Derailleur jams when rolling backwards
 

If I roll my bike backwards, fairly quickly the following happens:
1) I hear a little 'tick' sound like a link catching on a jockey wheel or something
2) The derailleur starts to bend forwards
3) The chain gets pulled onto bigger sprockets
4) The chain gets pulled off one of the jockey wheels (the 'middle' one), getting jammed between the derailleur cage and the cassette.

Presumably it's related to my recent repair of a broken derailleur hanger which necessitated:
1) Replacing the derailleur hanger
2) Straightening a dislodged jockey wheel
3) Removing some shredded metal from both sides of that jockey wheel
4) Replacing gear cable
5) Reassembling chain, which is now a couple of links shorter than ideal, and has two power links and one of those special pins.
6) Re-mounting and adjusting derailleur, which for some reason necessitated adjusting both hi and low limit screws.

Whew. So, any ideas what causes this issue? Just the shortened chain? I'm also having a massive chain skipping problem on exactly three combinations: granny gear (26) to 2nd, 3rd and 4th biggest sprocket (32, 28, 24). It doesn't just "skip", the chain leaps off the sprocket onto a completely different sprocket, then climbs back up again.

Bent derailleur maybe?

Steve

"...chain, which is now a couple of links shorter than ideal"

If your chain is too short to fit onto your big-big combo, you are riding your bike in a perilous condition. If you forget (and you will) and shift into that combination you risk severe damage to your derailleur and possibly your frame.

The first thing I'd recommend is to get a new chain and make sure it is long enough. Then worry about the derailleur.

+1

Never use a chain that's too short except in an emergency. Catastrophic failure will result, sooner or later. Given a broken RD hanger, it's certainly possible your RD is also damaged from your description of the crash. First replace the chain and see where that gets you.

I wonder if the RD was terminally damaged beyond what was obvious.

>I wonder if the RD was terminally damaged beyond what was obvious.

Yes, so do I. How can I tell? I'm in the lucky situation that I have an identical bike to compare against - I can't see anything obviously broken, bent, twisted etc - but it's sort of hard to tell. Any tips on what to look for?

First off make sure the chain is long enough to loop the big/big without pulling the RD up to it's stop post. The chain is so much stronger than anything it loops, so if over-tightened it'll destroy whatever it has to.

Now check that the RD is low enough (B screw setting) that the upper pulley never can touch a sprocket through the chain. If you thing yours might be too close, try lowering it a bit and see if it helps. You can always reset it later.

Here's a way to make a poor man's gauge for checking if the hanger and RD are vertical. It's not as good as the tool, but a damn sight cheaper. You'll need a reliable straight edge long enough to span the rear wheel, and a short ruler. Shift ti a larger sprocket, but it doesn't have to be the largest. Set the straight vertical against the wheel, and measure out to the top and bottom loop of the chain. The distance will be the same if all is truly vertical, otherwise you'll need to bend the hanger to correct. Note that this doesn't measure horizontal twist, but that isn't as common a problem.

Most hangers can be gently bent by putting an allen key in the RD's mounting bolt and horsing it over a bit.


Otherwise your problem could be caused by a stiff link in the chain or a stiff pulley, among other things.

Make sure the chain is routed through the RD pulleys correctly--the chain should not touch anything but the pulleys.

+100. it's a very common and easy error to route the chain outside the tab near the middle of the cage.

This will make a sawing sound when you ride, and jam the chain like yours when backpedaling or rolling backward.

oops
 

So glad I found this. I feel like an idiot, but I had routed my chain around that outside tab. I rode like this for quite a bit too, hope I didn't do any damage to my derailleur or chain. It seems to run like butter now though!

Don't roll backwards. lift the bike, instead, and turn it around.

For the problem when you move the bike backward: Certainly one should not be moving the bike backward just to do it, but it's inevitable one will do so at some point, and it should not cause serious problems if everything is adjusted and aligned properly. If you have an identical bike why just look? Switch out the derailleur first, then the wheel to isolate the cause.

