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Old 02-12-08, 02:21 PM   #1
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Chain falling off big ring

OK - here's my problem: I have a Rival compact double (50/34) with Rival FD/RD/brifters. I am trying to do stomps from 50/13 or 50/12 but when I attempt them in this gear combo, after approx. 2 pedal revolutions the chain will come off the big ring. I can do these stomps in 50/14 or higher, but I don't want to do them in an easier gear. I've taken the bike to the LBS and had the FD adjusted. The mechanic showed me that it had a slight bit of play in it and was possibly allowing the chain to come off under pressure.

Well, yea, it's still doing that. It will do it also when I am in 50/12-13 and going downhill. Do you guys have any suggestions as to what I could look at that might be causing this and a solution?

Thanks - wp.
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Old 02-12-08, 02:59 PM   #2
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Are you a real big guy? I suspect either flex in the cranks during your "stomps" or a slightly loose BB.
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Old 02-12-08, 03:07 PM   #3
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Checking for a loose bb sounds like a great idea. I'd also check your chainring bolts.

What's your frame? How 'bout your Q-factor and chainline? What chain have you got?
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Old 02-12-08, 04:43 PM   #4
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no, i'm not a big guy, i'm a gal (guess i should change my avatar back to what it was previously). my frame is an '06 fuji team rc. a new chain was put on in december, shimano 105, 10sp. guess i should look up what the Q-factor is huh? i don't have the tools to check the bb and not sure about the chainring bolts. everything on the bike is Rival except the cassette & chain.
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Old 02-12-08, 04:56 PM   #5
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I cannot apply full power at low rpm on the big chain ring without flexing the crank and dumping the chain to the outside. All of my equipment and adjustments are perfect (for what they are) so I can't sprint "stomp?" on the big chain ring.
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Old 02-12-08, 05:02 PM   #6
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A FD problem will cause derailing during shifts but if the problem happens with pedal pressure, not during shifts, then, as others are saying, you've probably got something loose in the BB or chainrings. Basically increased pedal pressure is causing the big chainring to cock to one side under pressure Standard stuff here: tighten chainring bolts (typically only requires an allen wrench), possibly tighten and/or repair BB. Remotely possible you've got a cracked frame or BB shell.

If the mechanic detected the "play", didn't he suggest how to get fix it? This sounds like one to have the bike shop fix if you don't have the tools or expertise.

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Old 02-12-08, 06:16 PM   #7
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well, the mechanic detected teh play in the FD, and tightened a screw somewhere, not sure where cause i didn't pay close enough attention. i'll check the chainring bolts this evening. i don't think i have tools or expertise to tighten/repair a bb.

remote possibility of cracked frame/bb shell - how would this happen (does it mean i get a new bike?? )? i've not been in any wrecks and the bike is babied.
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Old 02-12-08, 07:47 PM   #8
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remote possibility of cracked frame/bb shell - how would this happen (does it mean i get a new bike?? )? i've not been in any wrecks and the bike is babied.
It's remote so I woudln't worry about it. If there is a problem, the mfg will often step in and get you a new frame. Much more likely to be a worn/broken/misadjusted BB or loose chainrings.

What speeds are you going when you "stomp" on 50/12 gearing?

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Old 02-12-08, 07:55 PM   #9
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If you have a standard FD with your compact crank, it might be too high, allowing the chain to slip through. Check to make sure the FD is only about 1mm above your big ring. You could also see if tightening the high limit screw a little more (assuming that's what the shop did with the "play") helps, but if it causes the chain to not shift into the big ring or rubs during normal riding, that's no good.
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Old 02-12-08, 08:20 PM   #10
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speeds: starting off as slow as i can, around 2-2.5mph up to 20mph, for around 10-12seconds.

FD: are there different ones for sram rival FD's?
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Old 02-12-08, 09:23 PM   #11
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If you have a standard FD with your compact crank, it might be too high, allowing the chain to slip through. Check to make sure the FD is only about 1mm above your big ring. You could also see if tightening the high limit screw a little more (assuming that's what the shop did with the "play") helps, but if it causes the chain to not shift into the big ring or rubs during normal riding, that's no good.
Again, as she describes it, that problem doesn't occur DURING shifting. It occurs AFTER shifting when she just pushes down on the pedals hard in a high gear. This doesn't sound like a FD problem.

