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Chain Coming off Randomly / Front Derailleur
#1
HI Guys,.. Got a real nice Trek 7.4 from craigslist the other day and the bike does ride like a dream... at least from what I'm used to (Wal-mart!)... But it does seem to have a gremlin that I can't figure out.  The chain will randomly come off the front gear set to the inside.  I've checked the inner limit setting and it's set so that there's just barely  any clearance between the chain and the derailleur when both front and back gears are set to the lowest gear.  It won't do it everytime,.. I don't see how it's even coming off, but it's happened 3 times in 2 days and it's driving me nuts.  Back gears seems to be shifting ok... front shifts fine too except this problem...  Might be worth noting every time this happened, the rear was in 1st gear,.. but I don't see why that would be a problem,.. any help would be great.  Thanks  
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#2
Check for worn/bent chainrings, chain, loose Bottom bracket. A stretched or worn chain would be the first and least expensive place to start. If chain replacement does not do it. I would pull the crank and make sure it has the proper length Bottom bracket installed. Someone may have popped in a grossly long one just to sell it.
There are two kinds of people in the world, "Those who help themselves to people, and those who help people!"
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#3
OK,.. thanks,.. I'll check. I doubt if anything's worn, I think it's only been ridden a few times.. but worth a look
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#4
Check the height of the front derailleur also. If it is really high, it may allow the chain to drop even if the inner limit is set right.
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#5
(09-07-2015, 04:17 PM)DaveM Wrote:  Check the height of the front derailleur also. If it is really high, it may allow the chain to drop even if the inner limit is set right.

I did not take it that it was happening during the downshift Dave. I think he means it just happens while it is on the small ring and he never knows when it will do it.
There are two kinds of people in the world, "Those who help themselves to people, and those who help people!"
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#6
I did check that... Good idea... I'm not a bike mechanic but am a fast learner and do have a mechanical background... Is the proper height of the derailleur about 1-2 MM above the large chain ring ? I believe its already there... Don't think I could lower it hardly at all... I did look at bottom bracket and all seems good there.. Chain rings are straight... The problem is so random ... Its driving me nuts

(09-07-2015, 04:26 PM)painkiller Wrote:  
(09-07-2015, 04:17 PM)DaveM Wrote:  Check the height of the front derailleur also. If it is really high, it may allow the chain to drop even if the inner limit is set right.

I did not take it that it was happening during the downshift Dave. I think he means it just happens while it is on the small ring and he never knows when it will do it.
No.... Happens when downshifting from middle to small chain ring
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#7
Derailleur height sounds fine, 2mm above big ring is correct. roughly 1 to 1.5 from inside of cage when chain is on the lowest gears. Make sure lateral runout of the chainwheels is not excessive. If good I would replace chain with a different make than the on you have. i.e. if you have a Sram try a kmc or visa versa. Sometimes certain combinations while sounds good on paper may not always play well with each other. Also a chain swap is a fairly cheap place to start.
There are two kinds of people in the world, "Those who help themselves to people, and those who help people!"
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#8
(09-07-2015, 04:46 PM)painkiller Wrote:  Derailleur height sounds fine, 2mm above big ring is correct. roughly 1 to 1.5 from inside of cage when chain is on the lowest gears. Make sure lateral runout of the chainwheels is not excessive. If good I would replace chain with a different make than the on you have. i.e. if you have a Sram try a kmc or visa versa. Sometimes certain combinations while sounds good on paper may not always play well with each other. Also a chain swap is a fairly cheap place to start.

Interesting idea about the chain... I know i'm going to sound like a dunce... but what do you mean by chainwheel?? I can imagine the lateral runout is how true something is??
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#9
Chainwheel is your front crank/rings. from looking down from above, the side to side float while spinning would be lateral run out, how true or untrue they are. Like a warped disc brake rotor on your car.
There are two kinds of people in the world, "Those who help themselves to people, and those who help people!"
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#10
That's what I thought you meant. I already looked at that,.. OF course I didn't use a caliper,.. but i doubt it's that critical! Visually they look straight... I'll look into the chain idea, that's a possibility
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#11
If is was bad you can detect with the naked eye. You can play with derailleur height to see if it helps. sti type shifting really slaps the chain down sharply. Not all chainrings are created equal either. I think a chain swap is the right thing to do from what you are describing and the checks you have made.
There are two kinds of people in the world, "Those who help themselves to people, and those who help people!"
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#12
(09-07-2015, 05:37 PM)painkiller Wrote:  If is was bad you can detect with the naked eye. You can play with derailleur height to see if it helps. sti type shifting really slaps the chain down sharply. Not all chainrings are created equal either. I think a chain swap is the right thing to do from what you are describing and the checks you have made.

yea it does click pretty fast, ... thanks for the advise. I'll try a chain and see what happens. The bike looks basically brand new, so I was quite surprised there was any issues like this,.. but it's my baby now,.. I have to figure it out...
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#13
Please post back what you come up with, good luck!! Just so you get your chain length proper if you go that route. Never lay the new chain side by side along with the old chain to measure and cut. You will have many people give you this advice,IT IS BAD ADVICE! Reason being you will be assuming it was right to begin with and it may well be, however it may well be wrong too. Never assume anything. learn to size your new chain from scratch. Simply place the new chain on the Big ring in front and the big cog in back (do not run it thru the rear derailleur yet). pull tight and add one link.(one inner, one outer plate,= 1 link). if you are using a quik link connector you must include that as half of your link. make your cut accordingly. Doing right the first time will keep you from having multiple connections on one chain (especially the new chain). once cut, thread thru derailleur, pull together and connect. if you cannot fully engage a quik link. turn crank until quik link on top and snap the crank forward and it should lock in place.
There are two kinds of people in the world, "Those who help themselves to people, and those who help people!"
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