I have a mongoose mountain bike with 3 gears in the front and 7 in the
back. The rear derailleur is all bent up and needs to be replaced.
Is it easy to do? Do I need special tools? What kind of rear
derailleur should I buy? Are they any good cheap ones online?
It's easy once you get the idea. You need Allen wrenches (usually 5mm and
3mm) and a small screwdriver. It's good to replace the inner cable at the
same time, so you'll want decent cable cutters too. If you don't want to
break the chain, the rear plate of the cage needs to be removed via the
jockey wheel bolts. You'll find full instructions online for fitting and
Probably any Shimano long cage derailleur will be fine. Look for one
described as 8 or 9-speed. This will also work with 7-speed.
If you use your bike a lot I wouldn't go for a Shimano A* (Altus,
Alivio,...) because you may or may not be lucky ;-) I used one of the
"Deore" groups RD and got *much* better performance out of it - as well
in shifting accuracy as in little running resistance. Actually I only
had two A* and one Deore LX and I'll never again want an A* ;-)
Please, before you mount your new RD, check the alignment of the
derailleur hanger - if it's bent you'll never be able to adjust your
Some notes in addition to the good advice tendered so far.
* You can do a good job installing the first time out.
* Read the instructions thoroughly before installing.
* Probably should install a new chain.
* Heed the advice about making the cable run good.
* Be sure to set the B limit screw so that the near
jockey pulley is close to the largest gear in back.
* The high and low limit screw settings are important.
Get them right.
* The limit screw that stops the derailleur closest
to the spokes is critical. It must be set out far
enough to allow the chain onto the largest gear, but
tight enough to prevent the chain going into the spokes.
When you think it is correct, put the chain in the
smallest gear in front and the largest gear in back,
turn the crank and push the derailleur body with your
thumb to see if the chain slips over the top of the
largest gear and into the spokes. Adjust as necessary.
But if the derailleur was "all bent up" as described in the original
post, it's quite possible the chain might be twisted, or that it could
have been a separating link that caused it. In any event, checking the
chain for damage (twists or tight links) as well as wear would be a good
--Mike-- Chain Reaction Bicycles
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