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Old 02-12-08, 09:43 PM   #12
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Again, as she describes it, that problem doesn't occur DURING shifting. It occurs AFTER shifting when she just pushes down on the pedals hard in a high gear. This doesn't sound like a FD problem.

- Mark
That doesn't rule out an improperly aligned derailleur. I'm not saying the derailleur is the cause, but if it was aligned properly, it could be the cure. If it's 1mm above the chainring and the high limit is properly set, it can prevent the chain from jumping off. I guess you're right that it doesn't diagnose the problem, though.
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Old 02-12-08, 10:32 PM   #13
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Chains derail for two reasons.

One is when something (a "derailleur") pushes the chain sideways and causes said chain to derail.

The other is if the chain is not happy staying on the particular cog or chainring. A number of things can cause this:
1. Chain line (already mentioned). Put the bike in the 50x12. Is the chain "straight", i.e. on the chainring it'll be "straight". Compare that part of the chain with the part spanning the distance to the cassette. If it angles a lot then you have a chain line problem. A typical result is that the chain derails to the outside (if chain angles outward) or to the inside (if chain angles inward, unlikely for the gear combination you're using). If your chain is dropping to the outside, this is probably the culprit. Flexing frames usually dump the chain to the inside if you're in the small cog, ditto loose chainring bolts.

2. Bent chainring or tooth. If you back pedal slowly, does the chain hang up anywhere on the chainring? Put the chain on the small ring and back pedal the cranks again. Are all the teeth basically lined up, or are there a few that are dramatically "bad"? Keep in mind that some teeth are not straight on purpose (to help the chain derail when the derailleur is pushing on it). This is true for any chainring that has a smaller ring to its inside. This seems unlikely since you'd have chain drops regardless of the rear cassette.

3. Bent plate on chain. Back pedal slowly, watch the chain. It should be consistent, except maybe a master link or a different color pin. The plates should be even, parallel to each other, etc. No twists. This seems unlikely since you'll have chain drops in other gears.

FD adjustments won't matter unless it's off so much it's actually derailing the chain. Play in FD is normal in any non-top-line group, and even top line groups FD have play in them after a while. FD adjustments and play only affect the shifting bit. It does not determine whether or not your chain stays on after you've shifted and the chain is "happy".

Chainline can be affected by loose chainring bolts (just grab a 5 mm wrench, tighten the 5 bolts holding your chainring), loose cranks (grab a crank arm and wiggle it left/right - there should be no clunking/movement), or a flexing frame (frames do crack).

Without seeing the bike, and based on your description of the problem, it sounds like a chain line problem.

cdr
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Old 02-13-08, 06:26 AM   #14
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Thanks for explaining this stuff to me cdr! I might be able to check the chainring bolts today, as I have my bike with me at work today. If it is a chainline problem, is there a way to fix it? What causes this problem? I'm not sure about #2 or #3, how would you get those bent? Like I said, no wrecks and I take really good care of the bike. I will check those as well tho.

Hope I can figure this out (with the help of you guys of course! )
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Old 02-13-08, 09:07 AM   #15
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Q-factor... errm maybe that's a misnomer and I'm passing on bad data. But, basically (and this can affect chainline as well) it's the distance from the center of your bottom bracket shell to the center of your pedal. This also takes into account pedal/crank-to-chainstay clearance and is usually set by the manufacturer. HOWEVER it is possible to shim the bottom bracket w/ 1mm shims to change both the Q-factor and chainline (if the shims are applied behind the drive side bb cup).

as CDR mentioned chainline can also be affected by any looseness in the system (crank, chainring bolts, frame). As a stopgap measure you can also shim the affected chainring with a variety of sizes of shim this will have the effect of moving your chainline on one ring only.

