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Overshifting on rear sprocket a fluke?

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Overshifting on rear sprocket a fluke?

Old 10-03-12, 05:35 PM
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immunizer
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Overshifting on rear sprocket a fluke?

So I trashed my rear wheel today when I somehow overshifted my large sprocket while coming to a stop. The ride is a probably mid-80s Schwinn or Panasonic (or something else - it's custom powder-coated, the only decal is Columbus SLX tubing.) It's converted 27 1/4->700c cold-spread from (probably) 128mm to 132.5 to accommodate modern hubs. I'm running a 9sp 12-25 with Shimano Deore XT RD and 600 up front on a 52-42-30. The shifters are Raleigh-branded non-indexed bar-ends. The wheel is an REI special Alex rim (see a previous post for why that was necessary. This post concerns why another one is necessary).

I've got about 4,000 miles on the bike, 1,500+ on the wheel, and 500 miles since I went to the 9sp cassette and chain (the derailleur came with the bike, which was 8sp 11-32 when I got it). I very carefully set the limits when I swapped the cassette, as I didn't want this very thing to happen. I tried quickly shifting into the large sprocket from every chainring and couldn't get it to overshift once. Then I replaced my shift-cable housing and shift cable (I sprung for teflon, which may make me a chump) yesterday. I paid no particular attention to the limits when I did this, although I did verify that the derailleur worked as expected.

I had about 6 miles over two short, harsh trips (20+mph on rough roads) before I discovered a flat today. Turns out my rim-tape shifted - or I wasn't paying attention the last time I installed the tire (hundreds of miles ago) - and the tube was punctured where it meets the rim. So I fixed that, reinstalled the wheel, and called it straight enough when it didn't rub the brake--the same way I've reinstalled the wheel dozens of times.

Today, riding home, after running through the gears several times in traffic, I upshifted as I came to a stop only to have my chain become wedged between the cassette and the wheel. I didn't torque it much, but it was enough to shear off one spoke and severely damage several others. Once I unjammed the chain (carefully), I rode the bus home. Now I get to rebuild the wheel or buy a new one.

Once I got home, and without removing the wheel, I worked through the gears again. I shifted as quickly as I could from the small sprocket to the large one. I could not get it to overshift, or even come close! The best I did was at one point the chain didn't settle perfectly on the teeth at first, but it quickly dropped into position. When I originally adjusted the limits, I began in a position that would not allow the derailleur to attain the largest sprocket, and slowly relaxed the limit until it would just get it. I've made no changes to the limit adjustments since then. The only change is to the cable and housing.

So here's the question: how the hell did this happen? Did I somehow put my wheel on crooked? If so, why can't I duplicate the overshift? My understanding of how the limit adjustments work makes them independent of the cable: the derailleur will move only so far, no matter what happens to the cable. Am I wrong?

Here's what I think may have happened. I'm in the habit of shifting to my big sprocket in one fell swoop. I think that when I do this the chain develops a fair amount of lateral momentum. Until recently this wasn't a problem, as the housing was old and there was plenty of friction. But I just replaced these things with very low-friction components (if you believe the marketing on the Teflon cables). So the shift now happens even faster, meaning momentum is greater than it once was. It's also possible that I hit a bump at just the wrong time, which exacerbated the problem. Is this explanation plausible? Or is it just bull**** I'm telling myself, and really I'm just incompetent. (But please be constructive!)

Thanks for looking, and thanks for any thoughts you may have!
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Old 10-03-12, 05:56 PM
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Set the low limit screw on the derailuer so it doen't happen again. That is what controls how far the chain goes in. Roger
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Old 10-03-12, 06:14 PM
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It sounds like you adjusted the low limit correctly. Momentum usually won't do it if properly adjusted, likewise a bump, but yes, it's possible. It is also possible a foreign object like a twig was sitting in the cassette and caused the overshift, especially if you had not been in the large cog for a while. Could have happened when you fixed the flat. You apparently have checked things out thoroughly post-accident, so I think you're fine. You should check the chain from the rear as you pedal in the stand to make sure it does not "snake" or twist as a result of the stress of going into the spokes.
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Old 10-03-12, 06:32 PM
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I am no expert but it sounds like you just had some bad luck. I don't know if a dork disk would have helped but when I replace my cassett I will be putting on a dork disk just in case. I have about 4000 miles on my bike with out shifting into the spokes but I still would like to have more insurance.
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Old 10-03-12, 06:46 PM
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I prefer to call them "spoke protectors" and I put them on all of my bikes. They are cheap insurance against costly and possibly inconvenient damage. I don't much care what other, more elitist, riders think about them (or me).
You might check your derailleur for unsuspected damage which may have caused your problem. Limit screws typically don't go out of adjustment very often.

