jdonzella

Guys,

I am having a real crisis. I have a trek 5200, from the day I bought the bike, when shifting from the big to small front rings the chain would be thrown off the inner ring. This could happen even when I was in the middle coggs on the back and big ring. I had mechanics look at it and tell me it was adjusted right, so I leaved with it shifting very lightly. I thought I would uprgrade to a new crank FSA team carbon and get a new chanin and cassett ( all DA 9). No luck, then I went to a new DA 10 speed FD. Here I am still riding cautiously when sHifting and still throwing it off. I have cheeper beater bikes with sora components that I feel more confidnet with. Any suggestions, or is this bike doomed. Maybe the fram is a defect or the chain needs some links added. I can't pinpoint the problem, but it is very frustrating and makes racing difficult because I am riding gears that are not as comfortable for fear the chain might go off when shifting. Thanks!

superdex

Change mechanics. Sounds like the front derailleur needs some adjustment. Check this out --

https://www.parktool.com/repair_help/...railleur.shtml

MichaelW

Check the derailleur for height abve the large ring (1-2mm), and for alignment of the mech along the axis of the bike.
And get a better mechanic.

Kayakado

It sounds like an adjustment but you can get a device called a dog tooth that will help prevent the chain falling off. It is a curved canine shaped piece of plastic that attaches to the down tube at about the height of the top of the inner chain ring and is a mechanical aid to keep that chain from coming too far off the teeth (it doesn't touch the chain)

jdonzella

good to know! Thanks

EagleEye

Ditto. Unless you decide to tackle this on your own, you need an new/better mechanic.

spindog

Okay, I am certainly not a mechanic but had a similar problem that took my LBS an entire season to solve. (Don't get me wrong - they tried their hearts out but couldn't find a problem).

I have a Cannondale 2000si and since brand new, I constantly dropped the chain. I could not identify any specific gears or under a certain amount of load - but the chain constantly fell off. I never had a problem with any other bike and this drove me nuts because I was riding centuries every weekend and dropping the chain a minimum of 5-8 times during my rides.

My LBS was perplexed. They changed cassettes and pretty much rebuilt the bike with brand new components by the time we went back and forth week after week with the problem unsolved.

Finally a Cannondale rep came to inspect the bike and he determined that the frame wasn't straight -that it was off a tiny bit in the rear. My LBS had checked the frame for straightness from the get-go but didn't find a problem according to their measurements.

Cannondale supplied me with a new frame and I NEVER dropped the chain again - over 3,600 miles ago.

jdonzella

Nice!, I am afraid of that! I will look into it. Thanks

neil0502

For a little deeper understanding, let's turn to the Book of Sheldon, Chapter 10, Verse 3:

https://www.sheldonbrown.com/chainline/

If it's not that (low %age), here's Sheldon's take on adjusting your FD:

https://sheldonbrown.com/derailer-adjustment.html

You must--I repeat: must--report back and let us know what happened.

Good luck!

Retro Grouch 

Since this problem is persisting despite all of these new parts I have to think it's an adjustment issue. Since I assume the normal adjustments have already been done, try this: Dial out your HIGH gear limit screw about 1/8 turn. That just might fix it. Here's why I think it might work.

Lots of mechanics set cable tension and high limit screw right up against each other. That puts the derailleur into a bind like the lock nut on a hub. Now, when you try to shift into the small chainring, the derailleur initially binds then it shifts so hard that momentum carries the chain over the small chainring in spite of the low gear limit adjustment.

Can't hurt to try.

cheg

This will cure it as long as your downtube is round. It allows you to overshift a bit going to the small ring to make it more positive and not worry about loosing the chain.

neil0502

Nothing wrong with that sort of product, but in many cases, it addresses the symptom and not the problem.

If I were the OP, I'd get the FD dialed in perfectly, then see what happens. If he still has the problem, then I'd look at chainline/frame alignment. Only when I'd fixed the problem would I put on an "insurance gizmo."

Just my $0.02

cheg

Whatever floats your boat. In my experience, "dialed in perfectly" is a temporary condition. I don't like not downshifting reliably any more than dropping the chain and 50 miles of chipseal always does one or the other.

neil0502

Fair point, though--in your case--it sounds like you've got the adjustment and chainline issues well in hand, at least to start with.

My point was that, if there is a relatively major problem with the OP's ride (e.g., significant frame alignment/bb installation issue), maybe it's a better idea to uncover it and, if possible, resolve it first.

YMMV....

Corsaire

After you have tried evrything advised above, install a small gizmo which I have on my own bikes for few pocket dollars, my chain never came off again:

chain watcher by third eye

https://www.speedgoat.com/product.asp...at=310&brand=3

Corsaire

Portis

I had a similar problem on my MTB for awhile. I had a Bottom Bracket with too long of spindle in my bike. It made the chainline so poor that there was no way i could adjust the front der to get a good chainline. Despite my best efforts i could never adjust it to keep the chain from throwing off the granny ring.

I decided to get the proper spindle length for my bottom bracket and poof the problem was gone. Make sure your BB spindle is the proper lenth for your crankset. Consult the manufacturer of the crankset for specs.

sakarias

It is possible for a rider to drop the chain off the inner chain ring with even a properly adjusted derailleur. If you yank the lever to shift to the small ring fast enough the chain will move sideways faster than it will drop onto the small ring (exacerbated by a large difference in ring size on a crankset like the Shimano XT, for example), thus overshooting the ring. This happened to me the first time I shifted to the small ring, this Spring. The first time in two years that I've done that. And, derailleur adjustment was NOT the problem. So, there IS a bit of operator skill involved here, too.

meb

Clarification- by "it doesn't touch the chain" don't you mean it doesn't touch during normal operation but touches the chain while performing its function of preventing the chain from overshooting during downshift?