Personally shimming, while it works, seems a little MacGuyveresque to me. Not to say that it doesn't work. But, needing to use shims could be considered a symptom of a compatibility issue in the system as a whole. Without knowing if your bike CAME with the Rival gruppo installed on it I couldn't say if this was a bad pick on the part of the Manufacturer's Product Manager OR if the gruppo was installed after market if THAT has something to do with it. Looking at the Fuji website their listing for the 06 Team RC has a mix of shimano components.

That being said CDR does put it quite rightly, without having your bike physically in front of me it's hard to give a complete diagnosis for your problem.
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Old 02-13-08, 09:22 AM   #16
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I could see it being frame flex plus a FD high limit/indexing that has the chain just ready to pitch over. I get the same effect on my rear cogs when the bike gets a little out of tune - when I apply heavy torque during strong acceleration, I get mystery shifts. Doesn't happen after I tune the bike, and it doesn't happen during normal riding. But when I get my wiggly frame flexing, the extra slack or pressure on the cable is enough to shift on its own.

I can imagine the same thing happening on the FD too. But it could be a number of things. I'd first try the things you can do yourself, like properly adjusting the limit and indexing. Feel for wiggle in the cranks and BB. Drop the chain off the chain rings, spin the crank, and look for any misalignment in the chainring. If you still can't figure it out, take it to a good shop.
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Old 02-13-08, 05:31 PM   #17
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ok, decided to just go ahead and take the bike to the shop to see if they could determine the problem.

FD high limit screw was tightened and more tension put on the cable (i think). then i tried it out. result: chain off big ring again. i asked about a problem in the bb area. well guess what?? wiggle, wiggle. bb is loose. so this mechanic takes the crank off and inspects bearings, says they are ok. main mechanic comes back (gone when i get to lbs) and says to just tighten it up really good (after talking to someone at SRAM i think). i couldn't try it out 'cause it was pouring down rain here, but will try it out tomorrow.

i hope that fixed it! will let ya know tomorrow.
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Old 02-13-08, 07:01 PM   #18
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I always thought these new cartridge BB's were relatively insensitive to how tight they were in the BB shell since you're not preloading the bearings - you're just making sure the cartridge is snug in the BB. In any event, hope it is fixed. If not and the BB is still loose, I suspect you've got a bad BB, regardless of their declaring the bearings are Okay.

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Old 02-14-08, 12:16 PM   #19
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well...that was not the problem. chain still comes off. i'm gonna try another lbs this afternoon after work. maybe the rival cranks are flexy? seems like they shouldn't be that flexy tho...didn't see any cracks in the frame around the bb either. i think the chainring bolts (one or all) may be very slightly loose. will get those tightened as well (no tools at work).
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Old 02-15-08, 02:41 AM   #20
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no, i'm not a big guy, i'm a gal (guess i should change my avatar back to what it was previously). my frame is an '06 fuji team rc. a new chain was put on in december, shimano 105, 10sp. guess i should look up what the Q-factor is huh? i don't have the tools to check the bb and not sure about the chainring bolts. everything on the bike is Rival except the cassette & chain.
Sounds like the FD alignment either height or angle. Usually height is the culprit. Some mechanics set the FD too high. A bent chain ring or loose bolts are another possibility although less so...Those are the only things I can think of. A loose BB would be too obvious. Don't change your avatar if it is your pic. Some guys use women's pics for some weird reason...and as for Q-Factor I have been turning my own wrenches since HS (back when Jesus was a child) and have no idea what Q-Factor is (and don't care). Try reading Sheldon Brown on FD alignment and or Zinn, or another good book on mechanics to see how to do it. If you can find a good LBS with a Wrench from UBI you will probably get a better job done. Good luck.

