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Old 01-01-13, 01:57 PM   #1
moondog321
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Rear Derailleur life

How long, typically should a rear derailleur last? My SRAM Apex has 12,000 miles on it and the chain will sometimes jump from 50T to the 34T CR. Was wondering if it's the derailleur; chain and chainring is new.
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Old 01-01-13, 02:06 PM   #2
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The pulleys can be replaced, the pins and bushing life depends, as usual , on cleaning and relubrication.

Im still using the Campag Rally I got in 1975.
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Old 01-01-13, 02:15 PM   #3
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How long, typically should a rear derailleur last? My SRAM Apex has 12,000 miles on it and the chain will sometimes jump from 50T to the 34T CR. Was wondering if it's the derailleur; chain and chainring is new.
Except for mtb in very harsh conditions, a rear derailleur should last nearly forever. Eventually the 4 pantograph pivots develop play which degrades rear shifting but it's a slow process. You can't measure in time or miles because it's not a question how far you've ridden but how often you shift, which depends on where and how you ride.

Either way, the condition of the RD has nothing to do with your problem of shifting up front. Without knowing more I suspect that the problem may be warped chainrings (wobble as they turn) or a loose BB which allows the top of the ring move out under the stationary FD causing the shift. Check for BB play by putting the cranks vertical, and pushing either in or out watching for deflection of the chain within the FD cage. If you can cause the chainring to move enough for the chain to touch the cage you have your culprit.
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Old 01-01-13, 09:06 PM   #4
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How long, typically should a rear derailleur last? My SRAM Apex has 12,000 miles on it and the chain will sometimes jump from 50T to the 34T CR. Was wondering if it's the derailleur; chain and chainring is new.
FB is right of course, but you really should not need an expert to tell you that your problem cannot be related to the rear derailleur. As the chain comes forward off the rear cog and then goes directly to the chainwheel the rear derailleur can have nothing to do with whether the chain moves from one chainwheel to the other.
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Old 01-02-13, 12:22 AM   #5
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I would equate a rear derailleur with a Hobbit/Lord of the Rings elf; as long as it does not meet an unnatural demise, it should live for ever (unless it's the parallelogram pivots after about a million shifts).
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Old 01-02-13, 10:13 AM   #6
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I've found significant wear on some of my Campy RDs, after only 15,000 miles. It doesn't mean that they were shot, or usuable, but there was a substantial increase in the axial play at the main pivot, where the RD bolts to the frame.

When new, this amount of play is around .008 inch. I was able to install a .032 inch shim under the C-clip, reducing the play to a better than new .002 inch.

The point is that if you don't look for the wear or have any way to measure it, you won't find any. RDs won't perform like-new forever.

The OP's problem has nothing to do with the RD, however.
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Old 01-02-13, 11:01 AM   #7
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Originally Posted by moondog321 View Post
How long, typically should a rear derailleur last? My SRAM Apex has 12,000 miles on it and the chain will sometimes jump from 50T to the 34T CR. Was wondering if it's the derailleur; chain and chainring is new.
The chain passes over both the derailleur pulleys and the cassette before heading toward the chainwheels. A mis-aligned or loose rear derailleur would likely result in mis-shifts at the rear imho. Not the front. You may want to look at whether there is play in bottom bracket bearings, mis-aligned or not completely tightened chain wheels, bent right crank arm or spider. Or the more likely choice of something going on with the front dr.
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Old 01-02-13, 08:46 PM   #8
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I know it's not about SRAM but I've researched the longevity a little on derailleurs. Some people on the ultra distance forum swear that Shimano LX or XT can last longer than Deore and Deore longer than Alivio etc.

On second hand, someone even said he questioned customers for years and found out Deore needs to be adjusted less often than Alivio. Guess it's somewhat hard to avoid the "you get what you paid for" (unless it's on sale).
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Old 01-02-13, 09:02 PM   #9
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I know it's not about SRAM but I've researched the longevity a little on derailleurs. Some people on the ultra distance forum swear that Shimano LX or XT can last longer than Deore and Deore longer than Alivio etc.

