front chain rings

[jman69]

Hi there, just a quick question . When i''m riding and want to change the front chain ring from the small one to the big one (two only on the front)it moves over ok but always drops back to the small one. Why is this happening? and what can i do to prevent it?? Thanks in advance.

 

[roubaixtuesday]

You probably need to adjust the cable tension and/or upper limit screw.

Park tool have an excellent article and video on this.

https://www.parktool.com/blog/repair-help/front-derailleur-adjustment

 

[Punkawallah]

What type of gear change mechanism does the bike have?

 

[Sharky]

A couple of thoughts ...
Is it the front mech moving back by itself, causing the chain to drop onto the small ring?
or
Is it alignment of the chain causing the chain itself to drop onto the small ring?
Does it happen when the chain is on all the sprockets at the back or just the larger ones?

 

[jman69]

Hi Sharky, the mech seems to be returning back to the small ring on it's own. The front derailer is a Shimarno RX100 (two rings) and the rear is a Shimarno RSX (8 Cogs)

 

[Ajax Bay]

need to adjust the cable tension and/or upper limit screw.

This ^^^

 

[T4tomo]

A couple of thoughts ...
Is it the front mech moving back by itself, causing the chain to drop onto the small ring?
or
Is it alignment of the chain causing the chain itself to drop onto the small ring?
Does it happen when the chain is on all the sprockets at the back or just the larger ones?

if the latter its just a set up issue with cable tension and/or upper limit screw as others have pointed out.

if the actual mech cage is moving without you touching the level, its a bigger issue and potentially your mech is knackered.
the only time I have come across that is with down tube friction shifters that weren't nipped up tight enough, but I assume you have modern indexed shifters?

 

[jman69]

youre right,they are indexed shifters. On the weekend i'll have a go at thightening the cable and adjust the stop screw and see if that helps. Other wise??

 

[Sharky]

If the front mechanism is moving back to the inner by itself, the stop screw can't be at fault.
The front mech is spring loaded to return to the inner ring, but the clutch mechanism in the lever should hold the cable to stop it doing so.

So my thought would be there is something in the lever that is not quite right. Could be broken or stuck. If the latter, a good squirt of wd40 might help.
The cable should be tight when on the inner ring, but as you can move the FD to the big ring , it can't be that slack.

Is it a new cable? Could it be that it wasn't routed correctly?

Hope you fix it - good luck.

 

[Ajax Bay]

If the front mechanism is moving back to the inner by itself, the stop screw can't be at fault.

Beg to differ. As @roubaixtuesday implied as the immediate answer:
* if the upper limit screw is too 'tight' (screwed in) then the STI will never make it to the correct 'index' position (over the large chainring) and when the lever's released the cage will just drop back left (see symptom described in OP), or
* if the upper limit screw is not preventing the FD sending its cage right then the cause is inadequate cable tension. This time the STI does click but the release of the lever allows the cage to fall back left (a bit, but enough to unship the chain down/back to the small ring).

 

[Sharky]

Both real possibilities above. Let's hope so. Will be interested to know the outcome.

 

[boydj]

youre right,they are indexed shifters. On the weekend i'll have a go at thightening the cable and adjust the stop screw and see if that helps. Other wise??

There should be a little slack in the cable when the chain is on the inner ring. Even with the stop screws set correctly, a too tight cable will pull the chain back to the inner ring.

 

[Ajax Bay]

There should be a little slack in the cable when the chain is on the inner ring.

Making a habit of this: beg to differ. When the cage is sitting correctly above the small ring, the cable should be just in tension (so not even "a little slack"). Then when the STI lever is applied the mech will smoothly and surely shift the cage to above the large ring, without having to take up "a little slack". The cable should not be "too tight" but it shouldn't be slack, ever (except at the moment when shifting down when there'll be a moment of slack as the cage springs left.

 

[boydj]

Making a habit of this: beg to differ. When the cage is sitting correctly above the small ring, the cable should be just in tension (so not even "a little slack"). Then when the STI lever is applied the mech will smoothly and surely shift the cage to above the large ring, without having to take up "a little slack". The cable should not be "too tight" but it shouldn't be slack, ever (except at the moment when shifting down when there'll be a moment of slack as the cage springs left.

Sorry, but that's not my experience when setting up my triple, which I've done several times now. There's not a lot of slack, but the cable is definitely not tight it's just about 'not in tension'. Of course this may depend on the particular bike and it's only a starting point for the user to get their own setup working properly, because there is a bit of trial and error getting triple front mechs working consistently.