Chain comes off front dérailleur

[Payneys]

Please can someone help. I've been doing recreational cycling - completed London Brighton twice and do only around50miles a week. No nothing about mechanics or dynamics of cycling etc. My problem is I just completed my first Sportive (Isle of Wight) and had a disater ( got lost ! Got a puncture ) all stuff I will learn from ! My problem is my chain kept coming off the front derailleur when changing down from the big ring to the small when climbing hill. I have the bike for 4 years it's a Scott speedster s 40 and never ever had this problem before I asked a mechanic at the sportive and he said it might be MY fault as the adjustment looked ok to him - he said change down early before the hills . I tried this ..... Chain came off . Can someone tell me if I am doing something wrong ? I am open to offending remarks about my ability ! I'm used to having people cycle past me often enough so I'm pretty thick skinned ! Its just that if I am doing something wrong why has it never occurred before and yes I do got up hills when training I live by the south downs in Sussex ( ok it's not the Pennines but nothing different to Isle of wight ) Thanks

 

[alfeng]

FWIW. The "mechanic" with whom you spoke may not have explained the situation sufficiently ...

Let me try again by briefly expandng that rather than simply stating that you "change down early before the hills" that[COLOR= #0000ff] BEFORE you engage the larger cogs in the rear that you move the chain to the inner chainring ...[/COLOR]

That is, when the chain is already on one of the larger cogs then the lateral 'tension' on the chain will be toward the bike's centerline & possibly-and-probably amplify the action of the front derailleur ...

Whereas, when the chain is on one of the smaller cogs then the 'tension' on the chain will not be inward and will actually impede the inward movement of the chain to a small extent.

ANOTHER partial remedy is to change the front derailleur to one which is designed for a Triple ([COLOR= #ff0000]this is not an option if your bike has SRAM shifters & derailleurs[/COLOR]) ...

• that is, a larger inner cage will help to shepherd the chain as it moves down to the inner chainring

It is ALSO important to ensure that the actual chain is the right TYPE ([COLOR= #808080]i.e., 9-speed with a 9-speed FRONT DERAILLEUR, regardless of the number of cogs in the rear ... 10-speed with a 10-speed FRONT DERAILLEUR ... 8-speed with an 8-speed, etc.[/COLOR]) ...

• so, if your bike has a 10-speed drivetrain and someone installed too wide a chain, then presumably the stops are set slightly wider than they should be & THAT might excacerbate any flaws in the original engineering specs ...
• et cetera

Hope that helps.

 

[jpr95]

I'd say your front derailleur probably still needs some more fine-tuning on the limit stops. There's only so much a mechanic can adjust with the bike on a repair stand. He may have it working flawlessly on the stand, but that is not the same as shifting while on the fly while you're stomping on the pedals at a low cadence--the bike and its components start to flex and change how things work.

If you pedal with a low cadence, that can cause poor shifting, and also puts far more stress on your chain, cogs and derailleurs. In addition, if there is a large difference in the number of teeth between your large and small front rings, that can make it more prone to throwing the chain. My old bike came with a 52-tooth and 40-tooth set in front. I changed the smaller ring to a 34-tooth ring, and subsequently threw the chain a few times if I didn't shift carefully. I might be able to make it better by some adjustment of the derailleur limit screws, but I don't ride that bike any more, so it's not a high priority.

 

[sitzmark]

Tuning, wear, and technique probably most responsible - maybe in that order. If you have had the bike 4 years prior to problems and haven't had any incidents that bent the cage, then adjustment, chain catcher, and maybe a new chain/chainrings are good places to start.

As chains wear they become more flexible laterally - especially if you run them at extreme angles (big/big. little.little). The front mech is spring loaded and when released with the index shifter, it releases inward with a fairly strong "snap" A worn chain can develop more turbulence during this process and not seat on the small ring, or miss it altogether.

If you can't get the problem tuned out, there are "chain catchers" which mount to the seat tube and prevent the chain from falling inward of the small ring. Rotor has had a nice chain catcher for a while and Sram just announced a similar one to integrate with their braze-on FDs. If your drivetrain is stock, I think it is Shimano Tiagra. Scott uses an FD clamp that works like a braze-on mount so the rotor or Sram catcher might work. Not sure if Shimano offers a chain catcher, but the generic ones will work too.

A new chain that is more stiff laterally may also smooth out your shifting issue. A heavily worn outer chainring could be a culprit, but if you don't have a LOT of miles on then less likely that it is part of the problem. Good luck!