Recurring derailleur problem, won't stay fixed
 

I just got a new Scott Speedster S20, with Shimano 105 components. A day after picking it up at a shop out of town, it stopped shifting down to the smaller chainring. The chain would just drop between the two rings. So, off to my LBS... They adjust the front derailleur (low limit screw) and everything's fine. A day or so later, same problem. Another LBS, adjust the FD (low limit, height and angle) everything's fine. Another day or so, can't shift to the smaller ring yet again. The original store says they can replace the derailleur, just wondering if it could be anything else before I schlep out there again. I'd hate to get home and have the same problem again on a bike with < 30 miles on it.

at this point i would look for other issues like bent teeth or rings. yes they can get bent when new during shipping or whatever. don't confuse bent teeth and shift gates.

No amount of adjusting limit screws will fix your problem. That's because the chain is moving in between the chainrings and the FD hasn't even reached the inner or outer limits yet. Limit screws would only come into play if you are throwing the chain off the outside onto your pedal, or off the inside onto the bottom-bracket shell.

Sounds like the chainrings aren't installed properly and have too large of a gap in between them. Inner chainrings typically have the teeth offset to one side and can be installed two different ways. One orientation has the teeth closer to the large ring and the other has the teeth further away. You may want to inspect your inner chainring and see if flipping it would move the teeth closer ot the large ring. Or there may be additional spacers that can be removed.

Also you want to be sure to lighten up on the pedal-force when shifting between chainrings so that there's less tension on the chain. That means shifting to the smaller ring well before hitting a hill, then adjust the rear as needed. Trying to shift from big-ring to small-ring while trying pedaling hard up a hill is very difficult for any front-derailleur. Although a pull-pull design with two cables can do this without any problems.

There's no *noticable* bend or misalignment to the teeth or rings... is there any real way to measure without taking the rings apart and using a straightedge?

The offset has the teeth closer to the large ring... and I've definitely tried shifting with little pedal-force, and my lack of overall conditioning makes it essential that I'm on the smaller ring as I get to a hill :)

I'll definitely ask the shop about bend & spacing on the rings/teeth though. Thanks.

As the others have said the chain is too narrow for the gap between the chainrings, as in a 10s chain with 9s crankset. Check that this is indeed the problem by trying to carefully hang the chain between the rings. If it can drop below the tip if the inner chainrings teeth the gap is too wide.

If it isn't a matter of chainring separation (or chain width) it's possible that the FD wire is a bit short, so the FD doesn't move all the way to the limit screw, or housing friction making the cage motion a bit sluggish so it doesn't shift crisply. This is most commonly the issue when the chain doesn't down shift well while on outer cassette sprockets.

It could also be that there's nothing wrong and you need to modulate how you shift a bit, shifting quickly when the chain comes from the ourside, and a bit more gingerly when it's coming from the inside. It's a subtle difference, but one that Shimano incorporated into their Di2 system.

Ah... the chain is not able to truly drop between the rings. Every time, it sits just barely on the teeth of the smaller ring, a small push is enough to get the chain to seat properly with the teeth. I'll pay more attention cadence & pedal force once it's fixed again. For now, it always drops to the slightly off position regardless of what I do (bike on a trainer).

The part I can't get my head around is the fact that that each "fix" works, but only for about 10 miles or so. Am I right to think that something is sort of slipping somewhere, undoing or negating the previous changes? I can't remember any consistency to when it starts happening, alas.

One other thing I maybe needed to mention is that it always takes two clicks of the shifter to switch to the smaller ring, just one to switch to the bigger. I believe this is normal with the 105's sometimes? Also, this is a 2010 model so not the newest 105. Thanks again.

Check your cable housing ends. What you are looking for is 1 or 2 stray litle wires sticking out of the housing end cap. If you find that it means that the cable housing is gradually shortening itself. When that happens you can adjust the derailleur and it will work for a short while but will require frequent readjustments.

I don't know where you live but maybe the bike is cooling and the lube in your cable housing is stiffening after a few minutes. First thing, have the shop strip the FD cable, clean and re-oil the housing using the lightest oil you can find. That'll ensure good crisp movement when cold.

