Hello, I was wondering how do I replace a rear derailleur. Like what do I need to know. Will any rear derailleur work? Can I put a 3x10 rear derailleur on a bike that is 3x7 and I just won't use the full range? I am asking because I am looking for one on craigslist. And anything else I should know?

Also, since I already need to replace my derailleur is it worth it to spend a little more oney and just upgrade to 3x11 speed? What is involved in doing that? Is it just cassette, chain, and the shifter?

Before I go further,m I remind you that the devil is always in the details. So while the general may be OK, your specific application may not be.

To start with derailleur indexing lives in the lever. The lever pulls a set amount of cable, and the RD translates that to the correct horizontal movement to match the sprocket spacing. So, the key it derailleur interchange is that it and the lever speak the same language, meaning that they use the same cable movement/RD movement ratios.

Companies use these same ratios for a while, then may change, so odds are that if you use the same brand RD form the same generation (ie. any 10s derailleur made within a few years as the levers) it will properly index with whatever the levers dictate.

So, it's not a question of using or not using some gears, as much as matching the lever, which matches the cassette.

There are also other more minor factors like whether the idler cage width being enough for your chain, but those aren't usually issues.

Since you have a 7s system, I suggest you stay as close as you can with a 7s or 8s derailleur of the era, which will most likely be less expensive than a more modern one.

A 10speed derailer Will Not be compatible with the 7speed shifter controls already on your bike.

for 10spd Shimano introduced a new 'Dyna-Sys' compatibility standard, the cable pull ratio is different from the 9spd and below groups.

Also
this is for the bike that got hit by a car and has a taco-ed rear wheel, right?

Good odds that the Derailer Hanger is bent or damaged, not just the derailer itself.

You really should take it to a repair shop to have the Hanger alignment checked and fixed.

Installing a new derailer onto a bent hanger isn't going to fix the problem....

How do you know it's an MTB?

I was about to say the same - Shimano road 10sp, except the current Tiagra, is backwards compatible to shimano sis.

Try replacing the derailleur hanger first, since from your description on your other thread, it sounds like maybe you bent it. Derailleur hangers are cheaper than derailleurs. If your derailleur is damaged, best to replace it with something similar, especially since 10 Speed and 11 Speed derailleurs are not compatible with 7, 8 or 9 speed shifters.

You beat me to it.

You have some good advice on this thread and the other one you recently started. You might want to go read a book, or maybe take a course on bike maintenance and repair. and also to read up a bit on the Sheldon Brown website, as there are numerous articles on bicycles there. I don't say this out of malice as I am not exactly a professional bike mechanic That said, from your threads, you seem to be coming at this with a very limited base of knowledge. And that is fine. We all started as newbies once, and the only way you figure stuff out is to read, talk with people, or tinker on your bike.

Now, the problem with switching the number of gears is, unless you have a pile of parts just laying around, it can get expensive quickly, and often more money than the quality of the bike merits spending. You might be able to switch from 3 x 7 to 3 x 8 or 3 x 9 by swapping out your wheel, cassette, chain, and right shifter. And since you know very little about bikes, along with these parts, you will have to factor in labor. Most people who go to 10 speed don't do a triple crank, so you might need to also factor in a new crankset and/or front derailleur. And the same is true of 11 speed.

Usually, the most cost effective thing is to swap the broken part for a similar or identical replacement. And when you start getting the desire to upgrade, ask yourself why? I still have a 20 year old bike with 3 x 7 shifters and you know what? 3 x 7 is more than enough gear combinations for most recreational riding and commuting. Though 3 x 7 is far from state of the art, I would remind you that enthusiast road riders were riding 7 speeds as recently as the mid 90s and 7 speed was still a thing for mountain biking several years beyond that. Then 8 speed had its reign at the top of the line through the mid 90s, then 9 speed in the late 90s, 10 speed had its moment in the sun from 2003 to 2011. And we are now living in the 11 speed era. But just because higher end new bikes have 11 speed shifters doesn't mean you need that. (and if you did, you probably should have budgeted more to begin with and just bought an 11 speed bike to begin with)

Rear derailleur "attributes":
Rear derailleur

What can be combined with what (compatibility):
4) Bicycle rear derailleur compatibility - Bike Gremlin

I'd not call 3x11 an "upgrade", but a "change". It has it's good and bad points.
For going to 3x11, you need: chain, cassette, derailleurs, shifters and, if it's a road bike cassette, you'll need an 11 speed freehub (usually requires a rear hub/wheel change). A MTB 11 speed cassette will fit an 8-9-10 speed freehub just fine.

How do you know it's not?

I just thought it might be useful to read this part again.

His other, very long thread indicates he has an MTB

What I believe is that you are posting to the wrong thread or totally misunderstand the issue.

Just keep it the way that it is. Far too expensive to justify, and far too complex for you to do, as you seem unable to follow instruction and advice very well at all.

Because if it was not you wouldn't be offering advice that is wrong for road components.