Chain dropping from the middle front ring to the small

[Bodhbh]

Using a hybrid triple on my commuter MTB (48-38-36) with a 34-11 cassette. It happens mainly while using the biggest 3 or 4 sprockets on the cassette, and the front mech is not rubbing. If I tighten the cable to bring the front mech closer to the middle ring (i.e. to block the chain dropping) it seems to offer enough resistance sometimes but always goes in the end.

I'm wondering if it's all about the chainline, as the front mech is not playing a part. I use an external BB with 2 x 5mm spacers on the right and 1 x 5mm spacer on the left. Thinking to swap them around and see what happens. Any other ideas?

 

[Rob3rt]

The only time I have had such issues was when the chainring was worn and it couldn't keep hold of the chain at extreme angles and it would fall off the big ring and either land on the small ring but grinding against the mech, or it would dump it completelly. What is the chainring looking like?

Possibly not the cause, but as it is a simple check (visual), it is worth having a look.

 

[Bodhbh]

The only time I have had such issues was when the chainring was worn and it couldn't keep hold of the chain at extreme angles and it would fall off the big ring and either land on the small ring but grinding against the mech, or it would dump it completelly. What is the chainring looking like?

Possibly not the cause, but as it is a simple check (visual), it is worth having a look.

The middle ring is pretty worn. From memory the problem has always existed and I've worked around it, but you might be right and it's just making it worst.

 

[Graham]

Worth checking that all the chain carrying parts are pointing straight up/down the bike. My thinking is: If the cassette or derailleur cage is 'pointing' towards the spokes, then as they move up towards the bigger sprockets, they will start to drag the chain off your middle chain ring. Similarly, if the chainring is not pointing straight that might also make the chain jump off. Probably not the cassette at fault, although if you've always had the same problem then maybe something was wrong from when the bike was put together.

If you have a 9 speed cassette, then I think sprocket 5 (middle one) should be directly behind the middle chainring (assuming all the others are spread evenly).

Failing that, sounds like your suggestion is a good one (unless you need the extra 5mm to clear something else!).

Let us know how you get on.

 

[RecordAceFromNew]

Using a hybrid triple on my commuter MTB (48-38-36) with a 34-11 cassette. It happens mainly while using the biggest 3 or 4 sprockets on the cassette, and the front mech is not rubbing. If I tighten the cable to bring the front mech closer to the middle ring (i.e. to block the chain dropping) it seems to offer enough resistance sometimes but always goes in the end.

I'm wondering if it's all about the chainline, as the front mech is not playing a part. I use an external BB with 2 x 5mm spacers on the right and 1 x 5mm spacer on the left. Thinking to swap them around and see what happens. Any other ideas?

Am not sure what chainset/bb you have, but if Shimano then their road/hybrid chainsets generally require no spacer either side and deliver a 45mm chainline, so if one puts 2x5mm spacers on the drive side then it would have given rise to a 55mm chainline which is WAY too much even for a mtb (which is typically 47.5mm or 50mm for a Shimano system). In fact I would be worried about running out of thread on the bb cups.

BTW what is your chainline (i.e. middle of frame to middle of middle ring)?

 

[Bodhbh]

Am not sure what chainset/bb you have, but if Shimano then their road/hybrid chainsets generally require no spacer either side and deliver a 45mm chainline, so if one puts 2x5mm spacers on the drive side then it would have given rise to a 55mm chainline which is WAY too much even for a mtb (which is typically 47.5mm or 50mm for a Shimano system). In fact I would be worried about running out of thread on the bb cups.

BTW what is your chainline (i.e. middle of frame to middle of middle ring)?

Sorry, I was a bit careless with the terminology - I am using a Deore ML590 mountain triple. Just as it's a 48/38/26 you'd call it a hybrid or trekking chainset. I didn't think to measure it the chainline tho - I will double check it.

 

[RecordAceFromNew]

I am using a Deore ML590 mountain triple.

If you are using that Deore chainset then you should use two of the 2.5mm (not 5mm) spacers supplied on the drive side and the remaining one on the non-drive side only if a) your BB shell is 68mm wide and b) you are not using a E type front mech, to give you a 50mm chainline.

 

[Bodhbh]

In the end, I just replaced the drivetrain... the middle ring was pretty sharkfinned and so where several of the sprockets on the back. The front mech shifter got jammed up with the cable - so just swapped that out with a spare too. Apart from the orginal issue, was getting chain suck, gears slipping at the back and the cables were pretty gone. So just did a big overhaul at the weekend. Not ridden in anger yet, but I would hope the orginal probably is gone.