On second hand, someone even said he questioned customers for years and found out Deore needs to be adjusted less often than Alivio. Guess it's somewhat hard to avoid the "you get what you paid for" (unless it's on sale).
Interesting info, but not relevant to the OPs problem, which is clearly a front end problem, having absolutely nothing to do with the RD.
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Old 01-02-13, 09:11 PM   #10
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As with anything mechanical it depends on maintence and proper useage without crashes or abuse.
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Old 01-03-13, 12:16 PM   #11
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FB is right of course, but you really should not need an expert to tell you that your problem cannot be related to the rear derailleur. As the chain comes forward off the rear cog and then goes directly to the chainwheel the rear derailleur can have nothing to do with whether the chain moves from one chainwheel to the other.
Well, it is possible that the OP was backpedaling when the chain jumped from the 50T to the 34T. If the derailleur is sloppy or the derailleur hanger is bent (like if you lay down the bike on the drive side), the reverse chain feed can be off, leading to auto-downshift when backpedaling.
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Old 01-03-13, 01:18 PM   #12
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thanks for the great feedback. i'll check out all your ideas.
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Old 01-03-13, 01:20 PM   #13
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Well, it is possible that the OP was backpedaling when the chain jumped from the 50T to the 34T. If the derailleur is sloppy or the derailleur hanger is bent (like if you lay down the bike on the drive side), the reverse chain feed can be off, leading to auto-downshift when backpedaling.
After rereading the op's post I think there are too many unknowns to figure out what happened. He could have been hammering hard and bb flex could have caused a jump.
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Old 01-03-13, 01:54 PM   #14
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I know it's not about SRAM but I've researched the longevity a little on derailleurs. Some people on the ultra distance forum swear that Shimano LX or XT can last longer than Deore and Deore longer than Alivio etc.

On second hand, someone even said he questioned customers for years and found out Deore needs to be adjusted less often than Alivio. Guess it's somewhat hard to avoid the "you get what you paid for" (unless it's on sale).
Sidetracking the thread a little, XT has ball bearings in the pulleys and Deore has bushings. That would account for a different useful life. Were it me, I'd buy aftermarket pulleys for Deore, and have the best of both worlds.
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Old 01-03-13, 03:18 PM   #15
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I think I've completely worn out one derailleur, and that was an OLD Suntour GT (from the late 70s) where the upper idler pulley wore off all its teeth so the chain couldn't shift anymore. the part was unobtanium so I replaced it with an Alivio.

I do tear my derailleurs down every couple of years, and clean and grease the bushings or bearings.
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Old 01-07-13, 11:33 AM   #16
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Checked the BB and its tight, no bend in chain ring, everything looks true. Yesterday when bunny hopping an object the chain jumped from the 50T CR to the 34T CR. this has happened before. Also, and I know this sounds weird but when shifting up from 3 to 4 on the cogs the chain jumped off the 50T to the 34T. Anymore ideas?
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Old 01-07-13, 12:32 PM   #17
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OK, that's new info. The rear derailleur does have something to do with this- it keeps the top run of the chain tensioned when freewheeling. So, either the springs aren't quite up to the job (possible, but it's unlikely to change- springs tend to work perfectly until they fail unless they're designed badly) or there's some stickage that allows slack on the top run.

It's possible that the tensioning arm pivots in the derailleur need lube, or the freehub needs lube. I'd bet on the freehub needing lube. Take the chain off, give the rear cluster a flick backwards, and it should spin really freely, like for seconds.

But that doesn't explain the front ghost shift when shifting from 3 to 4 cog. How's the chainline? The new chainring might be the culprit.
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Old 01-07-13, 12:41 PM   #18
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I still suspect the crank or FD. Here's how to diagnose and rule it in or out.

Shift to the inner ring, and use alcohol or acetone to completely clean the inner surface of the outer plate on the FD cage. Then paint it with whatever you have, ie. mail polish, touch up paint, or even a dry marker.

Go riding, with the bike on the outer ring, and do not under any circumstances shift to low, or trim the FD inboard. Do whatever it takes to cause the ghost shift, and ideally repeat it a few times. Now, while the chin is still on the inner ring look an the painted outer cage plate for evidence of chain contact (paint buffed off). This will confirm whether or not chain/FD cage contact caused the ghost shift.

It won't tell you why, but will rule in a crank/BB/FD problem in or out so you know where to look.
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Old 01-07-13, 12:50 PM   #19
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indeed, I'm having a hard time picturing how a 50-34 could drop down to the 34 and not IMMEDIATELY shift itself back up to the 50 if the FD is at all well adjusted and functioning properly. put the bike in 34, stop pedaling, and click the derailleur up to the 50 position, and it should be pretty darn obvious the chain would be scraping that derailleur hard until it popped back up.

maybe this problem is brifter or cable related.
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Old 01-07-13, 02:11 PM   #20
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Or maybe the adjustment on the FD is waaa-aaa-aay off, like not parallel with the rings and 1/2" above the top of the big ring or something.
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