Chain falling off front mech

[fraz101]

Its happening maybe once or twice per week, averaging about 100-125 miles per week at minute.

When shifting from big front cog to small approaching a junction or really slowing down quickly and shifting the chain will fall off over the small cog ?

I’ve adjusted the lower limit screw on the front mech a couple times now when the chain is on small front cog, biggest rear cog. But did it again yesterday. I adjusted again a little more inwards last night and did 45 miles this morning without a hitch……

Chain isn‘t stretched and mechs are well lubed and free.

Does this just happen sometimes or is there something I’m missing?

Shifting is perfect in all gears.

 

[geocycle]

It can happen, I was in your position until I fitted a dog fang chain keeper. Probably the most useful £5 spent on a bit of plastic. Once fitted the problem stopped so it must work. Fitting is a bit fiddly and you might need to source a longer bolt than provided.

 

[Paulus]

It can be just that you haven't found the sweet spot of the limit screw. Keep trying a quarter turn at a time until you find it.

 

[presta]

If your gears are well adjusted it shouldn't happen more than occasionally, but if it's bothersome you can fit a chain catcher:

 

[Peter Salt]

Its happening maybe once or twice per week, averaging about 100-125 miles per week at minute.

When shifting from big front cog to small approaching a junction or really slowing down quickly and shifting the chain will fall off over the small cog ?

I’ve adjusted the lower limit screw on the front mech a couple times now when the chain is on small front cog, biggest rear cog. But did it again yesterday. I adjusted again a little more inwards last night and did 45 miles this morning without a hitch……

Chain isn‘t stretched and mechs are well lubed and free.

Does this just happen sometimes or is there something I’m missing?

Shifting is perfect in all gears.

It shouldn't happen on a well-adjusted system. You say you've fiddled with the lower limit screw so that the FD is now more inwards? That's the opposite of what I would do. There should be a fraction of a mm between the chain and the inside plate when in easiest gear.

I'm assuming chain length, RD total capacity, FD position (height, rotation), etc. is all good.

 

[fraz101]

It shouldn't happen on a well-adjusted system. You say you've fiddled with the lower limit screw so that the FD is now more inwards? That's the opposite of what I would do. There should be a fraction of a mm between the chain and the inside plate when in easiest gear.

I'm assuming chain length, RD total capacity, FD position (height, rotation), etc. is all good.

On the 105 front mech there is a black (plastic?) part attached to the left hand of the mech cage, the chain is running on that in small/big cog. There is no gap, I figured if there was any gap I run the risk of it jumping off?

 

[Peter Salt]

On the 105 front mech there is a black (plastic?) part attached to the left hand of the mech cage, the chain is running on that in small/big cog. There is no gap, I figured if there was any gap I run the risk of it jumping off?

It shouldn't be running on it. Try leaving a credit card of space between the two.

 

[fraz101]

It shouldn't be running on it. Try leaving a credit card of space between the two.

Yes it was at that much of a gap but the chain still fell off hence me moving it slightly closer still

 

[Peter Salt]

Yes it was at that much of a gap but the chain still fell off hence me moving it slightly closer still

If the gap was like that then it's not the lower limit that's causing the issue. At lest in my opinion. I would look for problems elsewhere.

Maybe there's too much tension in the cable? When you put your lever in 'max minus half a click' position - do the 'tension indicator lines' align?

 

[fraz101]

If the gap was like that then it's not the lower limit that's causing the issue. At lest in my opinion. I would look for problems elsewhere.

Maybe there's too much tension in the cable? When you put your lever in 'max minus half a click' position - do the 'tension indicator lines' align?

Not sure what that means to be honest.

Will need to look into that

 

[Peter Salt]

Not sure what that means to be honest.

Will need to look into that

Probably because I'm explaining it very poorly.

The procedure:

Put the lever in the 'big ring' position - all the way. And then do the 'half click' down - you know, the click that doesn't make you shift down to the small ring but just moves the FD slightly inwards to make room for a bit of cross-chaining.

When in that position, look at your FD, specifically this part with the two lines I've highlighted with a red and blue arrow:



Are these lines... In-line?

 

[MichaelW2]

Check FD height and alighnment. Is the cage damaged or dented beyond factory original bulges and recesses?

 

[fraz101]

Check FD height and alighnment. Is the cage damaged or dented beyond factory original bulges and recesses?

No it’s all good.

 

[Ajax Bay]

Think we need pictures of the FD cage: one from the side in the large ring, and one from above the FD with the chain in the small ring and largest sprocket.
Maybe the wax on the chain is causing this increase in chain drop.

 

[fraz101]

Think we need pictures of the FD cage: one from the side in the large ring, and one from above the FD with the chain in the small ring and largest sprocket.
Maybe the wax on the chain is causing this increase in chain drop.

It’s been happening since before I started dipping the chain!

Dipping for life btw 😁