower part is usually not in play when riding typical gears.

In my opinion the derailleur could go down a bit. I know it worked before, but perhaps the pivots have developed play. Also check the inside of the outer cage for wear. A chain can rub its way right through the cage eventually, and if it has worn it at all it could compromise shifting. You can always cheat and bend the rear part in a bit. Usually does not affect other gears as the lower part is usually not in play when riding typical gears.

+1

I assume it's touching your seat tube now because you let out the low limit screw, but the shifting problem is due to drivetrain wear.

Regardless of what it "used to do", I think you have a bottom bracket spindle in that bike that is of the wrong length. That is probably one of the main factors here. I also agree that you can probably drop the derailleur height and get it to fall, but I am still convinced that your BB spindle length is not appropriate for that crankset.

Another thing I wonder about is about that derailleur. I wonder if it was designed for the tooth difference you have between the large and small ring. YOu have a 46 and probably a 22 or something to that effect. That would mean it would have to accomodate a 24 tooth difference.

What is the story with the crankset and the derailleur? Are they original?

It is touching because I let the lower limit out. It hits the seat tube before it can move the chain. I also moved the derailluer down a bit but it didn't help with this problem.

I will check the bottom bracket spindle. The derailluer and the crankset came with the bike, which was built up by the bike store where I bought it (about 6 years ago).

Since checking the spindle is beyond my wrenching skills, I will take it to the lbs.

Thanks for the help everyone. I will report back on what it turns out to be.

Did you try changing the angle? It's one bolt. Try it. Turn it so the back end comes out a couple degrees.

It is touching because I let the lower limit out. It hits the seat tube before it can move the chain. I also moved the derailluer down a bit but it didn't help with this problem.

I will check the bottom bracket spindle. The derailluer and the crankset came with the bike, which was built up by the bike store where I bought it (about 6 years ago).

Since checking the spindle is beyond my wrenching skills, I will take it to the lbs.

Thanks for the help everyone. I will report back on what it turns out to be.

Looking at your pictures everything looks correct. Could you have some debris in behind the derailer that isn't letting it move over far enough. The thing that has everyone flummoxed is the hitting the seat tube comment. That just should happen unless something was changed.

Another idea is to lube the pivots on the derailer and work them a bit. If things are dirty, it might hang up the shifting.

OK, took it to the local bike shop and after some initial head scratching, they fixed it. It appears, that the spindle length, front derailluer, and seat tube size have a very low tolerance when put together. I haven't had to adjust the front derailluer on this bike before, so I never realized just how little margin there was in the setup.

In fact, when it was in the work stand and correctly adjusted, it still didn't shift into the small chain ring. However, the mechanic said that once I got on the bike and loaded the rear shock, it would shift fine. So, we took it off the stand and I rode it around the parking lot and sure enough, it shifted perfectly. I would never have guessed this when I had it in the stand at home.

I recently got my shock Push'd and last weekend, when I noticed the problem was the first ride with the tuned shock. I'm wondering if there was enough difference in the shock pre-load to affect this. Just guessing, since this is beyond my level of expertise on the drive chain.

Thanks to everyone for offering suggestions, I really appreciate it.