You also need to see what happens when the problem first occurs, rather than just the sound. If the derailleur starts to pivot forward before anything else happens the problem has to be at the point where the chain meets the top derailleur pulley or between the two derailleur pulleys, the latter being very unlikely.

The short chain is not likely the issue, though it certainly should be corrected.

The derailleur is "hunting" due to not being properly aligned to the rear cog (not sprocket) that it is under. To have that occur only in certain cogs strongly implies an alignment issue, either the hanger or the derailleur itself is bent. You must first make sure that the hanger is aligned, and given the apparently severe trauma your derailleur went through (helps to know what happened) I would say you need to have it professionally aligned, as the hanger may well be twisted rather than just bent inward. If it is OK and you still have the problem again I would suggest switching out the derailleur from the identical bike.

About the only thing this ever damages is the mechanic's pride. Since it's on the slack side of the drive the only load is the RD cage spring, so if it's ignored for long enough, the chain might saw through the tab and solve the problem by itself, but even that doesn't matter since the tab isn't necessary.

Just a little quibble.

Technically the proper use of the term "cog" is a tooth or projection on a wheel or bar. I know that we call the rear sprockets "cogs" very commonly in the bicycle world but really it's a misuse of the term. So stevage is the more correct by referring to the.... er... sprocket as a "sprocket".

I've seen the chain derail and jam up the RD on many bikes that don't actually have anything at all wrong with them. It seems to occur moreso when the cross chaining is such that by backing up the chain is fed off the sprocket it's riding on and over to the next one or two. How far over it goes depends on the amount of the cross chaining angle at the moment. The same thing would happen if you back pedal the cranks at that point instead of pushing the bike backwards.

I did that once. So have many others.

Only once? You're good.

I've done it at least 6 times, but i usually catch it as soon as I join the chain and try to spin it. (Doh!)

I can only remember once. The memory is the first thing to go. :)

I suppose we could go on forever about proper terminology. Yes, saying "not sprocket" was incorrect as it technically is a sprocket, but yes, in the bicycle industry cog has become short for cog wheel (from Sheldon's glossary - Originally, "cog" referred to just a single tooth on a "cog wheel." Then "cog wheel" was shortened by popular usage to "cog). Several sources show cogwheel as the first or second definition. It's not as much a misuse as an adaptation, which is common with language.

If one uses sprocket it's necessary to specify front or rear (and many don't go to that trouble) which is why many prefer cog for the rear and chainwheel for the front in bicycle usage.

When everything is set up properly you can still get the chain to derail by walking the bike backwards or turning the cranks backwards if the chain position is out by the smallest amount... this little mis-alignment is something that does not cause problems when you are pedaling forward under load but will derail the chain when you back pedal under no load.

When I am setting up indexed drives I backpedal to test that the alignment in a given gear is correct as this can be masked when the bike is under load.

The odds of the chain derailing while the bike is in reverse will also increase when there is more deflection on the chain.

I'm glad to know I'm not the only one who has done this and it doesn't sound like I caused any damage, phew! I don't think I'll make that mistake again though....live and learn and ride!

Yes you will,

But at least you'll catch it much sooner.

Any mechanic who says he's never done that either hasn't replaced a lot of chains, or is a liar.

Cny' I only wanted to point out that we shouldn't call out folks for using the more accurate term even though it's normal or almost normal to use the colloquialism version in the cycling sport. Sorry if it sounded a little like preaching.

I call them "cogs" as well for the sake of clarity. But I cringe a little inside each time I do... :D

I did it once a long time ago... has become one of those measure twice cut once tasks that I do see a lot of people doing when they first learn to thread a chain.

I like the old open cage designs on Suntour rear derailleurs... it was one of those great design ideas that set them apart although offered a few models with a cage that could be opened manually for chain removal.

I did that as late as 10 days ago. Makes one feel incredibly stupid.

Yup. They're so cool!