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Old 10-03-12, 07:13 PM
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Do you have an exposed length of cable run where you can pull on the cable directly with all your might while pedaling to see if you can get it overshift then? Sometimes you can get it to overshift this way but not with the shift lever...
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Old 10-03-12, 10:52 PM
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Originally Posted by rhenning View Post
TL;DR. Set the low limit screw on the derailuer so it doen't happen again. That is what controls how far the chain goes in. Roger
Fixed that for you.

Originally Posted by cny-bikeman View Post
It sounds like you adjusted the low limit correctly. Momentum usually won't do it if properly adjusted, likewise a bump, but yes, it's possible. It is also possible a foreign object like a twig was sitting in the cassette and caused the overshift, especially if you had not been in the large cog for a while. Could have happened when you fixed the flat. You apparently have checked things out thoroughly post-accident, so I think you're fine. You should check the chain from the rear as you pedal in the stand to make sure it does not "snake" or twist as a result of the stress of going into the spokes.
I'd been into the large cog twice on that ride, so I'm not sure how debris could have entered, but it's a thought. I could have flicked something up into the wheel. At any rate, thanks for restoring my faith in my mechanical ability. I checked the chain for stiff links, but not for lateral play. I can't imagine having play without stiff links, but I'll certainly check after your suggestion.

Originally Posted by jim p View Post
I am no expert but it sounds like you just had some bad luck. I don't know if a dork disk would have helped but when I replace my cassett I will be putting on a dork disk just in case. I have about 4000 miles on my bike with out shifting into the spokes but I still would like to have more insurance.
Brilliant! Frankly it never even occurred to me to install one. I don't really care what the dudes on the fixies or racing bikes think of me while I pass them on my commuter. The only question is where I can get a solid one. I don't like the idea of the cheap, brittle plastic ones. I'm sure someone makes a solid, durable version. Any leads?

Originally Posted by dsbrantjr View Post
I prefer to call them "spoke protectors" and I put them on all of my bikes. They are cheap insurance against costly and possibly inconvenient damage. I don't much care what other, more elitist, riders think about them (or me).
You might check your derailleur for unsuspected damage which may have caused your problem. Limit screws typically don't go out of adjustment very often.
I'll give it another look, but it's been fine for thousands of miles and I've never crashed on it or otherwise abused it. The hanger and cage are both straight, and the pulleys run smoothly. Still, it's a possibility and I appreciate the thought.

Originally Posted by PHT View Post
Do you have an exposed length of cable run where you can pull on the cable directly with all your might while pedaling to see if you can get it overshift then? Sometimes you can get it to overshift this way but not with the shift lever...
I tried yanking on it where it runs along the chain stay, per your suggestion. The derailleur didn't move a bit from it's innermost position.

Thanks for all of the ideas (those of you who read my post, that is)!
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Old 10-03-12, 11:38 PM
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Originally Posted by immunizer View Post
Once I got home, and without removing the wheel, I worked through the gears again. I shifted as quickly as I could from the small sprocket to the large one. I could not get it to overshift, or even come close!
You need to pull on the cable instead of using the shifter.

So here's the question: how the hell did this happen? Did I somehow put my wheel on crooked? If so, why can't I duplicate the overshift?
Maybe the hanger is bent and one of the upper and lower pivots was a little sticky so the derailleur was in a different orientation when the over shift happened.

My understanding of how the limit adjustments work makes them independent of the cable: the derailleur will move only so far, no matter what happens to the cable.
Right.
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