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My derailleur kinked up when I was pedaling hard and became a twisted mass of metal. It has a derailleur in the back which is a Shimano Altus and has some numbers on it that Google didn't identify. I can't find a manual for the exact bike online, I think it is a 2003 model.

Does anyone know how to find the exact specifications for such a derailer? I am afraid that if I go to a bike repair shop, they will tell me that they (and they alone) have the right connections to get the part and then they will order one, and I will have to pay the price for a brand new one, plus shipping and then pay the guy 45 bucks an hour to install the thing.

I could probably get another bike like mine used for what I might pay the professional. (If I were very wealthy then I would support such people as bike repairmen, but I prefer to borrow tools from friends and fix things myself.

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    Can't really tell for sure without more info, but Altus appears to be a bog-standard derailer. You basically just need to match the number of speeds. – Daniel R Hicks Oct 2 at 22:28
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    Is it 7 speeds? The basic specs for that haven't changed since well before 2003. Any 7 speed Shimano derailleur ripped off a police auction bike or recycling center bike or whatever will work just the same. Bike repair co-op here has a crate of the things. – Affe Oct 2 at 22:36
  • IMHO replacing a damaged rear derailleur is actually likely to require some bike mechanic experience and specialized tools, so it may be worthwhile to pay someone to do it for you. For one, you want to make sure that non-replaceable parts of the frame have not been damaged beyond repair. Detailed photos of the damaged derailleur and derailleur hanger/rear chainstay area would help quite a lot. – Armand Oct 3 at 3:54
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    @Armand What specialist tools are needed? The whole job should be possible with some hex drivers, maybe a screwdriver. If the jockey wheels are rivetted in, then a chain tool is about as complex a tool as needed. If the cables need replacing, then care with cutting is necessary. – Criggie Oct 3 at 12:00
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    @Criggie I had a similar event happen on my bike about 10 years ago when an internal flaw on the Shimano derailleur body casting caused it to fail while riding, getting all caught in the spokes and bending the frame's integrated derailleur hanger. In such a situation, I'd want a derailleur alignment tool and some dropout alignment tools to get the frame fixed or decide it's trashed. OP's new comment suggests there may be a similar issue. – Armand Oct 4 at 3:50
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Good news. Shimano kept broad compatibility between their products, even between generations.

An Altus derailleur from circa 2002 may be from a 7 or 8 speed (number of gears in the rear cluster) drivetrain. You can actually use any Shimano 7, 8 or 9 speed derailleur with a 7 or 8 speed shifter. Shimano used the same derailleur actuation ratio (ratio of cable pulled to lateral derailleur movement) for all these drivetrains.

Remember you need to select a derailleur that has a sufficient total capacity (capacity required is (difference between max and min sprockets teeth) + (difference between max and min chainring teeth), and has a appropriate max and min largest sprocket size for the sprockets on your cassette.

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Argenti Apparatus' information is correct and there are several different derailleur models that would work as a drop-in replacement for your old Altus. You should stick with a Shimano derailleur model that will be marketed as a 7, 8 or 9 speed. Here, and in today's modern bike lingo, "speeds" refer to the number of rear cogs as opposed to the total number of gear options available in a bike with multiple chainrings, in which case multiplying the # of chainrings x # of rear cogs = #speed bike.. Anyway, you presumably have a Shimano shifter, which is designed to pull a specific amount of cable during a shift,. The derailleur is designed to move a certain distance for each mm of cable pull. This is termed a derailleur's "actuation ratio." The importance here is that differing brand's have differing actuation ratios by design. For a consumer shopping for a new derailleur, it means you must match brand of shifter with the same brand of derailleur. This is a somewhat simplistic explanation (believe it or don't) because exceptions exist but are not relevant to your described situation.

There are many options available besides a bike shop for one to purchase a derailleur, either new or used.. Arguably the most broad selection and convenient shopping takes place online. eBay, Amazon, and others are good places to search for derailleur's. Social networking sites such as Facebook, have pages or groups devoted to the sale and trading of bike parts. Many larger communities have bicycle co-ops which are places where people donate new or used bike parts that others can take or use for very low cost. Shimano Altus derailleur's are marketed toward entry level bikes and bicyclists. They are a lower tier of Shimano mountain bike components which simply means the material they are made from and the bikes they are found on are not particularly expensive. This is not to say that that it's poor quality or lacking in performance. In fact, Shimano derailleurs and other parts are known for high quality and performance up and down the line, and at today's prices a new Altus derailleur sells for between $20-30 new. One can expect to pay about 50% of that for a used version.

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