Cannot adjust triple front mech to work properly after renewing chain.

[Jon George]

The gory details:

Chain gauge tool suggests new chain, but not enough wear for a new cassette.

Front mech is a triple with an eight speed cassette. The LBS did not have a 8-speed chain in stock, but said a 9-speed would still work. (It turned out I had a new 8-speed in my 'stock', and I cannot discern any difference between to the problem that has presented itself.)

When I put on a new chain and the bike is upside down so I can attend to it, I can run through all the gears. When I go for a ride, however, I cannot engage the top chainring. This happens with both the new chains. If I adjust the tension so that it will engage, I cannot then disengage it.

I have:
Checked the cable; it's fitting into the handle bar gear change; and the smoothness of its run to the mech.
Checked that limits on the mech are adjusted correctly.
Put the old chain back on and it works fine.

I appreciate that a new chain (and I have replaced the chain on the this bike several times before) will be stiffer, but what the hell is going on? Do I need to put on a new cassette, as well?

 

[Smokin Joe]

The nine speed chain is slightly narrower than your old eight speed. If the front mech was only just shifting the old chain onto the big ring it may mean that the high gear stop screw needs letting out a tad, letting the mech move just that bit further.

That's the only thing I can think would be wrong.

 

[Jon George]

The nine speed chain is slightly narrower than your old eight speed. If the front mech was only just shifting the old chain onto the big ring it may mean that the high gear stop screw needs letting out a tad, letting the mech move just that bit further.

That's the only thing I can think would be wrong.

Thanks for this, but I did try a direct 8-speed replacement I remembered I had and I have spent ages adjusting the limits to make sure they are (I think) in the ideal position. It just plain baffles me ...

 

[raleighnut]

With a triple the only thing the tensioner does is position the mech correctly for the middle ring, the 'top' adjuster/limit screw needs to come out about 1/2 a turn.

 

[Racing roadkill]

Turn the ‘H’ screw on the front mech ( if it has no letter, it’s the one nearest the chainrigs) clockwise, until the mech is moving the chain out to the biggest ring.

 

[Cycleops]

Disconnect the FD cable and move the mech accoss by hand to check the limit screws are adjusted correctly. Then connect the cable and adjust it so the tension is sufficient to pull the derailleur onto the large ring. If the limit screws are set correctly the only thing that can stop the chain lifting onto the big ring is insufficient cable tension. Also make sure the cage clears the outside teeth on the chainring by about 2mm and it’s parallel to it.

 

[Broadside]

Good advice above. I would add that if the old chain ever jammed in the front derailleur while shifting then the FD cage could have widened and bent out of shape causing the issue you describe. You can simply bend/squeeze the side plates back in together to return it to shape, I use wide mouth plumbing pliers/wrench for this.

Can you post a picture looking down at the FD?

 

[raleighnut]

Disconnect the FD cable and move the mech accoss by hand to check the limit screws are adjusted correctly. Then connect the cable and adjust it so the tension is sufficient to pull the derailleur onto the large ring. If the limit screws are set correctly the only thing that can stop the chain lifting onto the big ring is insufficient cable tension. Also make sure the cage clears the outside teeth on the chainring by about 2mm and it’s parallel to it.

Whilst that advice holds true for a double front mech it is wrong for a triple, the only thing that 'indexes' the middle ring on a triple is the cable tension.

The shifter will 'overshoot' the big ring fractionally when pressed but upon release of pressure will drop back to allow clearance of the chain. it doesn't sit hard against the stop when riding. Try it yourself by pushing on the shift lever whilst in the big ring, you'll find the cage rubs but as you release the pressure that will stop.

 

[Jon George]

Cheers people - looks like I have a choice after the bobsleigh between going to the pub this afternoon, or more fettling. Oh, now about to go out for a ride with the old chain temporarily back in situ ...

 

[Jon George]

About five hours of 'investigation' in and I'm still baffled. I have set it up/adjusted/fettled so it shifts delightfully when it the bike is upside down (even better than it did before the problem manifested itself) - out on the road and it refuses to engage. Tweak it a smidgen to force it to engage and it won't disengage. I'm off to the LBS tomorrow for some expert eyes-on advice.

 

[User6179]

About five hours of 'investigation' in and I'm still baffled. I have set it up/adjusted/fettled so it shifts delightfully when it the bike is upside down (even better than it did before the problem manifested itself) - out on the road and it refuses to engage. Tweak it a smidgen to force it to engage and it won't disengage. I'm off to the LBS tomorrow for some expert eyes-on advice.

I had a 10 speed chain that would only work with the mech raised higher up the frame and even then it would only work in the big/middle rings, if I adjusted it to work in the small ring it would not shift to the big ring, Shimano compatibility charts showed the mech and chain was not compatible .

 

[Jon George]

Just like to thank @Smokin Joe , @raleighnut ,@Cycleops ,@Broadside and @Eddy for you responses. I had one last try before an excursion to the LBS and decided to try the new 8 speed chain I had as a spare again. Having not worked before, all the tweaking following your advice has meant this now works.

Incidentally, I was beginning to wonder if it was a worn middle ring and was going to try swapping the crankset I have on Patsy #3 The Hybrid as it is identical to the older one on Patsy #2 The CX and was new at the beginning of Autumn last year. Instead, having sorted the problem on #2, I decided to try the 9 speed on #3 as, it too - I have just discovered - needed a new chain. Works fine ... So, perhaps newer chainrings are in order for #2.

Lessons have been learnt.

 

[boydj]

When I rebuilt a triple it took ages to get the front change working properly. I found the trick was to get the starting point just right with the cable not very tight when on the small ring i.e. just a touch of slack. That's with the limit screws adjusted correctly.