I'm trying to figure out what happened today with my bike. I was backing it out of the racks at work, and I thought I heard a noise when backing it up (will come to that later), but started talking with a co walker and completely forgot about the noise , and went to start riding, and as soon as I make a complete turn of the crank, a horrible grinding noise starts, and the crank locks up. I look down to see the chain has literally worked itself into a knot pretty much, and has fallen off the crank, and is all sorts of jammed up in the rear derailleur. The derailleur is jammed so far up its at the back of its range. I broke the chain and assessed the damage, and I had to force the RD from its stuck position, but the spring is working now, and it doesnt appear to be damaged.

I should note the bike is an 84 cannondale, so there is no RD hanger, just a bolt mount.

Why would the chain have slipped off when coasting backwards? I was in the 2nd to highest cog, so I can't imagine that the RD came in contact with the spokes, as I had just climbed the parking lot hill in the largest cog right before that. The chain was pretty old, and maybe that was to blame? I bought a new chain now but am hesitant to put it on fearing that the same thing might happen again

Internal gear hub bike back up fine, your derailleur does not run backwards,
unless you pay strict attention.
pick the bike up and turn it around.
problem does not rise.

derailleur drivetrain's meant to go forwards, when pushed backwards the chain goes slack and easily comes off the cogs and rings, especially if you are in an extreme gear. just the way it is.

This can happen when there is a tight link in the chain. Had the problem chain been broken and re-joined with a new pin lately?

I think that this may have been the problem. There was a stiff link that I had lubed and tried to work back and forth, but the chain was just too old I guess. I have bought a new chain since, and will put it on today.

I just hope nothings wrong with the derailleur, it seems sort of bent in towards the spokes but it could just be an illusion.

Take it to a good mechanic and have the deralluer aligned. Check the new chain carefully for a stiff link.

Yeah I'm taking it in next week to have a headset installed, so I will just have them adjust the derraileur when its there.

Quick question though, I was going to put the chain on today, and I've read things about making sure the new chain is the right length, and when I hold them up side by side, they are close, about 1/2inch at the most difference. Should I go by length or amount of links since the old one is probably stretched.

Thanks

if "stretched" over 0.125" ie 12.0" has become 12.125" [measuring pin centers]
you will have to consider the teeth on wheel cluster is worn too.

1/2" IS THE STANDARD DISTANCE BETWEEN THE PIN CENTERS OF ONE LINK.

Say what???

I can roll my bike backwards all I want and NOTHING will happen. The chain will not bind or fall off the chainwheels or cogs. This is because there is no "direction" to a derailleur other than needing to have the chain travel in the appropriate direction so the derailleur can control the shifts.

If your chain falls off when you roll the bike backwards it would also fall off the cogs when you spin the pedals "backwards" and it DOES NOT DO THIS! If your bike does this then you have maintenance problems that need to be addressed rather than blaming a component that has nothing to do with this.

As for the OP, a tight link OR someone hit the shifter while the bike was parked. Either can cause the chain to jump off. Have the dropout alignment checked. You may also need to replace your cassette and chainwheels due to excessive chain wear.

The new chain should be the same number of links.

You might want to be very careful until you have the shop check to make sure the derailleur is still aligned. The last thing you want is for a bent derailleur or hanger to cause the derailleur to go into the spokes and ruin the derailleur and wheel, and maybe even the frame. If it were me, and I had any question that it was bent, I'd bring it in for a quick look-see. They should be able to advise you on the spot.

I have no idea if your chain is stretched or not, but what you should do is make the new chain the same number of links as the old one. If it's that much different w/ the same number of links, you have a serious problem with the old chain and maybe worn cassette and chain rings.

More likely, one chain is just 1 link shorter than the other.

Remember, that when you install a new chain (and measure the old one) you need to allow for the connecting link if you have one. The ends of the chain need to be both "inner" links if you're using a connecting link. If you're just pinning the ends together, one end needs to be inner, one outer. So make sure of those things when comparing the old chain to the new one. Like I said, 1/2" difference is most likely a link.

Such sound and fury, signifying nothing.

Even a pristine drivetrain in perfect alignment can do this if it's cross chained, especially using an 9 or 10 speed drivetrain. Take any new bike from your favorite shop, throw it in the big/big combo and spin the pedals backwards, see what happens. Or, as you go on to suggest, the rear derailleur got moved to a higher cog before rolling backwards.

Is that a maintenance problem?

