Chain jumping from highest gear under full load....

[chris_c201]

Hi all.

Struggling to find anything online and don't really want to bother lbs just now but....

Last couple of weeks 3 times now I've went to really hammer it on a segment (as in full sprint) and the chain has popped off the top ring.... Thought I'd snapped it first time.

It only happens when in big ring front, smallest cog on back and when trying to push flat out, if I push in any other gear or ride hard in that gear (not full sprint) it's fine.

After it happened twice last I fully degreased my drivetrain and checked components and everything looks fine including indexing, derailleur, jockey wheels etc. It's the original crank (cannondale synapse so fsa) that's done roughly 1700 miles and teeth look completely fine and the rear cassette and chain (shimano 105 genuine parts) has done about 200 miles.... Only thing I'm unsure on is although the cassette and chain is new but run in, not sure I've ever really went full power in those 200 miles....

Any thoughts? Only thing I could think is either the chain doesn't have enough tension either due to an extra link or the derailleur isn't taking the slack enough.... Neither appears obvious...

Its not the end of the world and if I wanst bored and being childish to hunt some local segments it would make no difference to my day to day riding.... But I am bored.... And lockdown has let me train quite a bit so I want to get up the leaderboards before I lose the fitness 😂😂

Any thoughts or suggestions appreciated!
Chris

[Justin S]

How many teeth is the chain surrounding the rear sprocket ? With a large chain ring and small sprocket, can cause the chain to only cover about 4 or 5 teeth. Maybe adjust the B tension screw a little to see if it moves the jockey wheel a little to cover another tooth , as it could just be jumping up.

[Mr E]

Bottom bracket flex?

[TheInternet]

If it's dropping off the chainring towards the pedal then I'd check the front derailleur is installed/set up correctly.

[TheInternet]

Mr E said:

Bottom bracket flex?

I'd expect flex/play to have a part in it at 'full power'.

[phil-sti]

Chain line isn’t correct and worn teeth on rear cassette.

[chris_c201]

Hadn't thought of bottom bracket flex.... There's no noise or flex that I have noticed but I'll check more closely.

Not sure how many teeth it covers but will have a wee look in the morning.

Rear cassette is fairly new and no visible signs of wear or damage....

Thanks for inputs so far, gives me a few more things to check out tomorrow!

Cheers
Chris

[chris_c201]

TheInternet said:

If it's dropping off the chainring towards the pedal then I'd check the front derailleur is installed/set up correctly.

I'm not 100% if where my legs are going at the point it pops (max power and hard to say for sure) but think there is a chance its as I'm pushing with my right and pulling with my left and the chain pops off to the right.

The front derailleur looks OK, shifting under normal conditions completely fine and I did dial it back in slightly after it happened first twice...

[gazza285]

Not known as the stiffest chainrings, or the hardest wearing. Can you pull the chain away from the front of the chainring much?

I have two bikes with FSA cranks on, and neither have FSA rings on due to their rapid wear.

[Mr E]

TheInternet said:

Mr E said:

Bottom bracket flex?

I'd expect flex/play to have a part in it at 'full power'.

I suspect it depends on number of directorships held?

[flight147z]

Did you change the chain when you changed the cassette?

A new cassette and an old chain isn't usually a good mix

Other thing to check would be the outer limit on the front derailleur. The outer limit should be set such that in the top trim position the outer plate is as close to the chain as possible, if that is set right there isn't any room for the chain to jump to the right

[chris_c201]

flight147z said:

Did you change the chain when you changed the cassette?

A new cassette and an old chain isn't usually a good mix

Other thing to check would be the outer limit on the front derailleur. The outer limit should be set such that in the top trim position the outer plate is as close to the chain as possible, if that is set right there isn't any room for the chain to jump to the right

Yes new chain at same time as cassette. The adjustment of the front derailleur was the first thing I done thinking the same thing... Will potentially dial it in a little further to ensure its as far in as it can go without rubbing...

[chris_c201]

gazza285 said:

Not known as the stiffest chainrings, or the hardest wearing. Can you pull the chain away from the front of the chainring much?

I have two bikes with FSA cranks on, and neither have FSA rings on due to their rapid wear.

Don't quite follow the question on pulling chain away?

Interesting point about the rings, the teeth look ok but might consider changing the ring depending how other tweaks go...

Thanks again for all input.

[flight147z]

chris_c201 said:

Yes new chain at same time as cassette. The adjustment of the front derailleur was the first thing I done thinking the same thing... Will potentially dial it in a little further to ensure its as far in as it can go without rubbing...

If you can't get it perfectly right then it's better to leave it rubbing a little when you are in the big ring at the front and the smallest cog at the back (given most people don't use that combination a lot)

Make sure your front derailleur is perfectly parallel before adjusting anything else otherwise you are wasting your time and you won't be able to eliminate rub across the cassette however much you tweak the limit screws

[gazza285]

chris_c201 said:

gazza285 said:

Not known as the stiffest chainrings, or the hardest wearing. Can you pull the chain away from the front of the chainring much?

I have two bikes with FSA cranks on, and neither have FSA rings on due to their rapid wear.

Don't quite follow the question on pulling chain away?

Interesting point about the rings, the teeth look ok but might consider changing the ring depending how other tweaks go...

Thanks again for all input.

[chris_c201]

gazza285 said:

👍 I'll check that this afternoon

[BOR]

Did you remove enough links from the new chain to get the same length as the old one ?

Take a look at the rear mech. Is the tensioner pulley at the end of its travel ?

[chris_c201]

I put it into LBS for the chain and cassette replacement so not 100%...

[chris_c201]

Done a few further checks and adjustments...

- Dialed in the adjustment in the front derailleur so it's tighter to the chain in big ring but not making contact... To answer one of the other questions it runs parallel to chain /crank so appears to be correctly aligned
- Tried pulling chain away from crank and there was a little daylight, adjusted the tension screw on rear derailleur and that appeared to pull it in a little

There is no play in the bottom bracket but appreciate me moving it about by hand is quite different to the forces its under during full pedaling.

I'll get out a few miles later and make sure running OK then maybe try a couple of sprints and see how it goes 👍

[Harpoon]

I did a chain and cassette change in March on my oldest road bike which now lives on the turbo. I can't remember exactly why I changed it, I think I was getting a bit of chain slip.

After the change the slip was even worse. Anything other than easy spinning on the big ring made horrific crunching noises and I expect a full sprint effort would have launched me across the garage. A new big ring on the chainset sorted things out. Strava's gear tracking was quite useful as it showed the original Ultegra ring had done just over 13,000km. Ultegra rings are bloody expensive though, so I ended up going with a 105 5750 replacement and putting up with a ugly looking chainset to save £50.