WalksOn2Wheels

First off, this is on a compact FSA SLK crank set and 105 cassette, RD and FD which came stock on my '06 Roubaix Comp.

I've been noticing the rear derailleur has been shifting a little funny, so I sat down with the bike today and did the full adjustment down to resetting the cable tension and the H and L screws as well as the "B" screw. That all looks fine and dandy, and the rear shifts perfectly now, but it didn't change the situation with my FD.

It doesn't happen all the time, but if I shift from the small ring to the big ring, it can drop the chain. After I set the RD, I went to the FD to check it out. I made some fine adjustments, but it was more or less perfect as it was. It's in line with the crank set and about 1mm clearance between the inside cage and chain on the small ring and largest rear cog, 1mm clearance from the outside cage to the chain on the big ring/smallest cog.

I kept trying to throw the chain and successfully did it a few times. If I do it slowly, it seems like the chain is just out of alignment to the point that the chain link plates on the inside of the chain are riding the teeth of the big chain ring. If you spin it fast enough, it just falls off.

So I guess I came to the conclusion that I need to be in the middle of the rear cassette in order for it to be lined up with the big chain ring, otherwise, the slight angle of the chain might pull it off of the chain ring. But it seems to me that I should be able to shift from the small chain ring to the biggest while in the 2nd or 3rd smallest cog in the rear without throwing a chain.

Am I missing something in the FD setup or do I just need to fix my shifting habits?

Edit: title should read "Limit screw OR shifting issue."

operator

I would try the limit screw first, if not you may have a cage rotation issue.

cyccommute 

It's probably a limit screw issue. I'd move the derailer outboard just a little on the inner limit. Trial and error it until you find the right spot.

FBinNY 

You should be able to shift to the outer with no problems regsrdless of which cassette sprocket the chain is on, except possibly from the innermost ones (cross chained).

It might be a matter of adjusting the outer limit if the chain only overshifts during the shift or immediately after, but in practice the FD shouldn't be used to keep the chain on the rings. If your chain is climbing up and overrunning the outer ring during normal pedaling (not shifting), there are a few possibilities to check.

First make sure the inner cage isn't touching the chain at all when the chain comes from the outer half of the cassette, that would be a limit screw issue. Otherwise, start by looking for chipped or bent teeth on the ring, or for a twisted or damaged link on the chain. Next, double check the chainline, though that should be OK if the bike had always been fine before.

Another possibility is a worn outer chainring. When the chain comes from an angle, the sides of the teeth keep the chain over a bit so the point of the next tooth can slide between the inner plates. This is especially critical on rings which have a cut down tooth for a shifting gate. On these the chain can move across a bit more at the gap, increasing the chances of the next tooth point hitting the edge of the plate and not sliding in smoothly. With wear the teeth get a bit thinner so the horizontal distance from the point of the tooth to the side of the tooth is less, so they become less able to pick up a chain coming from an angle.

You can correct for this somewhat, if you're skilled with a file. With the chain off, spin the crank, while holding a file at an angle against the inner edge of the outer ring (for a chain that falls off to the outside). This will effectively move the point of the teeth out a bit improving pickup from that angle.

Next time you replace the chain, look for one whose the inner plates are more flared to the open between the links forming a wider funnel to catch the sprocket teeth and guide them home. This is a subtle difference that varies among brands and models of chains.

Al1943

You could also try reducing the cable tension on the front derailleur. With the chain on the small chainring it is best to have no cable tension if you can make the shift to the big ring.

Have you checked the chain for stretch? A stretched chain won't mesh well with the chainring teeth and may try to climb off of the chainring.

Al

davidad

Make sure the der. is aligned with the chainwheel.

WalksOn2Wheels

Ok, I went back to it, measured and fine tuned the height and alignment. Both were fine, but I just got nitpicky with them. I also checked the chain for a stretch and it's fine.

Now the chain won't actually drop, but it will still ride the inside link plates on the teeth. After doing it over and over, I stopped whenever I saw it start to ride the teeth and I could always trace it back to one stubby tooth on the chain ring, so it looks like a few worn teeth are making some shifts precarious. I think the chain that FB mentioned could definitely help with the issue.

As it stands, the issue is now pretty minimal, so I'm not going to worry about it for now. But when it comes time for a chain, I'll have to give FB's recomendation a try.

Thanks all for the tips and pointers. This is the first time I've really tuned my own bike. Well, that is tuned it with some sort of guide and idea of what I was doing rather than just turning crap until it worked.

Al1943

What about the cable tension, did you check that?

WalksOn2Wheels

Sorry, for not addressing that. I did check the tension and actually reset it on both the front and rear derailleurs. The rear needed it bad. The front was fine, but I think a little of the fine adjustment with the height and resetting the cable helped a touch.