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Rear derailleur replacement

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mrshanno Offline
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Posts: 5
Joined: Jun 2012
Post: #1
Rear derailleur replacement
Hi,

I have a Giant bicycle and the rear derailleur has gotten bent...It is a shimano 7 speed RD-MR40. I can't seem to find a direct replacement. Any advice on a compatible replacement (keeping low to mid budget in mind)?

thanks
Jun 24, 2012 01:34 PM
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xerxes Offline
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Kent, UK
Posts: 609
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Post: #2
RE: Rear derailleur replacement
Jun 24, 2012 04:42 PM
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djembefora Offline
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Post: #3
RE: Rear derailleur replacement
(Jun 24, 2012 01:34 PM)mrshanno Wrote:  Hi,

I have a Giant bicycle and the rear derailleur has gotten bent...It is a shimano 7 speed RD-MR40. I can't seem to find a direct replacement. Any advice on a compatible replacement (keeping low to mid budget in mind)?

thanks

you can probably bend it back into place if nothing is actually broken. take a picture?
Jun 24, 2012 07:27 PM
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Joe_W Offline
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Post: #4
RE: Rear derailleur replacement
You cannot unbend metal. Sorry.
What is actually bent? The dérailleur or the hanger? The hanger can be fixed in a couple of minutes using the correct tools. When the dérailleur gets damaged you should always check the alignment of the hanger anyway.
Jun 25, 2012 06:52 AM
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djembefora Offline
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Post: #5
RE: Rear derailleur replacement
(Jun 25, 2012 06:52 AM)Joe_W Wrote:  You cannot unbend metal. Sorry.
What is actually bent? The dérailleur or the hanger? The hanger can be fixed in a couple of minutes using the correct tools. When the dérailleur gets damaged you should always check the alignment of the hanger anyway.

cannot unbend metal!? i have bent at least a half dozen derailleur cages and/or hangers back into functioning shape at our shop in the past month alone. are they as strong as they were before? no. are they a little bit more likely to break completely the next time you take a fall? yes. is this a cheap way to get your derailleur working perfectly again? yes!
Jun 25, 2012 06:59 AM
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mrshanno Offline
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Post: #6
RE: Rear derailleur replacement
I will take a picture and post it tonight to see if it can be salvaged...thanks for your help!

I know the integrated hanger on the steel frame its bent some, as well as the cage plate for the lower pulley on the derailleur itself.

In case it can't be fixed, thanks for the replacement suggestions.
Jun 25, 2012 07:43 AM
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mrshanno Offline
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Post: #7
RE: Rear derailleur replacement
In these pictures hopefully you can see the bent hanger in the first picture and the bent cage for the lower pulley in other pictures. In the last picture of the hanger, you can see where a screw appears to be missing. I apologize for the quality of the pictures, I can take them again if it's helpful.

Thanks again!

bent hanger

bent cage

bent cage

Missing screw on cage
Jun 25, 2012 09:43 AM
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xerxes Offline
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Kent, UK
Posts: 609
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Post: #8
RE: Rear derailleur replacement
The bent hanger should be easy enough, I've seen worse than that, here's how: http://www.utahmountainbiking.com/fix/rhanger.htm

In addition to the method shown in the link above, I would buy a couple of nuts to go on the bolt, then tighten them either side of the hanger, this will spread the load a bit and there's less chance of damaging the thread in the hanger.

I have also done it without a bolt, just using a large adjustable spanner nipped up either side of the hanger and using it to lever the hanger straight. Just do it gently and slowly and don't use a hammer on it.

The derailleur cage could probably be fixed as well, but given that you've lost a bolt and a similar new one would cost around $10.00, I would be inclined to replace it.
Jun 25, 2012 10:49 AM
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mrshanno Offline
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Post: #9
RE: Rear derailleur replacement
Thanks for all your help and the links...I certainly can fix the bent hanger using the method shown.

As for a replacment, I'm not sure if this is a "long cage", but do you think it would work (see link below)?

Shimano Tourney TX75
Jun 25, 2012 01:51 PM
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xerxes Offline
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Kent, UK
Posts: 609
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Post: #10
RE: Rear derailleur replacement
Should do the trick, shop around though, you might be able to find it a bit cheaper. Try Google shopping.
Jun 25, 2012 02:52 PM
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painkiller Online
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Peoria, IL. USA
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Post: #11
RE: Rear derailleur replacement
After seeing the pictures I will chime in now.
replace the derailleur. buy this tool first
http://www.parktool.com/product/derailleur-hanger-alignment-gauge-dag-2
do not dilly dack around with threaded rod and straight blades
This tool is critical for perfecting shifting imperfections and performance issues. Always start with making sure the wheel is true first
a nice set of calipers goes well too
use the 4 point method and align to 1mm to 2mm top to bottom and side to side
you will use this tool more often than you think
a must have for anyone wanting to be a bicycle mechanic at any level


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There are two kinds of people in the world, "Those who help themselves to people, and those who help people!"
Jun 25, 2012 05:20 PM
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xerxes Offline
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Kent, UK
Posts: 609
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Post: #12
RE: Rear derailleur replacement
It's quite an expensive tool, £50.00 is fine if you're a bike mechanic and going to get plenty of use out of it, but it's way too much to be economical for a one-off repair.
Jun 25, 2012 06:03 PM
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painkiller Online
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Peoria, IL. USA
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Post: #13
RE: Rear derailleur replacement
(Jun 25, 2012 06:03 PM)xerxes Wrote:  It's quite an expensive tool, £50.00 is fine if you're a bike mechanic and going to get plenty of use out of it, but it's way too much to be economical for a one-off repair.

for a one off repair? or a lifetime of tune-ups even if only used as a check gauge..MMmm sorry, Disagree on this one. the price is chump change when you consider the ease of use and how much time you save using it. not to mention the money it can make you over the years. Sure, one could use the spartan method and get the bike usable. I just wanted to give good advice as to how it is really done and the best way over the long run.
" Learn from a pro and become a pro, learn from an amateur and become an amateur " man! I hate that old saying. Sad


There are two kinds of people in the world, "Those who help themselves to people, and those who help people!"
Jun 25, 2012 08:06 PM
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mrshanno Offline
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Posts: 5
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Post: #14
RE: Rear derailleur replacement
Thanks to all who posted advice...I ended up taking it to a bike shop and for $20 including parts/labor I'm back in the game. I guess I could have done it myself like I planned for the learning experience, but decided I would feel better having him make all the adjustments, etc.

I appreciate it.
Jun 27, 2012 10:22 AM
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GeorgeET Offline
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Venice Beach, CA
Posts: 1,162
Joined: Apr 2010
Post: #15
RE: Rear derailleur replacement
Ye it always helps to have a good relationship with your local bike shop. Some do it yourself repairs can get costly. Also some items loose warranty when home installed.

Never Give Up!!!
Jun 27, 2012 10:42 AM
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