Well I had to drive into town today anyways, so I decided to bring my bike with me to see if they could possibly take a quick look at the derailleur. The back one is definitely bent, the long cage is bent inwards, and since its an old derailleur, they reccomended I buy a new one.

They offered to install one of the RD's that theye had in stock, but the only long cage they had was a $140 shimano (not sure what model, just saw the price tag and denied it). Being in a small town its like that sometimes. I have to use a long cage since I have a touring cassette, and the large rear cog has 30t.

I was going to order a new derailleur, but wasnt sure what to look for, never had to buy one before. Whats a decent long cage that will handle up to 30t, and reliable for touring? I don't need anything super fancy, just the basics will work.

I sure hope that the dropout isnt bent, there is no hanger on this bike, the RD attaches into a bolt hole right on the frame, so I can't imagine that it would have bent that entire dropout. Can I still even buy RDs that dont use a hanger anymore?

Sorry for all the questions, Im just trying to get my bike up and running so I dont have to drive to work

I'm going to guess and say some nice person shifted your bike for you while on the rack so when you rolled it backwards it wrapped it's self up and you didn't notice and when you took off it really jammed up.

Heres my situation. You can see the lower dropout is slanting inwards, problem is, I don't remember if it was like that normally...can anyone who owns an old cannondale (I think most of their dropouts were relatively the same) tell me if that bend is normal or not?
Thanks guys. Hopefully I get this all worked out before my tour thats coming up soon.

Very likely. There are those who think this is the ultimate in humor. Happened to me, just not quite as dramatically.

I straightened a bent derailleur hanger on a steel framed bike using a large adjustable wrench. Use small tweaks until it is straight!

It is bent!
For now so you can ride it take it off and put it in a vise and try to get it straightened out best you can but get a new one and replace asap once they bend its very weak and will do so again.
They are easy to find and don't cost much thats the good thing.

Well the thing is, its not a hanger, on these old school bikes they didnt have those. Its part of the actual frame, thats what I'm worried about.

Its an aluminum frame, so it would probably be pretty easy to bend back, but I'd like some more opinions before I jump into it and possible screw it up for good.
Thanks

IF the shifting guide pulley is not directly under the same cog the chain is on ,
then pushing the bike backwards turns the whole drivetrain, and derails the chain
between the cog and the guide pulley, tangling up the best otherwise adjusted derailleur mechanism.

Here is what you do , grab the bike's back brake, roll it up on its back wheel ,
do a little pirouette , on the back wheel,
and set the bike back down facing the other way and go Forward with the bike moving forward with you.

When you were in the bike shop why didn't you get them to straighten that out?

The derailleur is one thing (you decided to find a cheaper option which is fine.... $140 is a lot), but the bent part is a totally different matter that also has to be fixed. If it were me, I'd have a competent shop do it because I just don't know how simple or dangerous it is to straighten that part.

As for your question on what to buy to replace it, what's on there now? (make and model)?

Ok if were my frame I'd remove wheel and rear derailler as in your photo.
Lay the frame with that side up I'd get a solid heavy piece of steal or a large sledge hammer head old it on the bottom side tight up up to the hanger and take another hammer and tap on the bend it will flatten right up you must hold something solid against it same method a body and fender person would use to remove a dent, fast and simple.
Do not just grab it and pull it will bend in the wrong place and make it worse than it is and end up like a wave.

Yet another member member on this forum that I would not want with 100 miles of one of my bikes. Use there proper tools or have some on do it that has them.

Actually this method would be the last resort for someone with out the correct tools.

I actually do happen to have a hanger straightening tool in my tool box I only suggested the method for someone who does not want to buy one but still wants to do it them self it does work quite well you don't pound the hell out of it just light taps will correct it.

Got to agree with Milice - this is not a "dent" - it's a bent derailleur hanger, with threads and the same ALUMINUM as the frame. So if you pound on it you will likely ruin the threads or produce a crack. Bending it back will not be an easy task, as aluminum is not ductile like steel. If doable I would not advise trying it on your own.

Last resorts for risky repairs are not advisable for non-mechanics anywhere but in the sticks. It's best to have a mechanic experienced in frame repair, or better yet a frame builder, do the work.

As for the derailleur, of course you can buy one that does not come with a hanger. The hanger goes with the frame nowadays, not the derailleur.

"Sound and fury?" Gimme a break.

There is so much voodoo science on this forum that it scares me. And it seems that the most prolific of these parties are the ones who won't tolerate any alternative viewpoints without abusive responses.