megacoupe

I bought a used Trek 7.1 FX that rode beautifully and gave me no problems the few times I rode it. I invited someone to take it for a spin and he shifted to the lowest gears so he could try out every speed, from bottom to top.

The chain came off the front cog, so he shifted back up, then got the chain partially around the cog. Normally, I would spin the pedals to get the chain to align the rest of the way, but the rider just jumped on and started pedaling. The next thing I know, the bike locks up and the cage on the rear derailleur bent back and up, getting stuck between the frame and the RD. I tried pulling it back down, but it's locked in pretty good (and I was afraid to apply too much pressure and break it).

A few things:

1. Why did the cage bend back? Something to do with the chain not being completely wrapped around the front cog? If so, why did it cause a problem near the rear cassette? Or did the rider make a mistake in shifting everything down and then trying to ride the bike?

1a. What is the correct way to shift? One small shift at a time or is jumping around to any gear you want okay?

2. Can the cage be bent back down and still work? With some effort, I might be able to get it out of the stuck position it's in now, but will it still work and shift gears correctly? Even if it won't work, should I try this anyways for the experience?

3. If it can't be bent back, does the whole rear derailleur need to be changed, or just the cage?

4. How much does it typically cost to fix or replace this sort of problem? Would it be better to buy the rear derailleur myself and just pay the LBS for the installation?

5. Finally, if I do need to buy a new RD, is it worth the few extra bucks to upgrade from the Shimano Acera to a Shimano Deore? To be honest, I loved the shifting on this bike before the RD cage bent.


Sorry about all the questions, but I'm still new to biking and trying to learn as much as I can.

bradtx

megacoupe, There are two combinations, chainring/casette that should be avoided, big-big and little-little. Generally just to reduce wear, but also because bad things can happen.

1. Possibly attempting to shift to the big-big combination.
1a. You can shift across the freewheel or cassette if you wish.
2. Maybe.
3. Generally just as cheap and easier to change the complete RD.
4. I don't know.
5. Either would be fine.

Brad

megacoupe

I'm looking at both Shimano Aceras and Deores and I can't figure out which one is compatible with my 7.1fx. Does it matter if it's a long or medium cage? I know that my current RD is an Acera m360; which model of Deore could replace it?

cyclist2000

Well the Acera m360 RD is an sgs rear derailluer, so if you do replace it with a Deore 591has the same rear cog max capacity so it should work fine as capacity goes, I don't know about cable pull, and working with an 8 speed. Someone else may be able to enlighten you about that.

I don't know if you know this but a lot of information can be found on shimano.com, look at the techdocs under cycling. They have information on derailuer capacities and set-up instructions. This will help to see if different parts are interchangeable.

bikeman715

try removing the derailleur from the bike. that should allow you to move the cage .

LarDasse74

You can replace your 8 speed derailleur with a Shimano 9 speed derailleur. They are 100% compatible. Deore will work perfectly.

Why did this happen? There are a couple possible reasons I can think of. Perhaps there was a damaged link in the chain that grabbed the rear derailleur as it came around; perhaps the bike was cross-chained (little-little) and the bottom of the chain ran off the bottom of the derailleur cage and got stuck; perhaps a peice of debris or a rock got stuck in the cage and this was just very bad luck.

You will likely need to replace the derailleur hanger on the frame as well as the derailleur.