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Old 10-03-09, 12:55 PM   #1
bobthib
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Catastrophic Rear derailer failure!

Got a BD Immortal Pro CF bike back in March. Probably have about 2K on it, and love every minute. Well, at least until this morning.

I was stopped at a stop light about 6:30 AM on my way for a metric century. The light changed, and as I started to pull away there was this god-awful "sprang" and the chain came off. I hobbled back to the curb, and what do you know. My Ultegra rear derailer was a mangled mess hanging off by the cable.

I've had no problem with the derailer save a minor adjustment after about 300 miles. There was not warning or other indications. I always down shift when I'm approaching a stop, so I was in low gear.

Any idea why this happened? I'm really bummed, and I need to get it fixed by next weekend.
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Old 10-03-09, 12:59 PM   #2
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Likely the limit screw wasn't set right and allowed the der into the spokes. Check the hanger that it was screwed to for damage, it might need to be replaced too. If not, just get another rear der and install. If you don't have tools/knowledge for that, take it in to a shop. I doubt this will be covered as a warranty issue.
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Old 10-03-09, 01:00 PM   #3
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Maybe you kicked it up a gear or two when stopped?
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Old 10-03-09, 01:50 PM   #4
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Maybe you kicked it up a gear or two when stopped?
No, more likely it is what JoelS suggested. Don't know how the limit screw might have gotten out of adj. The only RD problem i've had since I got the bike was a barrel adjustment to cleanup shifting. A bike wrench in our club did that for me after the bike had about 500 miles on it.

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Old 10-03-09, 02:13 PM   #5
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I've had limit screws creep. They do need to be checked periodically.
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Old 10-03-09, 04:29 PM   #6
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I've had limit screws creep. They do need to be checked periodically.
I've never had the limit screws on any derailleur change spontaneously but I did have a rear derailleur ( a week old Ultegra 9-speed! ) destroyed when the chain grabbed a piece of steel wire thrown up by the front wheel and ran (or tried to anyway) it through the derailleur. Fortunately, only the derailleur was trashed and the dropout was ok.
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Old 10-03-09, 04:32 PM   #7
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Don't the adjustment screws come backed-up with hard plastic "lock washers" that prevent unintended movement? At least my Sachs Rival has them.
Anyway, I'd take a serious look at the hanger. Forces needed to bend a derailler will definitely affect the hanger.
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Old 10-03-09, 05:36 PM   #8
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I've never had the limit screws on any derailleur change spontaneously but I did have a rear derailleur ( a week old Ultegra 9-speed! ) destroyed when the chain grabbed a piece of steel wire thrown up by the front wheel and ran (or tried to anyway) it through the derailleur. Fortunately, only the derailleur was trashed and the dropout was ok.
I'm with HR, something got jammed in there and broke it.
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Old 10-03-09, 06:21 PM   #9
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I had a Shimano RD break off due to a flaw in its casting in the area of the attachment bolt. My LBS expert said he'd seen that once or twice before for Shimanos from the mid-90s.
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Old 10-03-09, 08:12 PM   #10
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You may have dumped the chain when you down shifted and tangled it in the der. Check the deraileur hanger. It can be bent in that type of accidnet.
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Old 10-03-09, 08:48 PM   #11
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Don't the adjustment screws come backed-up with hard plastic "lock washers" that prevent unintended movement? At least my Sachs Rival has them.
Anyway, I'd take a serious look at the hanger. Forces needed to bend a derailler will definitely affect the hanger.
That's not a lock washer. That's a threaded insert that holds the bolt in place. Without that your derailleur bolt would fly off the derailleur. Most derailleur limit screws have some sort of threadlocking compound on them. They don't usually unscrew themselves. If your rear derailleur runs close to the spokes even when properly adjusted, only a minor derailleur hanger misalignment (from dropping the bike on its side) may be enough to push the derailleur into the spokes on gear change.
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Old 10-03-09, 11:00 PM   #12
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I'll use the dark and cruel words for what your bike may benefit from if your RD just plain likes running close to the spokes: Dork-Disc. Pizza-Pan. Spoke-Protector. I'm talking about one of those plastic or metal pie-plates that fits between the cassette/freewheel and the spokes. They are designed to prevent just this from happening.

Other than that - get the derailleur-hanger checked - and rechecked anytime you might knock your bike over in the drive-side. And check those limit-screws often. It would be ashame to hurt another Ultegra RD.

As for a new Ultegra, here's a brand new 6500 one for under $40 + shipping:

http://www.cambriabike.com/shopexd.a...EAR+DERAILLEUR
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Old 10-04-09, 08:24 AM   #13
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Indeed the hanger is bent too. It's replaceable. Chain is damaged too.
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Old 10-04-09, 08:54 AM   #14
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Quote:
Originally Posted by bobthib View Post
No, more likely it is what JoelS suggested. Don't know how the limit screw might have gotten out of adj. The only RD problem i've had since I got the bike was a barrel adjustment to cleanup shifting. A bike wrench in our club did that for me after the bike had about 500 miles on it.
That's the tipoff. RD trim doesn't change with use, except sometimes for a slight adjustment, as new cables settle.

Here's what might have happened. At some point, either because of a crash, dropping the bike, or rough handling the RD hanger got bent inward slightly. This threw off the trim, which you corrected, but it also affected the limit screw positions which you didn't correct. End result is with the inner limit now slightly inboard, it just needed a hard shift into low to overshift into the spokes, either the RD itself, or just the chain which jammed against the spokes.

Lesson --- Whenever you suddenly find the trim off, correct both the trim with the barrel adjuster and check the limits. This is especially if the RD doesn't shift crisply into high, requiring the outer limit to be backed off. If the outer limit is too far inboard, probably the inner limit is too.
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Old 10-04-09, 09:10 AM   #15
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My opinion:

1. Bent derailleur hanger. That can happen pretty easily. It's common. Almost always the bottom of the hanger gets bent in. If you try to adjust for the bent hanger by adjusting the cable tension and limit screws, the cage can go into the spokes.

