Front mech won't go to smallest cog - any tips?

[swee'pea99]

Got a friend a Carrera Subway after reading lots of positive comments, and he's very happy with it, except there's one slight problem. The front mech will only select the mid and largest ring - it won't go to the small.

I've cleaned and lubricated, slackened off the low screw as far as it will go and even disconnected the cable, so it's as free as it could possibly be. But even manipulating it by hand, it's simply impossible to get the cage close enough to the frame to flip the chain onto that small ring. The mech simply doesn't seem to have adequate range. (It does on the other hand have the ability to go a lot further in the other direction than it needs - ie, it could probably select another two rings outside the biggest one if they were there.)

I'm totally baffled. The thing was only a little over a year old, and I think it unlikely that the previous owner changed the mech, so the one it has (a SRAM, by the way) is probably the one it was factory fitted with. But it just doesn't seem to go far enough. Any ideas/suggestions appreciated.

 

[Ravenbait]

Is it set up correctly position-wise on the frame? Last time I had a problem that the screws failed to fix it was because the mech's position on the frame was wrong.

I suppose it depends on whether this has always been a problem. Bizarre. Hard to say without seeing it.

Sam

 

[Gerry Attrick]

Correct me if I'm wrong but I believe your bike has a square taper bottom bracket. Now what can happen if the bike has been poorly assembled, the centre screw overtightened or the chainset removed and replaced many times is that the square hole in the chainset becomes worn or oversized. This positions the chainset too near the frame so the mech can't do its business.

You must totally eliminate the other causes suggested above first as the only cure for this is a new chainset.

 

[HLaB]

I'd try checking it set up correctly first like suggested above but the other thing I wouldn't put past hellfrauds is that they have set it up with a double mech rather than a tripple because that's what they had lying around!

 

[RecordAceFromNew]

It is possible that the previous owner changed the bottom bracket (e.g. with one that is too short) and/or chainset, which resulted in what you are experiencing. It sounds like the front mech is bottoming out or clashing with the frame before it can get to the inner ring.

If I were you I would first check what the chainline is (i.e. distance from centre of frame to middle ring, it should be 47.5mm or thereabout). Also it would be useful if you could provide the make/model details of the front mech and chainset, and if possible to spot what type of bb it is. Also just in case what front shifter is there?

 

[swee'pea99]

Thanks all. I obviously need to provide more info, which unfortunately I can't do 'cos he lives some way from me, but I will try to find out & repost. FWIW I think it's set a bit too high - bicycletutor reckons 2mm or so between the large ring and the cage...I think his is more like 5 or 6. And come to think of it, when I was thinking about shifting it down, I found the allen nut holding the whole mech chewed out in the middle, which suggests some less than expert DIY has taken place at some point. My friend may be able to supply some of the extra info - I'll send him a link and see if anything comes of it. Thanks again.

 

[heretic]

My money's on the b/b axle being too short. If it ever worked from new has the b/b been changed?

 

[CAADHEAD]

Assuming lowering the front mech doesn't work and all other adjustments have been checked, cheapest and easiest solution(if you have tools) if its a cartridge bottom bracket is to remove both crank arms, undo the right side cup and fit a spacer between it and the bottom bracket shell, generally between 1 and 2mm thick, then tighten up the left hand cup as it will now move in to take up the spacers thickness, refit crank arms. The chainset will now sit out a little further hopefully enough to allow the chain to now drop onto the granny gear Its a little trickier with older cup and cone bottom brackets as sometimes the width of the driveside spacer will not allow enough thread on the left hand cup to refit the lockring, but you could get away with it in the short term

 

[cyberknight]

I assume you have checked the high /low stops are set up right and the cable tension is set up ok?
I have always found that whoever makes halfords carrera bikes tends to set the front mech to high but that would not stop the chain moving, it just makes it easier to fall off !.

Is the cable tension correct ? If its going to far over it could mean the cable tension needs adjusting , my old subway 1 had a tension adjuster near the grip shift , or you could do it manually where the cable is held in place on the front mech ...

http://bicycletutor.com/adjust-front-derailer/