Originally Posted by
flashpoint145
Okay, so first off, if you're changing out a cassette, you need to change your chain also. They both wear the same/evenly, or should. Both also can cause the shifting difficulties as you described.
Next, gear numbers. The speed is dictated both by your shifters and casstte, and the rear derallieur's capacity. Generally the shifters determine this based upon their indexing. Thus if you havent changed your cassette yet or have taken it to a bike shop only for this, it's likely an 8 speed. For beginners, determine this based upon how many cogs there are in the cassette. To change the speeds of a bicycle you have to upgrade pretty much your entire drivetrain, you cant just buy a 10 speed cassette and expect it to work. It would skip and autoshift like crazy. You will need an 8 speed cassette.
Next compatibility. Shimano and SRAM are interchangable. Campagnolo is it's own thing. Thus, you can easily buy a SRAM cassette for to replace a shimano, and vice versa, but not campy. The compatibility is based upon what wheel setup you have as the cassette fits on the wheel and the wheel manufacturers design the wheels to be compatible with a specific drivetrain manufacturer, either SRAM, Shimano, or Campy. You will need a Shimano or SRAM cassette.
Next, the cassette numbers. The number range is variable and based upon what the rider plans to do. For example 11-23 is a common time trial cassette. 12-32 is a common mountain biking cassette. The smaller the number, the harder/bigger/faster gear. The larger number, the lower/easier gear. The dash indicates the range. A wider range generally indicates a climbing cassette, and a smaller range is more for flat roads or time trials. Generally 12-25 is a universal all purpose road cassette. The choice is totally yours what cassette you want, but I hope I spelled it out. The sizes that you can find range from 11 in the smallest to 35 in the biggest, and there are dozens of combinations of those. JUST BECAUSE YOU HAVE ONE SIZE DOES NOT MEAN IT HAS TO BE EXACTLY THE SAME WHEN YOU UPGRADE. For example, if you have a 12-26, you can easily switch to 11-23 no problem...provided it's still the same brand (sram/shimano, campy)
Essentially, the numbers don't matter mechanically, it's just your preference and where/what you ride.
You need special tools to remove and install the cassette, and also a special tool for replacing the chain. There are multiple videos on youtube on how to do this, and you can't really mess the replacement of these parts up. You can get all the tools you need off amazon for probably $15: Chain whip, cassette tool, chain breaker.
Also to note: Skipping and autoshifting have dozens of causes, with teh cassette and derailleur being most common. You could have problems with shifters themselves, cables, bottom bracket, derailleur, etc. If the issue persists after replacing cassette and chain, I'd take it to a mechanic to look at.
You can try inspecting the rear derailleur on your own and adjusting the barrel adjuster (google: barrel adjuster) to diagnose and rule out tensioning problems.