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Old 02-15-08, 04:27 AM   #21
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I wish that I had this problem. You are just so strong that you rip the chain off when you put the power on. I am proud of you. I once upon a time had a bike that would throw the chain off when I shifted. My fix was to lower the front derailer and to bend the front of the derailer (outside plate) in toward the bike so that the chain would not jump off when shifting. This might be worth a try if nothing else seems to solve the problem. What this does is cause the derailer to hit the chain when the chain rises to come off the big ring unless there is too much force on the chain the derailer will not allow it to fall off the outside. But if there is enough force to push the derailer out of the way then the chain will continue to fall off.
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Old 02-15-08, 04:30 AM   #22
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If you can find a good LBS with a Wrench from UBI you will probably get a better job done.
Sorry, but that's no guarantee. I worked with a guy that was a UBI graduate...and it didn't make him a master mechanic any more than having a driver's license makes one a good driver. No disrespect intended, I know UBI is a good program, but there's more to it than the piece of paper.

If I could look at the bike, things I would be checking for are (and several have already been mentioned) in no particular order:

-Proper FD alignment. Tighten the cage gap from the large ring to barely clear, check rotation, check for a bent cage. If all is well, then adjust stops, check cable condition and cable tension.

-Check for a bent chainring. Look down at the FD while slowly turning the cranks. It should be the same distance from the cage through the whole rotation.

-Inspect chainring for burrs and/or bent teeth.

-Check bottom bracket setup and "adjustment". Although this type of crank really has no adjustment per se other than side loading the bearings, the spindle is a slip fit through a plastic sleeve lining the bearing, and the chainline is determined by how many and where the plastic spacers and seals are added (This varies by crank type, BB type and BB shell width). Even though the bearings can appear to be smooth, the interface can wear causing creaks if dirt gets into the gap, or cause play in the spindle. If the crank is torqued properly yet play is felt when the crank is wiggled laterally, something is worn or not set up correctly. If the issue is the bottom bracket, replacing it should take the play out. If the issue is a worn spindle, unfortunately the fix is to replace the crank (one big weakness of the design and a rough one to tell a client).

-Inspect the chain carefully for twisted or sticky links, or an improperly installed HG pin or master link.

-Check the crank for flex. Stand beside the bike with both wheels on the ground and with the crank at the bottom, push the pedal sideways with one foot. This demonstrates how much lateral flex the frame has as well as crank flex. Excess movement may indicate a crack in the frame. It helps to have a partner watch the chainring closely to see if it moves in relation to the FD cage.

-Tighten the chainring bolts. Remove them, grease or anti-seize the threads, then retorque to specs.

-Check the chainline. I won't go into detail, but the spacers added to the BB cups can change this. Even brand new frames can be misaligned.

-Inspect the FD for worn pivots or other damage. Lube the pivots for smooth operation.

Things I would not check are:

Q-factor. This is the virtual distance between the outside of the pedal faces from one another. This is set by the two-piece crank design and cannot be altered without changing the crank to another type, although one can check the distance of the crankarm to the chainstays visually by revolving the crank. They should be roughly equal and have adequate clearance if set up properly. A low Q-factor is desirable for comfort and efficiency, but has no effect on shifting, chainline does.

My apologies wolfpack for the mistaken gender and the short novel I just published...lol
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Old 02-15-08, 07:13 AM   #23
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Do I take it that the chain is slipping off the outside of the chain-ring? I just had a bike in with that very problem. It turned out to be the chain-line. The manufacturer had fitted a BB with an axle which was too long giving a dreadful chain-line. This was fine and worked while the chain and chain-ring were new the chain remained on the ring but, when they wore, the extreme chain-line caused the chain to fall off to the outside. I had to shim the block and fit a shorter BB to fix the problem.
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Old 02-17-08, 10:49 AM   #24
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Status update, please. Inquiring minds want to know!
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Old 02-17-08, 05:31 PM   #25
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ok, here's the requested update:

the last lbs i went to, the guy has probably 30yrs of experience...he likened what what happening to trying to start your car off in 4th gear (stick shift) - it's going to chug, chug, chug and not do well. pretty much it's too much torque/power to try and start from a very slow speed (2-3mph) in my 50/12-14 gears. if i want to do these stomps, i should probably be going a little faster and it may not fall off to the outside (but it might). my 50T ring is also slightly worn, with about 3200mi on it in the past 10mo.

not really anything i can do about it i think, except to not do these from close to trackstand...i still need to check the chainring bolts and some of the other stuff mentioned above....
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