2. Foreign matter in the lower chain run. If a stick or something gets caught in the lower chain run, it catches on the derailleur cage and bends it into the spokes. Back in my mountain biking days even my buds who always bought the good stuff stopped buying XTR rear derailleurs once they realized they were always only 1 twig away from disaster.

I've never encountered limit screw creep although I've never felt the need to mark them some way so I'd know for sure. Frankly, that doesn't sound very logical to me.

A broken derailleur casting would be obvious. I only remember having seen 1 so they're not common.
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Old 10-04-09, 09:11 AM   #16
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Originally Posted by HillRider View Post
I've never had the limit screws on any derailleur change spontaneously but I did have a rear derailleur ( a week old Ultegra 9-speed! ) destroyed when the chain grabbed a piece of steel wire thrown up by the front wheel and ran (or tried to anyway) it through the derailleur. Fortunately, only the derailleur was trashed and the dropout was ok.
I agree. Limit screws don't creep. I've never run across one that isn't tight in the derailer body.

Why when something happens with a derailer, the very first thing that people start talking about is the stupid limit screws? Limit screw problems are the last thing that goes wrong with a derailer. Once set, the limit screws will never be a concern again.

bobthib, I assume that when you say the "chain came off" that it came off the chainring. If this is the case and the drivetrain was under tension, the sudden release of pressure on the chain could have caused the spring in the derailer to go backwards. If you were under a fair amount of tension, such as taking off from a light, the release could be violent enough for the derailer backlash to carry it up into contact with the cassette. Once there, the derailer could be twisted around the cassette and mangled.

It sounds complicated but if you shift a bike on a stand you can see the derailer bouncing around a bunch when you shift off a ring...especially the big one.
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Old 10-04-09, 09:17 AM   #17
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That's the tipoff. RD trim doesn't change with use, except sometimes for a slight adjustment, as new cables settle.

Here's what might have happened. At some point, either because of a crash, dropping the bike, or rough handling the RD hanger got bent inward slightly. This threw off the trim, which you corrected, but it also affected the limit screw positions which you didn't correct. End result is with the inner limit now slightly inboard, it just needed a hard shift into low to overshift into the spokes, either the RD itself, or just the chain which jammed against the spokes.

Lesson --- Whenever you suddenly find the trim off, correct both the trim with the barrel adjuster and check the limits. This is especially if the RD doesn't shift crisply into high, requiring the outer limit to be backed off. If the outer limit is too far inboard, probably the inner limit is too.
Rear derailer trim doesn't change with use? It changes all over the place as the cable stretches. That's what the barrel adjuster is for...to adjust the derailer trim so that the derailer doesn't try to hop between gears.

If he had a bent derailer prior to this problem, his shifting would have been poor. He wouldn't have been able to shift into some gears and the bike would have chattered on each shift.

bobthib didn't say where the chain came off but if it had come off into the spokes, the damage would have been far more than a twisted derailer. A chain jammed between the wheel and the cassette is a memorable event and not likely to be reported as 'the chain came off'.
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Old 10-04-09, 09:20 AM   #18
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Quote:
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As for a new Ultegra, here's a brand new 6500 one for under $40 + shipping:

http://www.cambriabike.com/shopexd.a...EAR+DERAILLEUR
Yes!!! Based on your earlier posting I ordered one of these from Cambria and it came a couple of days ago. Thanks for the lead as it will replace a very high mileage 105 rd on one of my bikes during this winter's overhaul.

The 6500 rd destroyed in the incident I described above died (or was assassinated) several years ago so this isn't a direct replacement.
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Old 10-04-09, 10:37 AM   #19
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Thanks for all the feedback and ideas. After drowning my sorrows in a bottle if wine (whine?) last night, I gathered up all the evidence and in the sober light of day, I pieced together an explanation.

I had just wiped my brow as I sat at the light. I had a small 6" x 6" glass cleaning rag in the same pocket as my hankey. Apparently when I pulled out the hankey the small rag fell onto the chain or somewhere and eventually found it's way into the RD.

The RD pulleys got sucked into the rear wheel. The RD broke in half as the 2 pivot pins were ripped out of the housing. A spoke is bent and the chain has a bent link, and the hanger is bent.

It was dark, but the intersection was well lit, and I didn't see anything in the road except that small rag. I picked it up and stuffed it in my pocket again. It wasn't until this AM that I looked at it. It's chewed up, and has chain marks and grease on it. A smoking gun!

So HR and DD and who ever else suggested a foreign object you were right on. Bottom line: be careful what you carry in your pockets, and don't carry small loose objects. Funny thing is, I ALWAYS put that little rag in the ziplock bag with my phone so it stays dry. The one time I didn't.....



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Old 10-05-09, 05:06 PM   #20
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Makes sense, get it back together and go ride!
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Old 10-06-09, 07:38 AM   #21
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The wrench in my bike club just called. He's got the bike fixed and I can pick it up tonight. Replaced the RD with a identical used one, straighten the RD hanger, and replaced the bent link in the chain, and trued the wheels. $75 out the door.

My only concern is a chip in CF the seat stay. I'm thinking I should get that fixed, or at least looked at. What do you think?
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Old 10-06-09, 09:06 AM   #22
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Is the chip a superficial scratch in the clear coat or did it actually gouge the structural carbon? The answer to that is important.
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Old 10-06-09, 01:21 PM   #23
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It is a little more than superficial. I get the bike back 2night from the wrench. I'll ask him what he thinks, but he's a bike mechanic, not a frame builder.
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