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Rear derailleur problem
I have a new bike and suddenly I can't shift to the 3 largest sprockets on the cassette. When I do the the chain skips around a lot and won't catch. Is this something that can be adjusted with the low gear limit stop screw or do I have a bigger problem? I'm a bit hesitant to start attempting adjustments since I'm new at this.

This video might help: http://bikeride.com/adjust-front-derailer/

I'm not sure what part of the front derailer video will help with a problem on the cassette - not that I'm not positive that the front isn't the problem, just I'm not sure what adjustment would help.

My fault, I completely mis-read your message. Here's the video for rear derailleur adjustments: http://bikeride.com/adjust-rear-derailleur/

I was recently biking and I shifted from second gear to first and my rear derailleur caught my spokes and snapped right off. The bike is a Mongoose and the derailleur is a Shimano Altus.
I need to know what replacement I can buy and how to install it.

First question: How did the derailleur catch your spokes? Perhaps the first question on how to solve this. Did it get bent somehow? You'll want to determine how this happened before you replace anything, so it doesn't repeat itself with your replacement.
As for the replacement, the first thing that will be useful is the model #. You should find that somewhere on the main body of the rear derailleur ("Shimano Altus" isn't too useful, honestly). As for what to replace it with, generally you should be okay as long as it is replaced with a similar derailleur, especially the same bracket since it will need to fit on your frame.
Derailleur removal instructions (this would be easier with a picture of the rear of your bike to be specific, so these are very generic):<br />
1) Remove the chain from the bike and unloop it from the derailleur parts. Video tutors are on this site for that. You will want to double-check the chain as well to see whether it has been damaged or not (same goes for the spokes).
2) You will then likely need to remove the derailleur guard (if there is one).
3) Then remove the rear wheel, as it is likely the axle is put through some part of the bracket and the axle nut is tightened down onto the derailleur. A video tutor is on this site for that as well.
4) Disconnect the shifter cable from the derailleur.
5) Then you will find a bolt and nut through the derailleur bracket. Remove this and be careful to not pull on the bracket as you do, and collect all the parts (this will be useful at least to determine a good replacement). The derailleur and its bracket should come off.
Repeat the process in reverse to put the new one back on.

Why is it that they make adult bikes that'll generally work for 5'9" or above, yet when you pedal these same bikes they only work for someone who is 5'4" or so?

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