If that doesn't do the trick, you can tweak the FD angle. Back in the day we used to toe in the outer cage plate fro snappier action, but today's FD cages are too brittle for that, so try bringing the heel out a bit so the shift comes from the front. Compensate by backing off the inner limit a hair. Then work on your touch a bit, don't back off the pedal pressure as much, allowing the increased chain tension spring the chain across more than if if it's slack. BTW- don't get carried away, too much chain tension is how you shift beyond the inner ring and drop the chain.

A good point but for a different reason. If the housing is extruding through the ferrule, that would slacken the cable and worsen shifting up to the larger ring rather than to the smaller as the OP reports.

But if the housing is being pinched at the ferrule that would increase cable friction, and might be a factor.

When I've taken off the inner ring, I have found super thin washers on the bolt, between the rings. These washers pushed my inner ring just a tiny bit further from the outer ring...

I would check to see if your crankset contains similar washers, and try shifting without them.

I would check the cable very closely. The fact that it works and then stops working indicates to me a cable issue. if the cable is under the bar wrap you may have to remove the wrap to see everything. if the ferrels are not seated on the shifter and all along the way it could be slipping or not fully releasing like it should. The cable may also be over tensioned as a result.

Are you shifting from large to small when you're in a small rear cog? If so, that could contribute to the problem. Is it a brand new bike? If not, make sure the cable guides under the BB are clean. I've seen many (if not most) shifting problems occur because of junk on these guides, especially from sports drinks dripping down from water bottles, or road gunk. I'd also be suspicious of a barrel adjuster (I really don't like them), but if the barrel adjuster loosens, it would tend to allow the chain to drop down to the small ring more easily, so I'd rule that out unless you're fussing with it.

Another issue that a lot of shops don't recognize is the physical positioning of the FD. Make sure the derailleur's outer cage is very close to the outer chainring teeth (2-3mm max):

http://www.parktool.com/uploads/imag...p/ft_der_2.jpg
http://www.parktool.com/uploads/imag...p/ft_der_2.jpg

If the FD is too high above the rings, it has to move a larger distance laterally in order to bend the chain the same amount.

Also sounds like a cable issue to me .... binding or sticking somewhere.
Try this:
Next time it malfunctions, let go of the shifter & stop riding. Pull gently on the front derailleur's bare cable running down the downtube. When you pull, is it tight at first, but then you feel something give making it go slack (bad), or is it already slack ? If it's already slack, will the front derailleur move the rest of the way if you give it a little push with your finger on the cage ?

I was shifting from a middle-ish cog. On the trainer now (warmed up from NYC cold) it does shift properly from the two largest cogs, not from any of the others. It's brand new, so there shouldn't be any dirt/grime clogging the cable guides.

Tried this too. The cable is reasonably slack once it tries to switch to the smaller chainring and I can't feel anything give when I pull on it a bit. Once the problem happens, I can't move the front cage at all by pushing on it.

I really appreciate all of the ideas and input (ah, so much to learn). I'll try to get them to inspect & replace the cable too.

On a new bike, it's more likely to be a rough cable-end, or a crimp causing the problem.

When you shift to the small ring, and shift into the biggest cog, how close is the chain to the left side of the front derail cage ?


Did the LBS guys take it for a test ride to experience the problem ? Or was it a 2 minute workstand check ?

The chain is pretty close... Can't fit a dime between them without it pushing the chain out a *little* bit. And, alas, both shops just put it on the stand.

So, the original shop inspected the cable, replaced the inner ring, test rode... Still a problem. Also replaced the chain, test rode (them and me) and everything seems fine. For now, anyway. Will give it a full ride tomorrow and hope for the best. Thanks all for your input.

I keep getting dirt in the hole the FD cable runs through (behind the bottom bracket shell). Over time this will cause the cable to start binding up and mess up the shifting. The only way to clean it is to remove the cable and run something through the hole to clean it out.

anyway, it's important to clean it out.